linux-kernel - Re: a competition when some threads acquire futex

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Message-ID: <alpine.DEB.2.20.1709052131130.2393@nanos>
Date:   Tue, 5 Sep 2017 21:37:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "chengjian (D)" <cj.chengjian@...wei.com>
cc:     huawei.libin@...wei.com, mingo@...hat.com, peterz@...radead.org,
        dvhart@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: a competition when some threads acquire futex

On Tue, 5 Sep 2017, chengjian (D) wrote:

> int main(int argc, char** argv)
> {
>         pthread_t id1;
>         pthread_t id2;
> 
>         printf("main pid = %d\n", getpid( ));
> 
>         pthread_mutex_init(&mutex, NULL);

So this is a plain mutex, which maps to a plain futex.

> use perf ftrace to shows the graph of the function calls. We found that the
> process 17327 auquire the lock quickly after call futex_wake( ), so the
> process 17328 futex_wait( ) all the time.

The observed functions look correct for that futex type.

> diff --git a/kernel/futex.c b/kernel/futex.c
> index 3d38eaf..0b2d17a 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1545,6 +1545,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval,
> struct futex_pi_state *pi_
>   	spin_unlock(&hb->lock);
>  	wake_up_q(&wake_q);
> +        _cond_resched( );

What's less correct is the placement of the cond_resched() which you patch
into the function:

       wake_futex_pi() 

wake_futex_pi() is not even remotely involved in this problem, so I have a
hard time to understand how this patch 'solves' it.

Thanks,

	tglx