linux-kernel - Re: [locks] 6d390e4b5d: will-it-scale.per_process

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Message-ID: <CAHk-=whnqDS0NJtAaArVeYQz3hcU=4Ja3auB1Jvs42eADfUgMQ@mail.gmail.com>
Date:   Mon, 16 Mar 2020 10:26:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     NeilBrown <neilb@...e.de>, yangerkun <yangerkun@...wei.com>,
        kernel test robot <rong.a.chen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Bruce Fields <bfields@...ldses.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

On Mon, Mar 16, 2020 at 4:07 AM Jeff Layton <jlayton@...nel.org> wrote:
>
>
> +       /*
> +        * If fl_blocker is NULL, it won't be set again as this thread "owns"
> +        * the lock and is the only one that might try to claim the lock.
> +        * Because fl_blocker is explicitly set last during a delete, it's
> +        * safe to locklessly test to see if it's NULL. If it is, then we know
> +        * that no new locks can be inserted into its fl_blocked_requests list,
> +        * and we can therefore avoid doing anything further as long as that
> +        * list is empty.
> +        */
> +       if (!smp_load_acquire(&waiter->fl_blocker) &&
> +           list_empty(&waiter->fl_blocked_requests))
> +               return status;

Ack. This looks sane to me now.

yangerkun - how did you find the original problem?

Would you mind using whatever stress test that caused commit
6d390e4b5d48 ("locks: fix a potential use-after-free problem when
wakeup a waiter") with this patch? And if you did it analytically,
you're a champ and should look at this patch too!

Thanks,

              Linus