Messages in this thread | | | Date | Thu, 18 Aug 2016 17:30:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled |
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On Thu, Aug 18, 2016 at 05:18:43PM +0200, Peter Zijlstra wrote: > On Thu, Aug 18, 2016 at 04:27:35PM +0200, Peter Zijlstra wrote: > > On Wed, Aug 10, 2016 at 11:44:08AM -0700, Jason Low wrote: > > > @@ -556,8 +604,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > > > * other waiters. We only attempt the xchg if the count is > > > * non-negative in order to avoid unnecessary xchg operations: > > > */ > > > - if (atomic_read(&lock->count) >= 0 && > > > + if ((!need_yield_to_waiter(lock) || wakeups > 1) && > > > + atomic_read(&lock->count) >= 0 && > > > (atomic_xchg_acquire(&lock->count, -1) == 1)) > > > + if (wakeups > 1) > > > + clear_yield_to_waiter(lock); > > > + > > > break; > > > > > > /* > > > > There's some { } gone missing there... > > > > Also, I think I'll change it to avoid that extra wakeups > 1 condition.. > > Also, its broken, even if we should not trylock, we should still very > much xchg(-1) to mark the lock as having waiters.
Ah, no. Since need_yield_to_waiter() can only be true if there is an actual waiter, at which point count must already be -1. /me adds a comment.
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