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

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Date:   Tue, 10 Dec 2019 08:21:19 +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:

> On 12/09, Ingo Molnar wrote:
> >
> > Any consumed exclusive event is handled in finish_wait_exclusive() now:
> >
> > +               } else {
> > +                       /* We got removed from the waitqueue already, wake up the next exclusive waiter (if any): */
> > +                       if (interrupted && waitqueue_active(wq_head))
> > +                               __wake_up_locked_key(wq_head, TASK_NORMAL, NULL);
> 
> See my previous email, I don't think we need this...
> 
> But if we do this, then __wake_up_locked_key(key => NULL) doesn't look right.
> It should use the same "key" which was passed to __wake_up(key) which removed
> us from list.
> 
> Currently this doesn't really matter, the only user of prepare_to_wait_event()
> which relies on the "keyed" wakeup is ___wait_var_event() and it doesn't have
> "exclusive" waiters, but still.
> 
> Hmm. and it seems that init_wait_var_entry() is buggy? Again, currently this
> doesn't matter, but don't we need the trivial fix below?
> 
> Oleg.
> 
> --- x/kernel/sched/wait_bit.c
> +++ x/kernel/sched/wait_bit.c
> @@ -179,6 +179,7 @@ void init_wait_var_entry(struct wait_bit
>  			.bit_nr = -1,
>  		},
>  		.wq_entry = {
> +			.flags	 = flags,
>  			.private = current,
>  			.func	 = var_wake_function,
>  			.entry	 = LIST_HEAD_INIT(wbq_entry->wq_entry.entry),

Yeah, agreed.

Thanks,

	Ingo