Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: Deadlocks due to per-process plugging | Date | Wed, 11 Jul 2012 12:05:51 -0400 |
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Jan Kara <jack@suse.cz> 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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