linux-kernel - Re: [RFC PATCH] sched/wait: Make interruptible exclusive waitqueue wakeups reliable

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Date:   Tue, 10 Dec 2019 08:29:21 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>
Subject: Re: [RFC PATCH] sched/wait: Make interruptible exclusive waitqueue
 wakeups reliable


* Oleg Nesterov <oleg@...hat.com> wrote:

> > long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
> > {
> >         unsigned long flags;
> >         long ret = 0;
> >
> >         spin_lock_irqsave(&wq_head->lock, flags);
> >         if (signal_pending_state(state, current)) {
> >                 /*
> >                  * Exclusive waiter must not fail if it was selected by wakeup,
> >                  * it should "consume" the condition we were waiting for.
> >                  *
> >                  * The caller will recheck the condition and return success if
> >                  * we were already woken up, we can not miss the event because
> >                  * wakeup locks/unlocks the same wq_head->lock.
> >                  *
> >                  * But we need to ensure that set-condition + wakeup after that
> >                  * can't see us, it should wake up another exclusive waiter if
> >                  * we fail.
> >                  */
> >                 list_del_init(&wq_entry->entry);
> >                 ret = -ERESTARTSYS;
> 
> ...
> 
> > I think we can indeed lose an exclusive event here, despite the comment
> > that argues that we shouldn't: if we were already removed from the list
> 
> If we were already removed from the list and condition is true, we can't
> miss it, ret = -ERESTARTSYS won't be used. This is what this part of the
> comment above
> 
> 	 * The caller will recheck the condition and return success if
> 	 * we were already woken up, we can not miss the event because
> 	 * wakeup locks/unlocks the same wq_head->lock.
> 
> tries to explain.

Yeah, indeed - it assumes that the condition is stable from wakeup to 
wakee running - which as Linus said it must be, because otherwise 
exclusive waiters couldn't reliably exit the wait loop.

So there's no bug. How about the clarifying comment below?

Thanks,

	Ingo

 kernel/sched/wait.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index ba059fbfc53a..6783bac00b5c 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -290,6 +290,11 @@ long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_en
 		 * But we need to ensure that set-condition + wakeup after that
 		 * can't see us, it should wake up another exclusive waiter if
 		 * we fail.
+		 *
+		 * In other words, if an exclusive waiter got here, then the
+		 * waitqueue condition is and stays true and we are guaranteed
+		 * to exit the waitqueue loop and will ignore the -ERESTARTSYS
+		 * and return success.
 		 */
 		list_del_init(&wq_entry->entry);
 		ret = -ERESTARTSYS;