Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Matthew Wilcox]

On Tue, May 15, 2018 at 09:57:44AM -0400, Waiman Long wrote:
> > Afaict the whole .owner=NULL thing in release already stops the spinners
> > dead, and the above 'fixes' the debug splat. And this avoids exposing
> > that horrible interface and keeps the mucking private to
> > rwsem/percpu_rwsem.
> 
> Actually setting owner to NULL does not stop spinning. The code just
> assume that the lock is going to be freed and spin in the outer loop. We
> need some special value to indicate that spinning should be stopped. How
> about just exposing a special value for that in linux/rwsem.h? Any
> suggestion for a good name?

RWSEM_NO_OWNER

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Matthew Wilcox]

On Tue, May 15, 2018 at 09:57:44AM -0400, Waiman Long wrote:
> > Afaict the whole .owner=NULL thing in release already stops the spinners
> > dead, and the above 'fixes' the debug splat. And this avoids exposing
> > that horrible interface and keeps the mucking private to
> > rwsem/percpu_rwsem.
> 
> Actually setting owner to NULL does not stop spinning. The code just
> assume that the lock is going to be freed and spin in the outer loop. We
> need some special value to indicate that spinning should be stopped. How
> about just exposing a special value for that in linux/rwsem.h? Any
> suggestion for a good name?

RWSEM_NO_OWNER

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Waiman Long]

On 05/15/2018 04:51 AM, Peter Zijlstra wrote:
> On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
>> Signed-off-by: Waiman Long 
>> ---
>>  include/linux/percpu-rwsem.h | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
>> index b1f37a8..dd37102 100644
>> --- a/include/linux/percpu-rwsem.h
>> +++ b/include/linux/percpu-rwsem.h
>> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
>> percpu_rw_semaphore *sem,
>>  bool read, unsigned long ip)
>>  {
>>  lock_release(>rw_sem.dep_map, 1, ip);
>> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>>  if (!read)
>> -sem->rw_sem.owner = NULL;
>> -#endif
>> +rwsem_set_writer_owned_nospin(>rw_sem);
>>  }
>>  
>>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
>>  bool read, unsigned long ip)
>>  {
>>  lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
>> +if (!read)
>> +rwsem_set_writer_owned(>rw_sem, current);
>>  }
> So what's wrong with adding:
>
>   if (!read)
>   sem->rw_sem.owner = current;
>
> ?

Yes, we can certainly do that within a "#ifdef" block.

>
> Afaict the whole .owner=NULL thing in release already stops the spinners
> dead, and the above 'fixes' the debug splat. And this avoids exposing
> that horrible interface and keeps the mucking private to
> rwsem/percpu_rwsem.

Actually setting owner to NULL does not stop spinning. The code just
assume that the lock is going to be freed and spin in the outer loop. We
need some special value to indicate that spinning should be stopped. How
about just exposing a special value for that in linux/rwsem.h? Any
suggestion for a good name?

Cheers,
Longman

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Waiman Long]

On 05/15/2018 04:51 AM, Peter Zijlstra wrote:
> On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
>> Signed-off-by: Waiman Long 
>> ---
>>  include/linux/percpu-rwsem.h | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
>> index b1f37a8..dd37102 100644
>> --- a/include/linux/percpu-rwsem.h
>> +++ b/include/linux/percpu-rwsem.h
>> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
>> percpu_rw_semaphore *sem,
>>  bool read, unsigned long ip)
>>  {
>>  lock_release(>rw_sem.dep_map, 1, ip);
>> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>>  if (!read)
>> -sem->rw_sem.owner = NULL;
>> -#endif
>> +rwsem_set_writer_owned_nospin(>rw_sem);
>>  }
>>  
>>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
>>  bool read, unsigned long ip)
>>  {
>>  lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
>> +if (!read)
>> +rwsem_set_writer_owned(>rw_sem, current);
>>  }
> So what's wrong with adding:
>
>   if (!read)
>   sem->rw_sem.owner = current;
>
> ?

Yes, we can certainly do that within a "#ifdef" block.

>
> Afaict the whole .owner=NULL thing in release already stops the spinners
> dead, and the above 'fixes' the debug splat. And this avoids exposing
> that horrible interface and keeps the mucking private to
> rwsem/percpu_rwsem.

Actually setting owner to NULL does not stop spinning. The code just
assume that the lock is going to be freed and spin in the outer loop. We
need some special value to indicate that spinning should be stopped. How
about just exposing a special value for that in linux/rwsem.h? Any
suggestion for a good name?

Cheers,
Longman

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Waiman Long]

On 05/15/2018 01:42 AM, Amir Goldstein wrote:
> On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
> Waiman,
>
> Thanks for the fix. I will test it soon.
>
> For this commit message I suggest that you add parts of the reproducer
> found here:
> https://marc.info/?l=linux-fsdevel=152622016219975=2
>
> Thanks,
> Amir.
Sure. I will add that to the commit log.

Cheers,
Longman

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Waiman Long]

On 05/15/2018 01:42 AM, Amir Goldstein wrote:
> On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
> Waiman,
>
> Thanks for the fix. I will test it soon.
>
> For this commit message I suggest that you add parts of the reproducer
> found here:
> https://marc.info/?l=linux-fsdevel=152622016219975=2
>
> Thanks,
> Amir.
Sure. I will add that to the commit log.

Cheers,
Longman

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Tue, May 15, 2018 at 02:45:32PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > > > Afaict the whole .owner=NULL thing in release already stops the spinners
> > >
> > > Not really, the new writer will spin in this case, afaics.
> > >
> > > But this is another problem and probably we do not care. The new writer is
> > > almost impossible in this particular case, another freeze_super() should
> > > notice frozen != SB_UNFROZEN and return EBUSY.
> >
> > rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
> > && owner != RWSEM_READER_OWNED, which will fail for !owner.
> 
> Yep. So rwsem_spin_on_owner() goes to "out:" and returns
> !rwsem_owner_is_reader() == T.
> 
> IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
> the case when the new writer is going to do rwsem_set_owner() or up_write()
> has already called rwsem_clear_owner() but didn't do up_write() yet.
> 
> Probably makes sense, but the code is not very clean,

Arrgh, you're right... I hate this rwsem code.

Some day I'll finish the atomic_long_t version, which similar to mutex,
merges the owner and 'count' fields.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Tue, May 15, 2018 at 02:45:32PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > > > Afaict the whole .owner=NULL thing in release already stops the spinners
> > >
> > > Not really, the new writer will spin in this case, afaics.
> > >
> > > But this is another problem and probably we do not care. The new writer is
> > > almost impossible in this particular case, another freeze_super() should
> > > notice frozen != SB_UNFROZEN and return EBUSY.
> >
> > rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
> > && owner != RWSEM_READER_OWNED, which will fail for !owner.
> 
> Yep. So rwsem_spin_on_owner() goes to "out:" and returns
> !rwsem_owner_is_reader() == T.
> 
> IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
> the case when the new writer is going to do rwsem_set_owner() or up_write()
> has already called rwsem_clear_owner() but didn't do up_write() yet.
> 
> Probably makes sense, but the code is not very clean,

Arrgh, you're right... I hate this rwsem code.

Some day I'll finish the atomic_long_t version, which similar to mutex,
merges the owner and 'count' fields.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Peter Zijlstra wrote:
>
> > > Afaict the whole .owner=NULL thing in release already stops the spinners
> >
> > Not really, the new writer will spin in this case, afaics.
> >
> > But this is another problem and probably we do not care. The new writer is
> > almost impossible in this particular case, another freeze_super() should
> > notice frozen != SB_UNFROZEN and return EBUSY.
>
> rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
> && owner != RWSEM_READER_OWNED, which will fail for !owner.

Yep. So rwsem_spin_on_owner() goes to "out:" and returns
!rwsem_owner_is_reader() == T.

IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
the case when the new writer is going to do rwsem_set_owner() or up_write()
has already called rwsem_clear_owner() but didn't do up_write() yet.

Probably makes sense, but the code is not very clean,

> Or am I completely confused again?

Or me, I am not sure.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Peter Zijlstra wrote:
>
> > > Afaict the whole .owner=NULL thing in release already stops the spinners
> >
> > Not really, the new writer will spin in this case, afaics.
> >
> > But this is another problem and probably we do not care. The new writer is
> > almost impossible in this particular case, another freeze_super() should
> > notice frozen != SB_UNFROZEN and return EBUSY.
>
> rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
> && owner != RWSEM_READER_OWNED, which will fail for !owner.

Yep. So rwsem_spin_on_owner() goes to "out:" and returns
!rwsem_owner_is_reader() == T.

IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
the case when the new writer is going to do rwsem_set_owner() or up_write()
has already called rwsem_clear_owner() but didn't do up_write() yet.

Probably makes sense, but the code is not very clean,

> Or am I completely confused again?

Or me, I am not sure.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Tue, May 15, 2018 at 01:06:33PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > So what's wrong with adding:
> >
> > if (!read)
> > sem->rw_sem.owner = current;
> 
> Agreed, I have already suggested this change twice. Except we obviously
> need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
> or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

Right, details ;-)

> > Afaict the whole .owner=NULL thing in release already stops the spinners
> 
> Not really, the new writer will spin in this case, afaics.
> 
> But this is another problem and probably we do not care. The new writer is
> almost impossible in this particular case, another freeze_super() should
> notice frozen != SB_UNFROZEN and return EBUSY.

rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
&& owner != RWSEM_READER_OWNED, which will fail for !owner.

Or am I completely confused again?

> > and the above 'fixes' the debug splat.
> 
> Yes.
> 
> Waiman, can't we trivially fix the problem first? Then we can add the helpers
> and think about other improvements.

It is really simple; we're not going to add public (and EXPORT'ed to
boot) interfaces to rwsem for this.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Tue, May 15, 2018 at 01:06:33PM +0200, Oleg Nesterov wrote:
> On 05/15, Peter Zijlstra wrote:
> >
> > So what's wrong with adding:
> >
> > if (!read)
> > sem->rw_sem.owner = current;
> 
> Agreed, I have already suggested this change twice. Except we obviously
> need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
> or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

Right, details ;-)

> > Afaict the whole .owner=NULL thing in release already stops the spinners
> 
> Not really, the new writer will spin in this case, afaics.
> 
> But this is another problem and probably we do not care. The new writer is
> almost impossible in this particular case, another freeze_super() should
> notice frozen != SB_UNFROZEN and return EBUSY.

rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
&& owner != RWSEM_READER_OWNED, which will fail for !owner.

Or am I completely confused again?

> > and the above 'fixes' the debug splat.
> 
> Yes.
> 
> Waiman, can't we trivially fix the problem first? Then we can add the helpers
> and think about other improvements.

It is really simple; we're not going to add public (and EXPORT'ed to
boot) interfaces to rwsem for this.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Jan Kara wrote:
>
> Now this behavior upsets lockdep and that's why we fool it by telling the
> semaphore got released before returning to userspace (through
> percpu_rwsem_release() helper) and similarly we tell lockdep we've got the
> semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now
> Amir has discovered that also rwsem debugging code gets confused by this
> behavior

Yes, plus someone else has already reported the problem a month ago,

> and previously also someone noticed that rwsem spinning does not
> make sense and can be broken by this behavior.

Well, this doesn't really matter but again, freeze_super() checks
frozen == SB_UNFROZEN under sb->s_umount and only then does sb_wait_write(),
when the previous writer has already realeased this lock. So the new writer
will never spin after lockdep_sb_freeze_release() clears ->owner.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Jan Kara wrote:
>
> Now this behavior upsets lockdep and that's why we fool it by telling the
> semaphore got released before returning to userspace (through
> percpu_rwsem_release() helper) and similarly we tell lockdep we've got the
> semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now
> Amir has discovered that also rwsem debugging code gets confused by this
> behavior

Yes, plus someone else has already reported the problem a month ago,

> and previously also someone noticed that rwsem spinning does not
> make sense and can be broken by this behavior.

Well, this doesn't really matter but again, freeze_super() checks
frozen == SB_UNFROZEN under sb->s_umount and only then does sb_wait_write(),
when the previous writer has already realeased this lock. So the new writer
will never spin after lockdep_sb_freeze_release() clears ->owner.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Peter Zijlstra wrote:
>
> So what's wrong with adding:
>
>   if (!read)
>   sem->rw_sem.owner = current;

Agreed, I have already suggested this change twice. Except we obviously
need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

> Afaict the whole .owner=NULL thing in release already stops the spinners

Not really, the new writer will spin in this case, afaics.

But this is another problem and probably we do not care. The new writer is
almost impossible in this particular case, another freeze_super() should
notice frozen != SB_UNFROZEN and return EBUSY.

> and the above 'fixes' the debug splat.

Yes.

Waiman, can't we trivially fix the problem first? Then we can add the helpers
and think about other improvements.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Oleg Nesterov]

On 05/15, Peter Zijlstra wrote:
>
> So what's wrong with adding:
>
>   if (!read)
>   sem->rw_sem.owner = current;

Agreed, I have already suggested this change twice. Except we obviously
need to check CONFIG_RWSEM_SPIN_ON_OWNER (->owner doesn't exists otherwise)
or even CONFIG_DEBUG_RWSEMS to make the purpose more clear.

> Afaict the whole .owner=NULL thing in release already stops the spinners

Not really, the new writer will spin in this case, afaics.

But this is another problem and probably we do not care. The new writer is
almost impossible in this particular case, another freeze_super() should
notice frozen != SB_UNFROZEN and return EBUSY.

> and the above 'fixes' the debug splat.

Yes.

Waiman, can't we trivially fix the problem first? Then we can add the helpers
and think about other improvements.

Oleg.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Jan Kara]

On Tue 15-05-18 10:35:25, Peter Zijlstra wrote:
> On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> > The percpu_rwsem_release() is called when the ownership of the embedded
> > rwsem is to be transferred to another task. The new owner, however, may
> > take a while to get the ownership of the lock via percpu_rwsem_acquire().
> > During that period, the rwsem is now marked as writer-owned with no
> > optimistic spinning.
> 
> This does not explain the problem sufficiently to even begin considering
> if the proposed solution is sensible.

So the original problem is following: There is percpu_rw_semaphore in
super_block which is used to implement filesystem freezing (actually three
of them but that's not really substantial here). This semaphore is acquired
for writing when a fs is frozen (i.e., in response to a syscall) and we
return to userspace with this semaphore held. Later someone else calls
another syscall to unfreeze the filesystem which drops the semaphore.

Now this behavior upsets lockdep and that's why we fool it by telling the
semaphore got released before returning to userspace (through
percpu_rwsem_release() helper) and similarly we tell lockdep we've got the
semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now
Amir has discovered that also rwsem debugging code gets confused by this
behavior and previously also someone noticed that rwsem spinning does not
make sense and can be broken by this behavior. So these patches from Waiman
try to fix up all these problems...

Honza
-- 
Jan Kara 
SUSE Labs, CR

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Jan Kara]

On Tue 15-05-18 10:35:25, Peter Zijlstra wrote:
> On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> > The percpu_rwsem_release() is called when the ownership of the embedded
> > rwsem is to be transferred to another task. The new owner, however, may
> > take a while to get the ownership of the lock via percpu_rwsem_acquire().
> > During that period, the rwsem is now marked as writer-owned with no
> > optimistic spinning.
> 
> This does not explain the problem sufficiently to even begin considering
> if the proposed solution is sensible.

So the original problem is following: There is percpu_rw_semaphore in
super_block which is used to implement filesystem freezing (actually three
of them but that's not really substantial here). This semaphore is acquired
for writing when a fs is frozen (i.e., in response to a syscall) and we
return to userspace with this semaphore held. Later someone else calls
another syscall to unfreeze the filesystem which drops the semaphore.

Now this behavior upsets lockdep and that's why we fool it by telling the
semaphore got released before returning to userspace (through
percpu_rwsem_release() helper) and similarly we tell lockdep we've got the
semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now
Amir has discovered that also rwsem debugging code gets confused by this
behavior and previously also someone noticed that rwsem spinning does not
make sense and can be broken by this behavior. So these patches from Waiman
try to fix up all these problems...

Honza
-- 
Jan Kara 
SUSE Labs, CR

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.
> 
> Signed-off-by: Waiman Long 
> ---
>  include/linux/percpu-rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..dd37102 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
> percpu_rw_semaphore *sem,
>   bool read, unsigned long ip)
>  {
>   lock_release(>rw_sem.dep_map, 1, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>   if (!read)
> - sem->rw_sem.owner = NULL;
> -#endif
> + rwsem_set_writer_owned_nospin(>rw_sem);
>  }
>  
>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
>   bool read, unsigned long ip)
>  {
>   lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
> + if (!read)
> + rwsem_set_writer_owned(>rw_sem, current);
>  }

So what's wrong with adding:

if (!read)
sem->rw_sem.owner = current;

?

Afaict the whole .owner=NULL thing in release already stops the spinners
dead, and the above 'fixes' the debug splat. And this avoids exposing
that horrible interface and keeps the mucking private to
rwsem/percpu_rwsem.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.
> 
> Signed-off-by: Waiman Long 
> ---
>  include/linux/percpu-rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..dd37102 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
> percpu_rw_semaphore *sem,
>   bool read, unsigned long ip)
>  {
>   lock_release(>rw_sem.dep_map, 1, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>   if (!read)
> - sem->rw_sem.owner = NULL;
> -#endif
> + rwsem_set_writer_owned_nospin(>rw_sem);
>  }
>  
>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
>   bool read, unsigned long ip)
>  {
>   lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
> + if (!read)
> + rwsem_set_writer_owned(>rw_sem, current);
>  }

So what's wrong with adding:

if (!read)
sem->rw_sem.owner = current;

?

Afaict the whole .owner=NULL thing in release already stops the spinners
dead, and the above 'fixes' the debug splat. And this avoids exposing
that horrible interface and keeps the mucking private to
rwsem/percpu_rwsem.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.

This does not explain the problem sufficiently to even begin considering
if the proposed solution is sensible.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Peter Zijlstra]

On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.

This does not explain the problem sufficiently to even begin considering
if the proposed solution is sensible.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Amir Goldstein]

On Tue, May 15, 2018 at 8:42 AM, Amir Goldstein  wrote:
> On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
>
> Waiman,
>
> Thanks for the fix. I will test it soon.
>
> For this commit message I suggest that you add parts of the reproducer
> found here:
> https://marc.info/?l=linux-fsdevel=152622016219975=2
>

fsfreeze is happy with these changes.

You may add:
Tested-by: Amir Goldstein 

Thanks,
Amir.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Amir Goldstein]

On Tue, May 15, 2018 at 8:42 AM, Amir Goldstein  wrote:
> On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
>> The percpu_rwsem_release() is called when the ownership of the embedded
>> rwsem is to be transferred to another task. The new owner, however, may
>> take a while to get the ownership of the lock via percpu_rwsem_acquire().
>> During that period, the rwsem is now marked as writer-owned with no
>> optimistic spinning.
>>
>
> Waiman,
>
> Thanks for the fix. I will test it soon.
>
> For this commit message I suggest that you add parts of the reproducer
> found here:
> https://marc.info/?l=linux-fsdevel=152622016219975=2
>

fsfreeze is happy with these changes.

You may add:
Tested-by: Amir Goldstein 

Thanks,
Amir.

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Amir Goldstein]

On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.
>

Waiman,

Thanks for the fix. I will test it soon.

For this commit message I suggest that you add parts of the reproducer
found here:
https://marc.info/?l=linux-fsdevel=152622016219975=2

Thanks,
Amir.

> Signed-off-by: Waiman Long 
> ---
>  include/linux/percpu-rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..dd37102 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
> percpu_rw_semaphore *sem,
> bool read, unsigned long ip)
>  {
> lock_release(>rw_sem.dep_map, 1, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> if (!read)
> -   sem->rw_sem.owner = NULL;
> -#endif
> +   rwsem_set_writer_owned_nospin(>rw_sem);
>  }
>
>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
> bool read, unsigned long ip)
>  {
> lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
> +   if (!read)
> +   rwsem_set_writer_owned(>rw_sem, current);
>  }
>
>  #endif
> --
> 1.8.3.1
>

Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem_release()

[Amir Goldstein]

On Mon, May 14, 2018 at 10:31 PM, Waiman Long  wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.
>

Waiman,

Thanks for the fix. I will test it soon.

For this commit message I suggest that you add parts of the reproducer
found here:
https://marc.info/?l=linux-fsdevel=152622016219975=2

Thanks,
Amir.

> Signed-off-by: Waiman Long 
> ---
>  include/linux/percpu-rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..dd37102 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct 
> percpu_rw_semaphore *sem,
> bool read, unsigned long ip)
>  {
> lock_release(>rw_sem.dep_map, 1, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> if (!read)
> -   sem->rw_sem.owner = NULL;
> -#endif
> +   rwsem_set_writer_owned_nospin(>rw_sem);
>  }
>
>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
> bool read, unsigned long ip)
>  {
> lock_acquire(>rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
> +   if (!read)
> +   rwsem_set_writer_owned(>rw_sem, current);
>  }
>
>  #endif
> --
> 1.8.3.1
>