On Wed 11-07-12 12:05:51, Jeff Moyer wrote:
> Jan Kara <jack@xxxxxxx> writes:
>
> > Hello,
> >
> > we've recently hit a deadlock in our QA runs which is caused by the
> > per-process plugging code. The problem is as follows:
> > process A process B (kjournald)
> > generic_file_aio_write()
> > blk_start_plug(&plug);
> > ...
> > somewhere in here we allocate memory and
> > direct reclaim submits buffer X for IO
> > ...
> > ext3_write_begin()
> > ext3_journal_start()
> > we need more space in a journal
> > so we want to checkpoint old transactions,
> > we block waiting for kjournald to commit
> > a currently running transaction.
> > journal_commit_transaction()
> > wait for IO on buffer X
> > to complete as it is part
> > of the current transaction
> >
> > => deadlock since A waits for B and B waits for A to do unplug.
> > BTW: I don't think this is really ext3/ext4 specific. I think other
> > filesystems can get into problems as well when direct reclaim submits some
> > IO and the process subsequently blocks without submitting the IO.
>
> So, I thought schedule would do the flush. Checking the code:
>
> asmlinkage void __sched schedule(void)
> {
> struct task_struct *tsk = current;
>
> sched_submit_work(tsk);
> __schedule();
> }
>
> And sched_submit_work looks like this:
>
> 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);
> }
>
> This eventually ends in a call to blk_run_queue_async(q) after
> submitting the I/O from the plug list. Right? So is the question
> really why doesn't the kblockd workqueue get scheduled?
Ah, I didn't know this. Thanks for the hint. So in the kdump I have I can
see requests queued in tsk->plug despite the process is sleeping in
TASK_UNINTERRUPTIBLE state. So the only way how unplug could have been
omitted is if tsk_is_pi_blocked() was true. Rummaging through the dump...
indeed task has pi_blocked_on = 0xffff8802717d79c8. The dump is from an -rt
kernel (I just didn't originally thought that makes any difference) so
actually any mutex is rtmutex and thus tsk_is_pi_blocked() is true whenever
we are sleeping on a mutex. So this seems like a bug in rtmutex code.
Thomas, you seemed to have added that condition... Any idea how to avoid
the deadlock?
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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