linux-kernel - Re: [PATCH] bdi: Fix another oops in wb

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Message-ID: <20180613164509.oeb3fsjylfpfzxuh@quack2.suse.cz>
Date:   Wed, 13 Jun 2018 18:45:09 +0200
From:   Jan Kara <jack@...e.cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        syzbot+4a7438e774b21ddd8eca@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        linux-block <linux-block@...r.kernel.org>
Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn()

On Wed 13-06-18 09:25:03, Linus Torvalds wrote:
> On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
> >
> > Since multiple addresses share bit_wait_table[256], isn't it possible that
> > cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by
> > hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from
> > wb_shutdown())? I think that we cannot be sure without confirming that
> > test_bit(WB_shutting_down, &wb->state) == false after returning from schedule().
> 
> Right.
> 
> That's _always_ true, btw. Something else entirely could have woken
> you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it
> just means "_signals_ don't wake me".
> 
> So every single sleep always needs to be in a loop. Always.

Agreed and in my patch it actually is in a loop - the one iterating the
list of active writeback structures. If we get a false wakeup, we find the
same structure in the list again and wait again...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR