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?
Cheers,
Jeff
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