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

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Date:   Thu, 14 Jun 2018 01:20:16 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Jan Kara <jack@...e.cz>
Cc:     Tejun Heo <tj@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+4a7438e774b21ddd8eca@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        linux-block@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn()

On 2018/06/13 23:46, Jan Kara wrote:
> On Wed 13-06-18 19:43:47, Tetsuo Handa wrote:
>> Can't we utilize RCU grace period (like shown below) ?
> 
> Honestly, the variant 1 looks too ugly to me. However variant 2 looks
> mostly OK. We can also avoid the schedule_timeout_uninterruptible(HZ / 10)
> from your patch by careful handling of the bit waitqueues. Also I'd avoid
> the addition argument to wb_writeback() and split the function instead. The
> patch resulting from your and mine ideas is attached. Thoughts?
> 
> 								Honza
> 

+static bool cgwb_start_shutdown(struct bdi_writeback *wb)
+	__releases(cgwb_lock)
+{
+	if (!wb_start_shutdown(wb)) {
+		DEFINE_WAIT(wait);
+		wait_queue_head_t *wqh = bit_waitqueue(&wb->state,
+						       WB_shutting_down);
+		bool sleep;
+
+		prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
+		sleep = test_bit(WB_shutting_down, &wb->state);
+		spin_unlock_irq(&cgwb_lock);
+		if (sleep)
+			schedule();
+		return false;
+	}
+	spin_unlock_irq(&cgwb_lock);
+	return true;
+}

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().