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Groups > linux.kernel > #1500349
| From | Will Deacon <will.deacon@arm.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation |
| Date | 2016-10-13 17:20 +0200 |
| Message-ID | <srPXB-5cV-45@gated-at.bofh.it> (permalink) |
| References | <spEWB-2dt-5@gated-at.bofh.it> <spFpD-2pN-17@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
Hi Peter,
Just one comment below.
On Fri, Oct 07, 2016 at 04:52:48PM +0200, Peter Zijlstra wrote:
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -54,8 +54,10 @@ EXPORT_SYMBOL(__mutex_init);
> * bits to store extra state.
> *
> * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
> + * Bit1 indicates unlock needs to hand the lock to the top-waiter
> */
> #define MUTEX_FLAG_WAITERS 0x01
> +#define MUTEX_FLAG_HANDOFF 0x02
>
> #define MUTEX_FLAGS 0x03
>
> @@ -71,20 +73,48 @@ static inline unsigned long __owner_flag
>
> /*
> * Actual trylock that will work on any unlocked state.
> + *
> + * When setting the owner field, we must preserve the low flag bits.
> + *
> + * Be careful with @handoff, only set that in a wait-loop (where you set
> + * HANDOFF) to avoid recursive lock attempts.
> */
> -static inline bool __mutex_trylock(struct mutex *lock)
> +static inline bool __mutex_trylock(struct mutex *lock, const bool handoff)
> {
> unsigned long owner, curr = (unsigned long)current;
>
> owner = atomic_long_read(&lock->owner);
> for (;;) { /* must loop, can race against a flag */
> - unsigned long old;
> + unsigned long old, flags = __owner_flags(owner);
> +
> + if (__owner_task(owner)) {
> + if (handoff && unlikely(__owner_task(owner) == current)) {
> + /*
> + * Provide ACQUIRE semantics for the lock-handoff.
> + *
> + * We cannot easily use load-acquire here, since
> + * the actual load is a failed cmpxchg, which
> + * doesn't imply any barriers.
> + *
> + * Also, this is a fairly unlikely scenario, and
> + * this contains the cost.
> + */
> + smp_mb(); /* ACQUIRE */
As we discussed on another thread recently, a failed cmpxchg_acquire
will always give you ACQUIRE semantics in practice. Maybe we should update
the documentation to allow this? The only special case is the full-barrier
version.
Will
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[PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
[PATCH -v4 4/8] locking/mutex: Allow MUTEX_SPIN_ON_OWNER when DEBUG_MUTEXES Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
[PATCH -v4 7/8] locking/mutex: Simplify some ww_mutex code in __mutex_lock_common() Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:10 +0200
[PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 12:50 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 15:30 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Boqun Feng <boqun.feng@gmail.com> - 2016-10-17 15:50 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-17 18:00 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-19 19:40 +0200
ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Davidlohr Bueso <dave@stgolabs.net> - 2016-10-24 04:00 +0200
Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-24 15:30 +0200
Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-24 16:30 +0200
Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Eric Wheeler <bcache@lists.ewheeler.net> - 2016-10-25 19:00 +0200
Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop) Kent Overstreet <kent.overstreet@gmail.com> - 2016-10-25 19:50 +0200
Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:20 +0200
[PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Will Deacon <will.deacon@arm.com> - 2016-10-13 17:30 +0200
Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:40 +0200
Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:30 +0200
[PATCH -v4 2/8] locking/mutex: Rework mutex::owner Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:30 +0200
Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Davidlohr Bueso <dave@stgolabs.net> - 2016-10-12 20:40 +0200
Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Jason Low <jason.low2@hpe.com> - 2016-10-12 22:00 +0200
Re: [PATCH -v4 2/8] locking/mutex: Rework mutex::owner Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
[PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Will Deacon <will.deacon@arm.com> - 2016-10-13 17:20 +0200
Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-17 11:30 +0200
Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:40 +0200
Re: [PATCH -v4 5/8] locking/mutex: Add lock handoff to avoid starvation Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:10 +0200
[PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:50 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-07 18:00 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-07 18:20 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Thomas Gleixner <tglx@linutronix.de> - 2016-10-08 14:10 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Thomas Gleixner <tglx@linutronix.de> - 2016-10-08 16:20 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-08 18:50 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-08 16:20 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-18 14:50 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Peter Zijlstra <peterz@infradead.org> - 2016-10-18 15:00 +0200
Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery Chris Wilson <chris@chris-wilson.co.uk> - 2016-10-18 15:00 +0200
[PATCH -v4 3/8] locking/mutex: Kill arch specific code Peter Zijlstra <peterz@infradead.org> - 2016-10-07 17:40 +0200
Re: [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-07 17:50 +0200
Re: [PATCH -v4 0/8] locking/mutex: Rewrite basic mutex Jason Low <jason.low2@hpe.com> - 2016-10-11 21:10 +0200
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