The patch titled
md: fix some small races in bitmap plugging in raid5
has been added to the -mm tree. Its filename is
md-fix-some-small-races-in-bitmap-plugging-in-raid5.patch
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: md: fix some small races in bitmap plugging in raid5
From: NeilBrown <neilb@xxxxxxx>
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: Neil Brown <neilb@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
drivers/md/raid5.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)
diff -puN drivers/md/raid5.c~md-fix-some-small-races-in-bitmap-plugging-in-raid5 drivers/md/raid5.c
--- a/drivers/md/raid5.c~md-fix-some-small-races-in-bitmap-plugging-in-raid5
+++ a/drivers/md/raid5.c
@@ -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/config.h>
#include <linux/module.h>
@@ -93,7 +117,7 @@ static void __release_stripe(raid5_conf_
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 {
@@ -1274,9 +1298,9 @@ static int add_stripe_bio(struct stripe_
(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);
}
@@ -2919,7 +2943,7 @@ 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);
_
Patches currently in -mm which might be from neilb@xxxxxxx are
origin.patch
generic_file_buffered_write-deadlock-on-vectored-write.patch
md-possible-fix-for-unplug-problem.patch
md-set-desc_nr-correctly-for-version-1-superblocks.patch
md-delay-starting-md-threads-until-array-is-completely-setup.patch
md-fix-resync-speed-calculation-for-restarted-resyncs.patch
md-fix-a-plug-unplug-race-in-raid5.patch
md-fix-some-small-races-in-bitmap-plugging-in-raid5.patch
md-fix-usage-of-wrong-variable-in-raid1.patch
md-unify-usage-of-symbolic-names-for-perms.patch
md-require-cap_sys_admin-for-re-configuring-md-devices-via-sysfs.patch
md-fix-will-configure-message-when-interpreting-md=-kernel-parameter.patch
md-include-sector-number-in-messages-about-corrected-read-errors.patch
md-dm-reduce-stack-usage-with-stacked-block-devices.patch
lockdep-annotate-sunrpc-code.patch
-
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
