dm: fix race in dm_start_io_acct

After commit 82f6cdcc ("dm: switch dm_io booleans over to proper
flags") dm_start_io_acct stopped atomically checking and setting
was_accounted, which turned into the DM_IO_ACCOUNTED flag. This opened
the possibility for a race where IO accounting is started twice for
duplicate bios. To remove the race, check the flag while holding the
io->lock.

Fixes: 82f6cdcc ("dm: switch dm_io booleans over to proper flags")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm.c

@@ -555,6 +555,10 @@ static void dm_start_io_acct(struct dm_io *io, struct bio *clone)
 		unsigned long flags;
 		/* Can afford locking given DM_TIO_IS_DUPLICATE_BIO */
 		spin_lock_irqsave(&io->lock, flags);
+		if (dm_io_flagged(io, DM_IO_ACCOUNTED)) {
+			spin_unlock_irqrestore(&io->lock, flags);
+			return;
+		}
 		dm_io_set_flag(io, DM_IO_ACCOUNTED);
 		spin_unlock_irqrestore(&io->lock, flags);
 	}