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

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Date:   Thu, 14 Jun 2018 06:04:04 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Jan Kara <jack@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 2018/06/14 1:45, Jan Kara wrote:
> 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...

Indeed. I overlooked that wb = list_first_entry() will select same wb again
if cgwb_remove_from_bdi_list() is not yet called. Well, we could update
"(in which case we also wait for it to finish)" part or move the body of
cgwb_start_shutdown() to cgwb_bdi_unregister() so that it becomes clear
that false wake-up is not a problem in this case.