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Commit ae3c20cc authored by NeilBrown's avatar NeilBrown Committed by Linus Torvalds
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[PATCH] md: fix some small races in bitmap plugging in raid5 



The comment gives more details, but I didn't quite have the sequencing write,
so there was room for races to leave bits unset in the on-disk bitmap for
short periods of time.

Signed-off-by: default avatarNeil Brown <neilb@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 7c785b7a

drivers/md/raid5.c

+27 −3
Original line number Diff line number Diff line
@@ -18,6 +18,30 @@ 
 * Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

 */



/*

 * BITMAP UNPLUGGING:

 *

 * The sequencing for updating the bitmap reliably is a little

 * subtle (and I got it wrong the first time) so it deserves some

 * explanation.

 *

 * We group bitmap updates into batches.  Each batch has a number.

 * We may write out several batches at once, but that isn't very important.

 * conf->bm_write is the number of the last batch successfully written.

 * conf->bm_flush is the number of the last batch that was closed to

 *    new additions.

 * When we discover that we will need to write to any block in a stripe

 * (in add_stripe_bio) we update the in-memory bitmap and record in sh->bm_seq

 * the number of the batch it will be in. This is bm_flush+1.

 * When we are ready to do a write, if that batch hasn't been written yet,

 *   we plug the array and queue the stripe for later.

 * When an unplug happens, we increment bm_flush, thus closing the current

 *   batch.

 * When we notice that bm_flush > bm_write, we write out all pending updates

 * to the bitmap, and advance bm_write to where bm_flush was.

 * This may occasionally write a bit out twice, but is sure never to

 * miss any bits.

 */



#include <linux/module.h>

#include <linux/slab.h>
@@ -92,7 +116,7 @@ static void __release_stripe(raid5_conf_t *conf, struct stripe_head *sh) 
				list_add_tail(&sh->lru, &conf->delayed_list);

				blk_plug_device(conf->mddev->queue);

			} else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&

				   conf->seq_write == sh->bm_seq) {

				   sh->bm_seq - conf->seq_write > 0) {

				list_add_tail(&sh->lru, &conf->bitmap_list);

				blk_plug_device(conf->mddev->queue);

			} else {
@@ -1273,9 +1297,9 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx, in 
		(unsigned long long)sh->sector, dd_idx);



	if (conf->mddev->bitmap && firstwrite) {

		sh->bm_seq = conf->seq_write;

		bitmap_startwrite(conf->mddev->bitmap, sh->sector,

				  STRIPE_SECTORS, 0);

		sh->bm_seq = conf->seq_flush+1;

		set_bit(STRIPE_BIT_DELAY, &sh->state);

	}
@@ -2918,7 +2942,7 @@ static void raid5d (mddev_t *mddev) 
	while (1) {

		struct list_head *first;



		if (conf->seq_flush - conf->seq_write > 0) {

		if (conf->seq_flush != conf->seq_write) {

			int seq = conf->seq_flush;

			spin_unlock_irq(&conf->device_lock);

			bitmap_unplug(mddev->bitmap);