Re: [PATCH RT] fs: jbd2: pull your plug when waiting for space

[Mike Galbraith]

On Mon, 2014-03-10 at 13:47 -0400, Theodore Ts'o wrote: 
> On Fri, Feb 21, 2014 at 02:54:12PM +0100, Mike Galbraith wrote:
> > 
> > ext3: dbench1 queues a buffer, blocks on journal mutex, it's plug is not
> > pulled.  dbench2 mutex owner is waiting for kjournald, who is waiting for
> > the buffer queued by dbench1.  Game over.
> 
> Where is in ext3/4 are we calling some function which could end up
> blocking on kjournald while we have the I/O queue plugged?  That
> sounds suspicious and potentially wrong.

I don't have the crash dumps and analysis handy, this was quite some
time ago.  Problem is that..

static inline void sched_submit_work(struct task_struct *tsk)
{
        if (!tsk->state || tsk_is_pi_blocked(tsk))
                return;
        /*
         * If we are going to sleep and we have plugged IO queued,
         * make sure to submit it to avoid deadlocks.
         */
        if (blk_needs_flush_plug(tsk))
                blk_schedule_flush_plug(tsk);
}

..tsk_is_pi_blocked(tsk) leaves us with IO queued, dependency on which
can (_did_ for ext[34] and xfs that I recall) end up with our waker
waiting on our IO.  There were other deadlock scenarios, not only the
one in the quoted text.

-Mike

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