raid5: fix 2 bugs in ops_complete_biofill

1/ ops_complete_biofill tried to avoid calling handle_stripe since all the
state necessary to return read completions is available.  However the
process of determining whether more read requests are pending requires
locking the stripe (to block add_stripe_bio from updating dev->toead).
ops_complete_biofill can run in tasklet context, so rather than upgrading
all the stripe locks from spin_lock to spin_lock_bh this patch just
unconditionally reschedules handle_stripe after completing the read
request.

2/ ops_complete_biofill needlessly qualified processing R5_Wantfill with
dev->toread.  The result being that the 'biofill' pending bit is cleared
before handling the pending read-completions on dev->read.  R5_Wantfill can
be unconditionally handled because the 'biofill' pending bit prevents new
R5_Wantfill requests from being seen by ops_run_biofill and
ops_complete_biofill.
Found-by: Yuri Tikhonov <yur@emcraft.com>
[neilb@suse.de: simpler fix for bug 1 than moving code]
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/raid5.c

@@ -514,7 +514,7 @@ static void ops_complete_biofill(void *stripe_head_ref)
 	struct stripe_head *sh = stripe_head_ref;
 	struct bio *return_bi = NULL;
 	raid5_conf_t *conf = sh->raid_conf;
-	int i, more_to_read = 0;
+	int i;
 
 	pr_debug("%s: stripe %llu\n", __FUNCTION__,
 		(unsigned long long)sh->sector);
@@ -522,16 +522,14 @@ static void ops_complete_biofill(void *stripe_head_ref)
 	/* clear completed biofills */
 	for (i = sh->disks; i--; ) {
 		struct r5dev *dev = &sh->dev[i];
-		/* check if this stripe has new incoming reads */
-		if (dev->toread)
-			more_to_read++;
 
 		/* acknowledge completion of a biofill operation */
-		/* and check if we need to reply to a read request
+		/* and check if we need to reply to a read request,
+		 * new R5_Wantfill requests are held off until
+		 * !test_bit(STRIPE_OP_BIOFILL, &sh->ops.pending)
 		 */
-		if (test_bit(R5_Wantfill, &dev->flags) && !dev->toread) {
+		if (test_and_clear_bit(R5_Wantfill, &dev->flags)) {
 			struct bio *rbi, *rbi2;
-			clear_bit(R5_Wantfill, &dev->flags);
 
 			/* The access to dev->read is outside of the
 			 * spin_lock_irq(&conf->device_lock), but is protected
@@ -558,7 +556,6 @@ static void ops_complete_biofill(void *stripe_head_ref)
 
 	return_io(return_bi);
 
-	if (more_to_read)
 	set_bit(STRIPE_HANDLE, &sh->state);
 	release_stripe(sh);
 }