Messages in this thread Patch in this message | | | Date | Mon, 23 Apr 2018 22:55:14 +0200 | From | Andrea Parri <> | Subject | Re: [PATCH] locking/rwsem: Synchronize task state & waiter->task of readers |
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Hi Waiman,
On Mon, Apr 23, 2018 at 12:46:12PM -0400, Waiman Long wrote: > On 04/10/2018 01:22 PM, Waiman Long wrote: > > It was observed occasionally in PowerPC systems that there was reader > > who had not been woken up but that its waiter->task had been cleared.
Can you provide more details about these observations? (links to LKML posts, traces, applications used/micro-benchmarks, ...)
> > > > One probable cause of this missed wakeup may be the fact that the > > waiter->task and the task state have not been properly synchronized as > > the lock release-acquire pair of different locks in the wakeup code path > > does not provide a full memory barrier guarantee.
I guess that by the "pair of different locks" you mean (sem->wait_lock, p->pi_lock), right? BTW, __rwsem_down_write_failed_common() is calling wake_up_q() _before_ releasing the wait_lock: did you intend to exclude this callsite? (why?)
> So smp_store_mb() > > is now used to set waiter->task to NULL to provide a proper memory > > barrier for synchronization.
Mmh; the patch is not introducing an smp_store_mb()... My guess is that you are thinking at the sequence:
smp_store_release(&waiter->task, NULL); [...] smp_mb(); /* added with your patch */
or what am I missing?
> > > > Signed-off-by: Waiman Long <longman@redhat.com> > > --- > > kernel/locking/rwsem-xadd.c | 17 +++++++++++++++++ > > 1 file changed, 17 insertions(+) > > > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > > index e795908..b3c588c 100644 > > --- a/kernel/locking/rwsem-xadd.c > > +++ b/kernel/locking/rwsem-xadd.c > > @@ -209,6 +209,23 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, > > smp_store_release(&waiter->task, NULL); > > } > > > > + /* > > + * To avoid missed wakeup of reader, we need to make sure > > + * that task state and waiter->task are properly synchronized. > > + * > > + * wakeup sleep > > + * ------ ----- > > + * __rwsem_mark_wake: rwsem_down_read_failed*: > > + * [S] waiter->task [S] set_current_state(state) > > + * MB MB > > + * try_to_wake_up: > > + * [L] state [L] waiter->task > > + * > > + * For the wakeup path, the original lock release-acquire pair > > + * does not provide enough guarantee of proper synchronization. > > + */ > > + smp_mb(); > > + > > adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment; > > if (list_empty(&sem->wait_list)) { > > /* hit end of list above */ > > Ping! > > Any thought on this patch? > > I am wondering if there is a cheaper way to apply the memory barrier > just on architectures that need it.
try_to_wake_up() does:
raw_spin_lock_irqsave(&p->pi_lock, flags); smp_mb__after_spinlock(); if (!(p->state & state))
My understanding is that this smp_mb__after_spinlock() provides us with the guarantee you described above. The smp_mb__after_spinlock() should represent a 'cheaper way' to provide such a guarantee.
If this understanding is correct, the remaining question would be about whether you want to rely on (and document) the smp_mb__after_spinlock() in the callsite in question (the comment in wake_up_q()
/* * wake_up_process() implies a wmb() to pair with the queueing * in wake_q_add() so as not to miss wakeups. */ does not appear to be suffient...).
Andrea
> > Cheers, > Longman >
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