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 <[EMAIL PROTECTED]>
### Diffstat output
./drivers/md/raid5.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)
diff .prev/drivers/md/raid5.c ./drivers/md/raid5.c
--- .prev/drivers/md/raid5.c 2006-06-27 12:17:33.000000000 +1000
+++ ./drivers/md/raid5.c 2006-06-27 12:17:33.000000000 +1000
@@ -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);
}
@@ -2920,7 +2944,7 @@ 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);
-
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