Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Wed, Jan 15, 2014 at 08:48:44AM +0100, Peter Zijlstra wrote: > On Wed, Jan 15, 2014 at 08:44:20AM +0100, Peter Zijlstra wrote: > > On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: > > > The mutex_can_spin_on_owner() function should also return false if the > > > task needs to be rescheduled. > > > > > > > While I was staring at mutex_can_spin_on_owner(); don't we need this? > > > > kernel/locking/mutex.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > > index 4dd6e4c219de..480d2f437964 100644 > > --- a/kernel/locking/mutex.c > > +++ b/kernel/locking/mutex.c > > @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex > > *lock) > > > > rcu_read_lock(); > > owner = ACCESS_ONCE(lock->owner); > > - if (owner) > > + if (owner) { > > That is, its an unmatched barrier, as mutex_set_owner() doesn't include > a barrier, and I don't think i needs to; but on alpha we still need this > read barrier to ensure we do not mess up this related load afaik. > > Paul? can you explain an unpaired read_barrier_depends? That is an impressive one! ;-) My rationale for the code without smp_read_barrier_depends() is that (1) the task struct was already exposed to readers and (2) the check is heuristic in nature -- if we miss the assignment to ->on_cpu due to memory order (or for any other reason), we just sleep unnecessarily. If we did need full ordering (which I do -not- believe that we do at the moment) then the above ACCESS_ONCE() can become rcu_dereference() and mutex_set_owner() needs an smp_store_release(). So if we need a barrier here (which again I believe we do not), then there needs to be a paired barrier in mutex_set_owner(). Thanx, Paul > > + smp_read_barrier_depends(); > > retval = owner->on_cpu; > > + } > > rcu_read_unlock(); > > /* > > * if lock->owner is not set, the mutex owner may have just acquired > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Wed, Jan 15, 2014 at 08:48:44AM +0100, Peter Zijlstra wrote: On Wed, Jan 15, 2014 at 08:44:20AM +0100, Peter Zijlstra wrote: On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: The mutex_can_spin_on_owner() function should also return false if the task needs to be rescheduled. While I was staring at mutex_can_spin_on_owner(); don't we need this? kernel/locking/mutex.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c219de..480d2f437964 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) rcu_read_lock(); owner = ACCESS_ONCE(lock-owner); - if (owner) + if (owner) { That is, its an unmatched barrier, as mutex_set_owner() doesn't include a barrier, and I don't think i needs to; but on alpha we still need this read barrier to ensure we do not mess up this related load afaik. Paul? can you explain an unpaired read_barrier_depends? That is an impressive one! ;-) My rationale for the code without smp_read_barrier_depends() is that (1) the task struct was already exposed to readers and (2) the check is heuristic in nature -- if we miss the assignment to -on_cpu due to memory order (or for any other reason), we just sleep unnecessarily. If we did need full ordering (which I do -not- believe that we do at the moment) then the above ACCESS_ONCE() can become rcu_dereference() and mutex_set_owner() needs an smp_store_release(). So if we need a barrier here (which again I believe we do not), then there needs to be a paired barrier in mutex_set_owner(). Thanx, Paul + smp_read_barrier_depends(); retval = owner-on_cpu; + } rcu_read_unlock(); /* * if lock-owner is not set, the mutex owner may have just acquired -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Wed, Jan 15, 2014 at 08:44:20AM +0100, Peter Zijlstra wrote: > On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: > > The mutex_can_spin_on_owner() function should also return false if the > > task needs to be rescheduled. > > > > While I was staring at mutex_can_spin_on_owner(); don't we need this? > > kernel/locking/mutex.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > index 4dd6e4c219de..480d2f437964 100644 > --- a/kernel/locking/mutex.c > +++ b/kernel/locking/mutex.c > @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex > *lock) > > rcu_read_lock(); > owner = ACCESS_ONCE(lock->owner); > - if (owner) > + if (owner) { That is, its an unmatched barrier, as mutex_set_owner() doesn't include a barrier, and I don't think i needs to; but on alpha we still need this read barrier to ensure we do not mess up this related load afaik. Paul? can you explain an unpaired read_barrier_depends? > + smp_read_barrier_depends(); > retval = owner->on_cpu; > + } > rcu_read_unlock(); > /* >* if lock->owner is not set, the mutex owner may have just acquired -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: > The mutex_can_spin_on_owner() function should also return false if the > task needs to be rescheduled. > While I was staring at mutex_can_spin_on_owner(); don't we need this? kernel/locking/mutex.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c219de..480d2f437964 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) rcu_read_lock(); owner = ACCESS_ONCE(lock->owner); - if (owner) + if (owner) { + smp_read_barrier_depends(); retval = owner->on_cpu; + } rcu_read_unlock(); /* * if lock->owner is not set, the mutex owner may have just acquired -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
The mutex_can_spin_on_owner() function should also return false if the task needs to be rescheduled. Signed-off-by: Jason Low --- kernel/locking/mutex.c |3 +++ 1 files changed, 3 insertions(+), 0 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c..85c6be1 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -212,6 +212,9 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) struct task_struct *owner; int retval = 1; + if (need_resched()) + return 0; + rcu_read_lock(); owner = ACCESS_ONCE(lock->owner); if (owner) -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
The mutex_can_spin_on_owner() function should also return false if the task needs to be rescheduled. Signed-off-by: Jason Low jason.l...@hp.com --- kernel/locking/mutex.c |3 +++ 1 files changed, 3 insertions(+), 0 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c..85c6be1 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -212,6 +212,9 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) struct task_struct *owner; int retval = 1; + if (need_resched()) + return 0; + rcu_read_lock(); owner = ACCESS_ONCE(lock-owner); if (owner) -- 1.7.1 -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: The mutex_can_spin_on_owner() function should also return false if the task needs to be rescheduled. While I was staring at mutex_can_spin_on_owner(); don't we need this? kernel/locking/mutex.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c219de..480d2f437964 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) rcu_read_lock(); owner = ACCESS_ONCE(lock-owner); - if (owner) + if (owner) { + smp_read_barrier_depends(); retval = owner-on_cpu; + } rcu_read_unlock(); /* * if lock-owner is not set, the mutex owner may have just acquired -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC 1/3] mutex: In mutex_can_spin_on_owner(), return false if task need_resched()
On Wed, Jan 15, 2014 at 08:44:20AM +0100, Peter Zijlstra wrote: On Tue, Jan 14, 2014 at 04:33:08PM -0800, Jason Low wrote: The mutex_can_spin_on_owner() function should also return false if the task needs to be rescheduled. While I was staring at mutex_can_spin_on_owner(); don't we need this? kernel/locking/mutex.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4dd6e4c219de..480d2f437964 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -214,8 +214,10 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock) rcu_read_lock(); owner = ACCESS_ONCE(lock-owner); - if (owner) + if (owner) { That is, its an unmatched barrier, as mutex_set_owner() doesn't include a barrier, and I don't think i needs to; but on alpha we still need this read barrier to ensure we do not mess up this related load afaik. Paul? can you explain an unpaired read_barrier_depends? + smp_read_barrier_depends(); retval = owner-on_cpu; + } rcu_read_unlock(); /* * if lock-owner is not set, the mutex owner may have just acquired -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/