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@xxxxxxx> > > 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(). -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
