Messages in this thread |  | | Date | Thu, 12 Jul 2001 14:53:11 +1000 | From | Andrew Morton <> | Subject | Re: Is Swapping on software RAID1 possible in linux 2.4 ? |
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Neil Brown wrote: > > --- drivers/md/raid1.c 2001/07/12 02:00:35 1.1 > +++ drivers/md/raid1.c 2001/07/12 02:01:42 > @@ -83,6 +83,7 @@ > cnt--; > } else { > PRINTK("raid1: waiting for %d bh\n", cnt); > + run_task_queue(&tq_disk); > wait_event(conf->wait_buffer, conf->freebh_cnt >= cnt); > } > } > @@ -170,6 +171,7 @@ > memset(r1_bh, 0, sizeof(*r1_bh)); > return r1_bh; > } > + run_task_queue(&tq_disk); > wait_event(conf->wait_buffer, conf->freer1); > } while (1); > } > > This is needed anyway to be "correct", as you should always unplug > the queues before waiting for IO to complete.
The problem with this approach is the waitqueue - you get several tasks on the waitqueue, and bdflush loses the race - some other thread steals the r1bh and bdflush goes back to sleep.
Replacing the wait_event() with a special raid1_wait_event() which unplugs *each time* the caller is woken does help - but it is still easy to deadlock the system.
Clearly this approach is racy: it assumes that the reserved buffers have actually been submitted when we unplug - they may not yet have been. But the lockup is too easy to trigger for that to be a satisfactory explanation.
The most effective, aggressive, successful and grotty fix for this problem is to remove the wait_event altogether and replace it with:
run_task_queue(tq_disk); current->policy |= SCHED_YIELD; __set_current_state(TASK_RUNNING); schedule();
This can still deadlock in bad OOM situations, but I think we're dead anyway. A combination of this approach plus the PF_FLUSH reservations would work even better, but I found the PF_FLUSH stuff was sufficient.
> Mind you, if I was really serious about being > gentle on the memory allocation, I would use > kmem_cache_alloc(bh_cachep,SLAB_whatever) > instead of > kmalloc(sizeof(struct buffer_head), GFP_whatever)
get/put_unused_buffer_head() should be exported API functions.
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