[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: Neil Brown <neilb@suse.de> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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@ -18,6 +18,30 @@
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* Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/*
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* BITMAP UNPLUGGING:
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*
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* The sequencing for updating the bitmap reliably is a little
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* subtle (and I got it wrong the first time) so it deserves some
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* explanation.
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*
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* We group bitmap updates into batches. Each batch has a number.
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* We may write out several batches at once, but that isn't very important.
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* conf->bm_write is the number of the last batch successfully written.
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* conf->bm_flush is the number of the last batch that was closed to
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* new additions.
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* When we discover that we will need to write to any block in a stripe
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* (in add_stripe_bio) we update the in-memory bitmap and record in sh->bm_seq
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* the number of the batch it will be in. This is bm_flush+1.
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* When we are ready to do a write, if that batch hasn't been written yet,
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* we plug the array and queue the stripe for later.
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* When an unplug happens, we increment bm_flush, thus closing the current
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* batch.
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* When we notice that bm_flush > bm_write, we write out all pending updates
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* to the bitmap, and advance bm_write to where bm_flush was.
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* This may occasionally write a bit out twice, but is sure never to
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* miss any bits.
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*/
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#include <linux/module.h>
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#include <linux/slab.h>
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@ -92,7 +116,7 @@ static void __release_stripe(raid5_conf_t *conf, struct stripe_head *sh)
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list_add_tail(&sh->lru, &conf->delayed_list);
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blk_plug_device(conf->mddev->queue);
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} else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&
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conf->seq_write == sh->bm_seq) {
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sh->bm_seq - conf->seq_write > 0) {
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list_add_tail(&sh->lru, &conf->bitmap_list);
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blk_plug_device(conf->mddev->queue);
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} else {
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@ -1273,9 +1297,9 @@ static int add_stripe_bio(struct stripe_head *sh, struct bio *bi, int dd_idx, in
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(unsigned long long)sh->sector, dd_idx);
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if (conf->mddev->bitmap && firstwrite) {
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sh->bm_seq = conf->seq_write;
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bitmap_startwrite(conf->mddev->bitmap, sh->sector,
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STRIPE_SECTORS, 0);
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sh->bm_seq = conf->seq_flush+1;
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set_bit(STRIPE_BIT_DELAY, &sh->state);
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}
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@ -2918,7 +2942,7 @@ static void raid5d (mddev_t *mddev)
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while (1) {
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struct list_head *first;
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if (conf->seq_flush - conf->seq_write > 0) {
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if (conf->seq_flush != conf->seq_write) {
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int seq = conf->seq_flush;
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spin_unlock_irq(&conf->device_lock);
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bitmap_unplug(mddev->bitmap);
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