Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
Hi all, On Wed, 16 Sep 2020 15:02:33 +0200 pet...@infradead.org wrote: > > On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote: > > > > > > - Original Message - > > > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote: > > > > On Tue, 2020-09-15 at 16:30 +0200, pet...@infradead.org wrote: > > > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote: > > > > > > I think this happened because seqcount_##lockname##_init() is > > > > > > defined > > > > > > at > > > > > > function rather than macro, so when the seqcount_init() gets expand > > > > > > in > > > > > > > > > > Bah! I hate all this :/ > > > > > > > > > > I suspect the below, while more verbose than I'd like is the best > > > > > option. > > > > > > > > Stephen, can you add this patch for now until Peter beats you to it? > > > > > > Did you verify it works? I only wrote it.. > > > > Yes, I did. > > Excellent, I'll stick a Tested-by from you on then. I'll add this into the tip tree merge today (unless the tip tree is updated in the mean time). -- Cheers, Stephen Rothwell pgpl8Iukb0goQ.pgp Description: OpenPGP digital signature
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
- Original Message - > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote: > > On Tue, 2020-09-15 at 16:30 +0200, pet...@infradead.org wrote: > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote: > > > > I think this happened because seqcount_##lockname##_init() is defined > > > > at > > > > function rather than macro, so when the seqcount_init() gets expand in > > > > > > Bah! I hate all this :/ > > > > > > I suspect the below, while more verbose than I'd like is the best > > > option. > > > > Stephen, can you add this patch for now until Peter beats you to it? > > Did you verify it works? I only wrote it.. Yes, I did.
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Tue, 2020-09-15 at 16:30 +0200, pet...@infradead.org wrote:
> On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > I think this happened because seqcount_##lockname##_init() is defined at
> > function rather than macro, so when the seqcount_init() gets expand in
>
> Bah! I hate all this :/
>
> I suspect the below, while more verbose than I'd like is the best
> option.
Stephen, can you add this patch for now until Peter beats you to it?
>
> ---
> include/linux/seqlock.h | 22 ++
> 1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index f73c7eb68f27..76e44e6c0100 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const
> seqcount_t *s)
> * @lock:Pointer to the associated lock
> */
>
> +#define seqcount_LOCKNAME_init(s, _lock, lockname) \
> + do {\
> + seqcount_##lockname##_t *s = (s); \
> + seqcount_init(&s->seqcount);\
> + __SEQ_LOCK(s->lock = (_lock)); \
> + } while (0)
> +
> +#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock,
> raw_spinlock)
> +#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s
> , lock, spinlock)
> +#define seqcount_rwlock_init(s, lock)seqcount_LOCKNAME_init(s
> , lock, rwlock);
> +#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock,
> mutex);
> +#define seqcount_ww_mutex_init(s, lock) seqcount_LOCKNAME_init(s
> , lock, ww_mutex);
> +
> /*
> * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
> * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
> @@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {
>
> \
> __SEQ_LOCK(locktype *lock); \
> } seqcount_##lockname##_t; \
> \
> -static __always_inline void \
> -seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)
> \
> -{\
> - seqcount_init(>seqcount);\
> - __SEQ_LOCK(s->lock = lock); \
> -}\
> - \
> static __always_inline seqcount_t * \
> __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)
> \
> {\
> @@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex,
> true, >lock->base, ww_mu
> __SEQ_LOCK(.lock= (assoc_lock)) \
> }
>
> -#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> +#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> #define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
> #define SEQCNT_MUTEX_ZERO(name, lock)SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
> #define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
>
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote: > On Tue, 2020-09-15 at 16:30 +0200, pet...@infradead.org wrote: > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote: > > > I think this happened because seqcount_##lockname##_init() is defined at > > > function rather than macro, so when the seqcount_init() gets expand in > > > > Bah! I hate all this :/ > > > > I suspect the below, while more verbose than I'd like is the best > > option. > > Stephen, can you add this patch for now until Peter beats you to it? Did you verify it works? I only wrote it..
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote: > > > - Original Message - > > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote: > > > On Tue, 2020-09-15 at 16:30 +0200, pet...@infradead.org wrote: > > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote: > > > > > I think this happened because seqcount_##lockname##_init() is defined > > > > > at > > > > > function rather than macro, so when the seqcount_init() gets expand in > > > > > > > > Bah! I hate all this :/ > > > > > > > > I suspect the below, while more verbose than I'd like is the best > > > > option. > > > > > > Stephen, can you add this patch for now until Peter beats you to it? > > > > Did you verify it works? I only wrote it.. > > Yes, I did. Excellent, I'll stick a Tested-by from you on then.
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in
Bah! I hate all this :/
I suspect the below, while more verbose than I'd like is the best
option.
---
include/linux/seqlock.h | 22 ++
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..76e44e6c0100 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const
seqcount_t *s)
* @lock: Pointer to the associated lock
*/
+#define seqcount_LOCKNAME_init(s, _lock, lockname) \
+ do {\
+ seqcount_##lockname##_t *s = (s); \
+ seqcount_init(&s->seqcount);\
+ __SEQ_LOCK(s->lock = (_lock)); \
+ } while (0)
+
+#define seqcount_raw_spinlock_init(s, lock)seqcount_LOCKNAME_init(s, lock,
raw_spinlock)
+#define seqcount_spinlock_init(s, lock)
seqcount_LOCKNAME_init(s, lock, spinlock)
+#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock,
rwlock);
+#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock,
mutex);
+#define seqcount_ww_mutex_init(s, lock)
seqcount_LOCKNAME_init(s, lock, ww_mutex);
+
/*
* SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
* seqprop_LOCKNAME_*()- Property accessors for seqcount_LOCKNAME_t
@@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {
\
__SEQ_LOCK(locktype *lock); \
} seqcount_##lockname##_t; \
\
-static __always_inline void\
-seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock) \
-{ \
- seqcount_init(>seqcount);\
- __SEQ_LOCK(s->lock = lock); \
-} \
- \
static __always_inline seqcount_t *\
__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
{ \
@@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true,
>lock->base, ww_mu
__SEQ_LOCK(.lock= (assoc_lock)) \
}
-#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
+#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
lock)
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> > On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > > Hi,
> > >
> > > Changelog-v2
> > >
> > >
> > > - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> > > sequence counters with associated locks, instead of v1
> > > seqcount_LOCKTYPE_t.
> > >
> > > - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> > > property accessors.
> > >
> > > - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> > > non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> > >
> > > Cover letter (v1)
> > > =
> > >
> > > https://lkml.kernel.org/r/20200828010710.5407-1-a.darw...@linutronix.de
> > >
> > > Preemption must be disabled before entering a sequence counter write
> > > side critical section. Otherwise the read side section can preempt the
> > > write side section and spin for the entire scheduler tick. If that
> > > reader belongs to a real-time scheduling class, it can spin forever and
> > > the kernel will livelock.
> > >
> > > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > > to higher latencies, and the write side sections will not be able to
> > > acquire locks which become sleeping locks (e.g. spinlock_t).
> > >
> > > To remain preemptible, while avoiding a possible livelock caused by the
> > > reader preempting the writer, use a different technique: let the reader
> > > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > > case, acquire then release the associated LOCKNAME writer serialization
> > > lock. This will allow any possibly-preempted writer to make progress
> > > until the end of its writer serialization lock critical section.
> > >
> > > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
> >
> > Reverting this patchset [1] from today's linux-next fixed a splat below. The
> > splat looks like a false positive anyway because the existing locking
> > dependency
> > chains from the task #1 here:
> >
> > >seqcount#2 ---> pidmap_lock
> >
> > [ 528.078061][ T7867] -> #1 (pidmap_lock){}-{2:2}:
> > [ 528.078078][ T7867]lock_acquire+0x10c/0x560
> > [ 528.078089][ T7867]_raw_spin_lock_irqsave+0x64/0xb0
> > [ 528.078108][ T7867]free_pid+0x5c/0x160
> > free_pid at kernel/pid.c:131
> > [ 528.078127][ T7867]release_task.part.40+0x59c/0x7f0
> > __unhash_process at kernel/exit.c:76
> > (inlined by) __exit_signal at kernel/exit.c:147
> > (inlined by) release_task at kernel/exit.c:198
> > [ 528.078145][ T7867]do_exit+0x77c/0xda0
> > exit_notify at kernel/exit.c:679
> > (inlined by) do_exit at kernel/exit.c:826
> > [ 528.078163][ T7867]kthread+0x148/0x1d0
> > [ 528.078182][ T7867]ret_from_kernel_thread+0x5c/0x80
> >
> > It is write_seqlock(>stats_lock) in __exit_signal(), but the
> > >seqcount#2
> > in read_mems_allowed_begin() is
> > read_seqcount_begin(>mems_allowed_seq),
> > so there should be no deadlock?
> >
>
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in
> that function, the lock_class_key of seqcount will be a static variable
> of seqcount_##lockname##_init() function, as a result, all
> seqcount_##lockname##_t in the same compile unit (in this case it's
> kernel/fork.c) share the same lock class key, and lockdep thought they
> are the same lock ;-)
>
Don't know how to fix this properly, but below is an ugly attemption,
only build test, just food for thoughts.
Regards,
Boqun
--->8
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..938a5053def3 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -84,14 +84,18 @@ static inline void __seqcount_init(seqcount_t *s, const
char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname }
+# define MSIOCU 8 /* MAX SEQCOUNT IN ON COMPILE UNIT */
/**
* seqcount_init() - runtime initializer for seqcount_t
* @s: Pointer to the seqcount_t instance
*/
# define seqcount_init(s) \
do {\
- static struct lock_class_key __key; \
- __seqcount_init((s), #s, &__key); \
+ static struct lock_class_key __key[MSIOCU]; \
+ static int idx = 0; \
+ \
+ BUG_ON(idx >= MSIOCU); \
+
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > Hi,
> >
> > Changelog-v2
> >
> >
> > - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> > sequence counters with associated locks, instead of v1
> > seqcount_LOCKTYPE_t.
> >
> > - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> > property accessors.
> >
> > - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> > non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> >
> > Cover letter (v1)
> > =
> >
> > https://lkml.kernel.org/r/20200828010710.5407-1-a.darw...@linutronix.de
> >
> > Preemption must be disabled before entering a sequence counter write
> > side critical section. Otherwise the read side section can preempt the
> > write side section and spin for the entire scheduler tick. If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> >
> > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > to higher latencies, and the write side sections will not be able to
> > acquire locks which become sleeping locks (e.g. spinlock_t).
> >
> > To remain preemptible, while avoiding a possible livelock caused by the
> > reader preempting the writer, use a different technique: let the reader
> > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > case, acquire then release the associated LOCKNAME writer serialization
> > lock. This will allow any possibly-preempted writer to make progress
> > until the end of its writer serialization lock critical section.
> >
> > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
>
> Reverting this patchset [1] from today's linux-next fixed a splat below. The
> splat looks like a false positive anyway because the existing locking
> dependency
> chains from the task #1 here:
>
> >seqcount#2 ---> pidmap_lock
>
> [ 528.078061][ T7867] -> #1 (pidmap_lock){}-{2:2}:
> [ 528.078078][ T7867]lock_acquire+0x10c/0x560
> [ 528.078089][ T7867]_raw_spin_lock_irqsave+0x64/0xb0
> [ 528.078108][ T7867]free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [ 528.078127][ T7867]release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [ 528.078145][ T7867]do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [ 528.078163][ T7867]kthread+0x148/0x1d0
> [ 528.078182][ T7867]ret_from_kernel_thread+0x5c/0x80
>
> It is write_seqlock(>stats_lock) in __exit_signal(), but the
> >seqcount#2
> in read_mems_allowed_begin() is
> read_seqcount_begin(>mems_allowed_seq),
> so there should be no deadlock?
>
I think this happened because seqcount_##lockname##_init() is defined at
function rather than macro, so when the seqcount_init() gets expand in
that function, the lock_class_key of seqcount will be a static variable
of seqcount_##lockname##_init() function, as a result, all
seqcount_##lockname##_t in the same compile unit (in this case it's
kernel/fork.c) share the same lock class key, and lockdep thought they
are the same lock ;-)
Regards,
Boqun
> [1] git revert --no-edit 0c9794c8b678..1909760f5fc3
>
> [ 528.077900][ T7867] WARNING: possible circular locking dependency detected
> [ 528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
> [ 528.077921][ T7867] --
> [ 528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
> [ 528.077942][ T7867] c01fce5570c8 (>seqcount#2){}-{0:0}, at:
> __slab_alloc+0x34/0xf0
> [ 528.077972][ T7867]
> [ 528.077972][ T7867] but task is already holding lock:
> [ 528.077983][ T7867] c56b0198 (pidmap_lock){}-{2:2}, at:
> alloc_pid+0x258/0x590
> [ 528.078009][ T7867]
> [ 528.078009][ T7867] which lock already depends on the new lock.
> [ 528.078009][ T7867]
> [ 528.078031][ T7867]
> [ 528.078031][ T7867] the existing dependency chain (in reverse order) is:
> [ 528.078061][ T7867]
> [ 528.078061][ T7867] -> #1 (pidmap_lock){}-{2:2}:
> [ 528.078078][ T7867]lock_acquire+0x10c/0x560
> [ 528.078089][ T7867]_raw_spin_lock_irqsave+0x64/0xb0
> [ 528.078108][ T7867]free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [ 528.078127][ T7867]release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [ 528.078145][ T7867]do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [ 528.078163][ T7867]
Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> Hi,
>
> Changelog-v2
>
>
> - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> sequence counters with associated locks, instead of v1
> seqcount_LOCKTYPE_t.
>
> - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> property accessors.
>
> - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
>
> Cover letter (v1)
> =
>
> https://lkml.kernel.org/r/20200828010710.5407-1-a.darw...@linutronix.de
>
> Preemption must be disabled before entering a sequence counter write
> side critical section. Otherwise the read side section can preempt the
> write side section and spin for the entire scheduler tick. If that
> reader belongs to a real-time scheduling class, it can spin forever and
> the kernel will livelock.
>
> Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> to higher latencies, and the write side sections will not be able to
> acquire locks which become sleeping locks (e.g. spinlock_t).
>
> To remain preemptible, while avoiding a possible livelock caused by the
> reader preempting the writer, use a different technique: let the reader
> detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> case, acquire then release the associated LOCKNAME writer serialization
> lock. This will allow any possibly-preempted writer to make progress
> until the end of its writer serialization lock critical section.
>
> Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
Reverting this patchset [1] from today's linux-next fixed a splat below. The
splat looks like a false positive anyway because the existing locking dependency
chains from the task #1 here:
>seqcount#2 ---> pidmap_lock
[ 528.078061][ T7867] -> #1 (pidmap_lock){}-{2:2}:
[ 528.078078][ T7867]lock_acquire+0x10c/0x560
[ 528.078089][ T7867]_raw_spin_lock_irqsave+0x64/0xb0
[ 528.078108][ T7867]free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[ 528.078127][ T7867]release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[ 528.078145][ T7867]do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[ 528.078163][ T7867]kthread+0x148/0x1d0
[ 528.078182][ T7867]ret_from_kernel_thread+0x5c/0x80
It is write_seqlock(>stats_lock) in __exit_signal(), but the
>seqcount#2
in read_mems_allowed_begin() is
read_seqcount_begin(>mems_allowed_seq),
so there should be no deadlock?
[1] git revert --no-edit 0c9794c8b678..1909760f5fc3
[ 528.077900][ T7867] WARNING: possible circular locking dependency detected
[ 528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
[ 528.077921][ T7867] --
[ 528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
[ 528.077942][ T7867] c01fce5570c8 (>seqcount#2){}-{0:0}, at:
__slab_alloc+0x34/0xf0
[ 528.077972][ T7867]
[ 528.077972][ T7867] but task is already holding lock:
[ 528.077983][ T7867] c56b0198 (pidmap_lock){}-{2:2}, at:
alloc_pid+0x258/0x590
[ 528.078009][ T7867]
[ 528.078009][ T7867] which lock already depends on the new lock.
[ 528.078009][ T7867]
[ 528.078031][ T7867]
[ 528.078031][ T7867] the existing dependency chain (in reverse order) is:
[ 528.078061][ T7867]
[ 528.078061][ T7867] -> #1 (pidmap_lock){}-{2:2}:
[ 528.078078][ T7867]lock_acquire+0x10c/0x560
[ 528.078089][ T7867]_raw_spin_lock_irqsave+0x64/0xb0
[ 528.078108][ T7867]free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[ 528.078127][ T7867]release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[ 528.078145][ T7867]do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[ 528.078163][ T7867]kthread+0x148/0x1d0
[ 528.078182][ T7867]ret_from_kernel_thread+0x5c/0x80
[ 528.078208][ T7867]
[ 528.078208][ T7867] -> #0 (>seqcount#2){}-{0:0}:
[ 528.078241][ T7867]check_prevs_add+0x1c4/0x1120
check_prev_add at kernel/locking/lockdep.c:2820
(inlined by) check_prevs_add at kernel/locking/lockdep.c:2944
[ 528.078260][ T7867]__lock_acquire+0x176c/0x1c00
validate_chain at kernel/locking/lockdep.c:3562
(inlined by) __lock_acquire at kernel/locking/lockdep.c:4796
[ 528.078278][ T7867]lock_acquire+0x10c/0x560
[ 528.078297][ T7867]___slab_alloc+0xa40/0xb40
seqcount_lockdep_reader_access at include/linux/seqlock.h:103
(inlined by) read_mems_allowed_begin at
[PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
Hi, Changelog-v2 - Standardize on seqcount_LOCKNAME_t as the canonical reference for sequence counters with associated locks, instead of v1 seqcount_LOCKTYPE_t. - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t property accessors. - Touch-up the lock-unlock rationale for more clarity. Enforce writer non-preemitiblity using "__seq_enforce_writer_non_preemptibility()". Cover letter (v1) = https://lkml.kernel.org/r/20200828010710.5407-1-a.darw...@linutronix.de Preemption must be disabled before entering a sequence counter write side critical section. Otherwise the read side section can preempt the write side section and spin for the entire scheduler tick. If that reader belongs to a real-time scheduling class, it can spin forever and the kernel will livelock. Disabling preemption cannot be done for PREEMPT_RT though: it can lead to higher latencies, and the write side sections will not be able to acquire locks which become sleeping locks (e.g. spinlock_t). To remain preemptible, while avoiding a possible livelock caused by the reader preempting the writer, use a different technique: let the reader detect if a seqcount_LOCKNAME_t writer is in progress. If that's the case, acquire then release the associated LOCKNAME writer serialization lock. This will allow any possibly-preempted writer to make progress until the end of its writer serialization lock critical section. Implement this lock-unlock technique for all seqcount_LOCKNAME_t with an associated (PREEMPT_RT) sleeping lock, and for seqlock_t. 8<-- Ahmed S. Darwish (5): seqlock: seqcount_LOCKNAME_t: Standardize naming convention seqlock: Use unique prefix for seqcount_t property accessors seqlock: seqcount_t: Implement all read APIs as statement expressions seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support seqlock: PREEMPT_RT: Do not starve seqlock_t writers include/linux/seqlock.h | 281 1 file changed, 167 insertions(+), 114 deletions(-) base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b -- 2.28.0
