Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20200308140314.GQ5972@shao2-debian> <1bfba96b4bf0d3ca9a18a2bced3ef3a2a7b44dad.camel@kernel.org>
 <87blp5urwq.fsf@notabene.neil.brown.name> <41c83d34ae4c166f48e7969b2b71e43a0f69028d.camel@kernel.org>
 <ed73fb5d-ddd5-fefd-67ae-2d786e68544a@huawei.com> <923487db2c9396c79f8e8dd4f846b2b1762635c8.camel@kernel.org>
 <36c58a6d07b67aac751fca27a4938dc1759d9267.camel@kernel.org>
 <878sk7vs8q.fsf@notabene.neil.brown.name> <c4ef31a663fbf7a3de349696e9f00f2f5c4ec89a.camel@kernel.org>
 <875zfbvrbm.fsf@notabene.neil.brown.name> <CAHk-=wg8N4fDRC3M21QJokoU+TQrdnv7HqoaFW-Z-ZT8z_Bi7Q@mail.gmail.com>
 <0066a9f150a55c13fcc750f6e657deae4ebdef97.camel@kernel.org>
 <CAHk-=whUgeZGcs5YAfZa07BYKNDCNO=xr4wT6JLATJTpX0bjGg@mail.gmail.com>
 <87v9nattul.fsf@notabene.neil.brown.name> <CAHk-=wiNoAk8v3GrbK3=q6KRBrhLrTafTmWmAo6-up6Ce9fp6A@mail.gmail.com>
 <87o8t2tc9s.fsf@notabene.neil.brown.name> <CAHk-=wj5jOYxjZSUNu_jdJ0zafRS66wcD-4H0vpQS=a14rS8jw@mail.gmail.com>
 <f000e352d9e103b3ade3506aac225920420d2323.camel@kernel.org>
 <877dznu0pk.fsf@notabene.neil.brown.name> <CAHk-=whYQqtW6B7oPmPr9-PXwyqUneF4sSFE+o3=7QcENstE-g@mail.gmail.com>
 <b5a1bb4c4494a370f915af479bcdf8b3b351eb6d.camel@kernel.org>
 <87pndcsxc6.fsf@notabene.neil.brown.name> <ce48ed9e48eda3c0f27d2f417314bd00cb1a68db.camel@kernel.org>
In-Reply-To: <ce48ed9e48eda3c0f27d2f417314bd00cb1a68db.camel@kernel.org>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 16 Mar 2020 10:26:59 -0700
Message-ID: <CAHk-=whnqDS0NJtAaArVeYQz3hcU=4Ja3auB1Jvs42eADfUgMQ@mail.gmail.com>
Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression
To:     Jeff Layton <jlayton@kernel.org>
Cc:     NeilBrown <neilb@suse.de>, yangerkun <yangerkun@huawei.com>,
        kernel test robot <rong.a.chen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        Bruce Fields <bfields@fieldses.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Mar 16, 2020 at 4:07 AM Jeff Layton <jlayton@kernel.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