Re: [PATCH] sched/completion: completion_done() should serialize with complete()
On 02/16, Peter Zijlstra wrote: > > On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote: > > Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when > > reading completion state" was not correct, without lock/unlock the code > > like stop_machine_from_inactive_cpu() > > > > while (!completion_done()) > > cpu_relax(); > > > > can return before complete() finishes its spin_unlock() which writes to > > this memory. And spin_unlock_wait(). > > > > While at it, change try_wait_for_completion() to use READ_ONCE(). > > So I share Davidlohrs concern Ah. I forgot to reply to Davidlohr's email. Sorry. > if we should not simply revert that > change; but given we've now gone over it detail I suppose we should just > keep the optimized version. Yes, I was going to say that of course I won't argue if we simply revert that commit. As he rigthly pointed the lockless check doesn't make sense performance-wise. However, this code needs a comment to explain why we can't simply check ->done and return, unlock_wait() is more documentation than optimization. But, > I did add a comment to your patch; and queued the below for > sched/urgent. Thanks! Now this logic is actually documented ;) unlock_wait() alone could confuse the reader too. Oleg. -- 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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
> Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
> reading completion state" was not correct, without lock/unlock the code
> like stop_machine_from_inactive_cpu()
>
> while (!completion_done())
> cpu_relax();
>
> can return before complete() finishes its spin_unlock() which writes to
> this memory. And spin_unlock_wait().
>
> While at it, change try_wait_for_completion() to use READ_ONCE().
So I share Davidlohrs concern if we should not simply revert that
change; but given we've now gone over it detail I suppose we should just
keep the optimized version.
I did add a comment to your patch; and queued the below for
sched/urgent.
---
Subject: sched/completion: completion_done() should serialize with complete()
From: Oleg Nesterov
Date: Thu, 12 Feb 2015 20:59:13 +0100
Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
reading completion state" was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()
while (!completion_done())
cpu_relax();
can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().
While at it, change try_wait_for_completion() to use READ_ONCE().
Fixes: de30ec47302c ("sched/completion: Remove unnecessary ->wait.lock
serialization when reading completion state")
Cc: waiman.l...@hp.com
Cc: raghavendra...@linux.vnet.ibm.com
Cc: d...@stgolabs.net
Cc: Nicholas Mc Guire
Cc: Linus Torvalds
Reported-by: "Paul E. McKenney"
Tested-by: "Paul E. McKenney"
Reported-by: Davidlohr Bueso
Signed-off-by: Oleg Nesterov
[peterz: Add a comment with the barrier]
Signed-off-by: Peter Zijlstra (Intel)
Link: http://lkml.kernel.org/r/20150212195913.ga30...@redhat.com
---
kernel/sched/completion.c | 19 +--
1 file changed, 17 insertions(+), 2 deletions(-)
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
* first without taking the lock so we can
* return early in the blocking case.
*/
- if (!ACCESS_ONCE(x->done))
+ if (!READ_ONCE(x->done))
return 0;
spin_lock_irqsave(>wait.lock, flags);
@@ -297,6 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion);
*/
bool completion_done(struct completion *x)
{
- return !!ACCESS_ONCE(x->done);
+ if (!READ_ONCE(x->done))
+ return false;
+
+ /*
+* If ->done, we need to wait for complete() to release ->wait.lock
+* otherwise we can end up freeing the completion before complete()
+* is done referencing it.
+*
+* The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
+* the loads of ->done and ->wait.lock such that we cannot observe
+* the lock before complete() acquires it while observing the ->done
+* after it's acquired the lock.
+*/
+ smp_rmb();
+ spin_unlock_wait(>wait.lock);
+ return true;
}
EXPORT_SYMBOL(completion_done);
--
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: [PATCH] sched/completion: completion_done() should serialize with complete()
On 02/16, Peter Zijlstra wrote: On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote: Commit de30ec47302c Remove unnecessary -wait.lock serialization when reading completion state was not correct, without lock/unlock the code like stop_machine_from_inactive_cpu() while (!completion_done()) cpu_relax(); can return before complete() finishes its spin_unlock() which writes to this memory. And spin_unlock_wait(). While at it, change try_wait_for_completion() to use READ_ONCE(). So I share Davidlohrs concern Ah. I forgot to reply to Davidlohr's email. Sorry. if we should not simply revert that change; but given we've now gone over it detail I suppose we should just keep the optimized version. Yes, I was going to say that of course I won't argue if we simply revert that commit. As he rigthly pointed the lockless check doesn't make sense performance-wise. However, this code needs a comment to explain why we can't simply check -done and return, unlock_wait() is more documentation than optimization. But, I did add a comment to your patch; and queued the below for sched/urgent. Thanks! Now this logic is actually documented ;) unlock_wait() alone could confuse the reader too. Oleg. -- 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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
Commit de30ec47302c Remove unnecessary -wait.lock serialization when
reading completion state was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()
while (!completion_done())
cpu_relax();
can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().
While at it, change try_wait_for_completion() to use READ_ONCE().
So I share Davidlohrs concern if we should not simply revert that
change; but given we've now gone over it detail I suppose we should just
keep the optimized version.
I did add a comment to your patch; and queued the below for
sched/urgent.
---
Subject: sched/completion: completion_done() should serialize with complete()
From: Oleg Nesterov o...@redhat.com
Date: Thu, 12 Feb 2015 20:59:13 +0100
Commit de30ec47302c Remove unnecessary -wait.lock serialization when
reading completion state was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()
while (!completion_done())
cpu_relax();
can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().
While at it, change try_wait_for_completion() to use READ_ONCE().
Fixes: de30ec47302c (sched/completion: Remove unnecessary -wait.lock
serialization when reading completion state)
Cc: waiman.l...@hp.com
Cc: raghavendra...@linux.vnet.ibm.com
Cc: d...@stgolabs.net
Cc: Nicholas Mc Guire der.h...@hofr.at
Cc: Linus Torvalds torva...@linux-foundation.org
Reported-by: Paul E. McKenney paul...@linux.vnet.ibm.com
Tested-by: Paul E. McKenney paul...@linux.vnet.ibm.com
Reported-by: Davidlohr Bueso d...@stgolabs.net
Signed-off-by: Oleg Nesterov o...@redhat.com
[peterz: Add a comment with the barrier]
Signed-off-by: Peter Zijlstra (Intel) pet...@infradead.org
Link: http://lkml.kernel.org/r/20150212195913.ga30...@redhat.com
---
kernel/sched/completion.c | 19 +--
1 file changed, 17 insertions(+), 2 deletions(-)
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
* first without taking the lock so we can
* return early in the blocking case.
*/
- if (!ACCESS_ONCE(x-done))
+ if (!READ_ONCE(x-done))
return 0;
spin_lock_irqsave(x-wait.lock, flags);
@@ -297,6 +297,21 @@ EXPORT_SYMBOL(try_wait_for_completion);
*/
bool completion_done(struct completion *x)
{
- return !!ACCESS_ONCE(x-done);
+ if (!READ_ONCE(x-done))
+ return false;
+
+ /*
+* If -done, we need to wait for complete() to release -wait.lock
+* otherwise we can end up freeing the completion before complete()
+* is done referencing it.
+*
+* The RMB pairs with complete()'s RELEASE of -wait.lock and orders
+* the loads of -done and -wait.lock such that we cannot observe
+* the lock before complete() acquires it while observing the -done
+* after it's acquired the lock.
+*/
+ smp_rmb();
+ spin_unlock_wait(x-wait.lock);
+ return true;
}
EXPORT_SYMBOL(completion_done);
--
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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Fri, 2015-02-13 at 13:56 -0800, Davidlohr Bueso wrote: > On Thu, 2015-02-12 at 20:59 +0100, Oleg Nesterov wrote: > > Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when > > reading completion state" was not correct, without lock/unlock the code > > like stop_machine_from_inactive_cpu() > > > > while (!completion_done()) > > cpu_relax(); > > > > can return before complete() finishes its spin_unlock() which writes to > > this memory. And spin_unlock_wait(). > > How about reverting the patch altogether? > > This was never a problem nor have I ever seen a performance issues in > completions that would merit these lockless checks. The commit changelog > has *zero* information, so I don't know if this was ever a real issue. > hmm I guess you're patch is more optimal tho, because we don't update the lock, less cacheline bouncing issues etc. -- 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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, 2015-02-12 at 20:59 +0100, Oleg Nesterov wrote: > Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when > reading completion state" was not correct, without lock/unlock the code > like stop_machine_from_inactive_cpu() > > while (!completion_done()) > cpu_relax(); > > can return before complete() finishes its spin_unlock() which writes to > this memory. And spin_unlock_wait(). How about reverting the patch altogether? This was never a problem nor have I ever seen a performance issues in completions that would merit these lockless checks. The commit changelog has *zero* information, so I don't know if this was ever a real issue. Thanks, Davidlohr -- 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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
> Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
> reading completion state" was not correct, without lock/unlock the code
> like stop_machine_from_inactive_cpu()
>
> while (!completion_done())
> cpu_relax();
>
> can return before complete() finishes its spin_unlock() which writes to
> this memory. And spin_unlock_wait().
>
> While at it, change try_wait_for_completion() to use READ_ONCE().
>
> Reported-by: "Paul E. McKenney"
> Reported-by: Davidlohr Bueso
> Signed-off-by: Oleg Nesterov
So I am having some difficulty reproducing the original problem, but
the patch passes rcutorture testing. So...
Tested-by: Paul E. McKenney
> --- x/kernel/sched/completion.c
> +++ x/kernel/sched/completion.c
> @@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
>* first without taking the lock so we can
>* return early in the blocking case.
>*/
> - if (!ACCESS_ONCE(x->done))
> + if (!READ_ONCE(x->done))
> return 0;
>
> spin_lock_irqsave(>wait.lock, flags);
> @@ -297,6 +297,11 @@ EXPORT_SYMBOL(try_wait_for_completion);
> */
> bool completion_done(struct completion *x)
> {
> - return !!ACCESS_ONCE(x->done);
> + if (!READ_ONCE(x->done))
> + return false;
> +
> + smp_rmb();
> + spin_unlock_wait(>wait.lock);
> + return true;
> }
> EXPORT_SYMBOL(completion_done);
>
--
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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
Commit de30ec47302c Remove unnecessary -wait.lock serialization when
reading completion state was not correct, without lock/unlock the code
like stop_machine_from_inactive_cpu()
while (!completion_done())
cpu_relax();
can return before complete() finishes its spin_unlock() which writes to
this memory. And spin_unlock_wait().
While at it, change try_wait_for_completion() to use READ_ONCE().
Reported-by: Paul E. McKenney paul...@linux.vnet.ibm.com
Reported-by: Davidlohr Bueso d...@stgolabs.net
Signed-off-by: Oleg Nesterov o...@redhat.com
So I am having some difficulty reproducing the original problem, but
the patch passes rcutorture testing. So...
Tested-by: Paul E. McKenney paul...@linux.vnet.ibm.com
--- x/kernel/sched/completion.c
+++ x/kernel/sched/completion.c
@@ -274,7 +274,7 @@ bool try_wait_for_completion(struct comp
* first without taking the lock so we can
* return early in the blocking case.
*/
- if (!ACCESS_ONCE(x-done))
+ if (!READ_ONCE(x-done))
return 0;
spin_lock_irqsave(x-wait.lock, flags);
@@ -297,6 +297,11 @@ EXPORT_SYMBOL(try_wait_for_completion);
*/
bool completion_done(struct completion *x)
{
- return !!ACCESS_ONCE(x-done);
+ if (!READ_ONCE(x-done))
+ return false;
+
+ smp_rmb();
+ spin_unlock_wait(x-wait.lock);
+ return true;
}
EXPORT_SYMBOL(completion_done);
--
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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Thu, 2015-02-12 at 20:59 +0100, Oleg Nesterov wrote: Commit de30ec47302c Remove unnecessary -wait.lock serialization when reading completion state was not correct, without lock/unlock the code like stop_machine_from_inactive_cpu() while (!completion_done()) cpu_relax(); can return before complete() finishes its spin_unlock() which writes to this memory. And spin_unlock_wait(). How about reverting the patch altogether? This was never a problem nor have I ever seen a performance issues in completions that would merit these lockless checks. The commit changelog has *zero* information, so I don't know if this was ever a real issue. Thanks, Davidlohr -- 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: [PATCH] sched/completion: completion_done() should serialize with complete()
On Fri, 2015-02-13 at 13:56 -0800, Davidlohr Bueso wrote: On Thu, 2015-02-12 at 20:59 +0100, Oleg Nesterov wrote: Commit de30ec47302c Remove unnecessary -wait.lock serialization when reading completion state was not correct, without lock/unlock the code like stop_machine_from_inactive_cpu() while (!completion_done()) cpu_relax(); can return before complete() finishes its spin_unlock() which writes to this memory. And spin_unlock_wait(). How about reverting the patch altogether? This was never a problem nor have I ever seen a performance issues in completions that would merit these lockless checks. The commit changelog has *zero* information, so I don't know if this was ever a real issue. hmm I guess you're patch is more optimal tho, because we don't update the lock, less cacheline bouncing issues etc. -- 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/
