Re: RAID1 might_sleep() warning on 3.19-rc7

Peter Zijlstra
2015-02-09T09:10:09Z

On Mon, Feb 09, 2015 at 12:13:57PM +1100, NeilBrown wrote:
> I had to re-read the code (And your analysis) a couple of times to be sure ...

Sorry :-)

> However, when io_schedule() explicitly calls blk_flush_plug(), then
> @from_schedule=false variant is used, and the unplug functions are allowed to
> allocate memory and block and maybe even call mempool_alloc() which might
> call io_schedule().
>
> This shouldn't be a problem as blk_flush_plug() spliced out the plug list, so
> any recursive call will find an empty list and do nothing.

Unless, something along the way stuck something back on, right? So
should we stick an:

WARN_ON(current->in_iowait);

somewhere near where things are added to this plug list? (and move the
blk_flush_plug() call inside of where that's actually true of course).

> Worst case is that a wait_event loop that calls io_schedule() (i.e.
> wait_on_bit_io()) might not block in the first call to io_schedule()
> if the unplugging needed to wait. Every subsequent call will block as
> required as there is nothing else to add requests to the plug queue.

Again, assuming @cond will not actually stick something on this list.
Which if we add the above we'll get warned about.

> It isn't that scheduling is "rare" - it is that it can only occur once in a
> loop which doesn't expect it.

With the above WARN stuck in, agreed.

> So I propose the following, though I haven't tested it.
>
> Signed-off-by: NeilBrown <neilb@suse.de>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e628cb11b560..b0f12ab3df23 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4374,6 +4374,11 @@ void __sched io_schedule(void)
>
> delayacct_blkio_start();
> atomic_inc(&rq->nr_iowait);
> + /* Any sleeping in blk_flush_plug() should not
> + * trigger the "do not call blocking ops" warning
> + * as it can only happen once in a wait_event loop.
> + */

Might I suggest the 'regular' multi-line comment style, and a reference
to the above WARN that makes everything actually work?

/*
* multi-line
* comments have an empty
* line at the start... As per CodingStyle ch. 8
*/

> + sched_annotate_sleep();
> blk_flush_plug(current);

Also, at this point, should we put it in blk_flush_plug()?

The only thing that really goes wrong then is if people 'forget' to put
a loop around io_schedule().