On Tue, 3 Jul 2012 15:51:11 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> Neil pointed out the bitmap write optimization in handle_stripe_clean_event()
> is unnecessary, because the chance one stripe gets written twice in the mean
> time is rare. We can always do a bitmap_startwrite when a write request is
> added to a stripe and bitmap_endwrite after write request is done. Delete the
> optimization. With it, we can delete some cases of stripe_lock.
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> ---
> drivers/md/raid5.c | 28 ++++++++--------------------
> 1 file changed, 8 insertions(+), 20 deletions(-)
>
> Index: linux/drivers/md/raid5.c
> ===================================================================
> --- linux.orig/drivers/md/raid5.c 2012-07-03 14:58:51.241382361 +0800
> +++ linux/drivers/md/raid5.c 2012-07-03 15:04:48.568889733 +0800
> @@ -2350,7 +2350,7 @@ static int add_stripe_bio(struct stripe_
> spin_lock_irq(&sh->stripe_lock);
> if (forwrite) {
> bip = &sh->dev[dd_idx].towrite;
> - if (*bip == NULL && sh->dev[dd_idx].written == NULL)
> + if (*bip == NULL)
> firstwrite = 1;
> } else
> bip = &sh->dev[dd_idx].toread;
> @@ -2427,7 +2427,6 @@ handle_failed_stripe(struct r5conf *conf
> int i;
> for (i = disks; i--; ) {
> struct bio *bi;
> - int bitmap_end = 0;
>
> if (test_bit(R5_ReadError, &sh->dev[i].flags)) {
> struct md_rdev *rdev;
> @@ -2451,10 +2450,9 @@ handle_failed_stripe(struct r5conf *conf
> /* fail all writes first */
> bi = sh->dev[i].towrite;
> sh->dev[i].towrite = NULL;
> - if (bi) {
> + if (bi)
> s->to_write--;
> - bitmap_end = 1;
> - }
> + spin_unlock_irq(&sh->stripe_lock);
>
> if (test_and_clear_bit(R5_Overlap, &sh->dev[i].flags))
> wake_up(&conf->wait_for_overlap);
> @@ -2473,7 +2471,6 @@ handle_failed_stripe(struct r5conf *conf
> /* and fail all 'written' */
> bi = sh->dev[i].written;
> sh->dev[i].written = NULL;
> - if (bi) bitmap_end = 1;
> while (bi && bi->bi_sector <
> sh->dev[i].sector + STRIPE_SECTORS) {
> struct bio *bi2 = r5_next_bio(bi, sh->dev[i].sector);
> @@ -2509,10 +2506,8 @@ handle_failed_stripe(struct r5conf *conf
> bi = nextbi;
> }
> }
> - spin_unlock_irq(&sh->stripe_lock);
> - if (bitmap_end)
> - bitmap_endwrite(conf->mddev->bitmap, sh->sector,
> - STRIPE_SECTORS, 0, 0);
> + bitmap_endwrite(conf->mddev->bitmap, sh->sector,
> + STRIPE_SECTORS, 0, 0);
> /* If we were in the middle of a write the parity block might
> * still be locked - so just clear all R5_LOCKED flags
> */
Thanks.
However this section - handle_failed_stripe - isn't correct.
bitmap_startwrite and bitmap_endwrite increment and decrement a
counter and so must be balanced.
We are now counting once for each list that is on either ->towrite or
->written. We bitmap_startwrite when we set ->towrite, we then move that to
->written. Then when we remove from ->written we bitmap_endwrite.
In the handle_failed_stripe case we may remove 0, 1, or 2 lists. So we
need to call bitmap_endwrite 0, 1, or 2 times.
So if towrite was not NULL, we want to call bitmap_endwrite
then if written was not NULL we want to call it again.
Also I think I'd prefer it if this patch were before "add a per-stripe lock".
It is best to first get rid of use of device_lock first, then change some of
the remaining ones to stripe_lock. Changing some to stripe_lock, then
discarding them seems messy.
Thanks,
NeilBrown
> @@ -2713,9 +2708,7 @@ static void handle_stripe_clean_event(st
> test_bit(R5_UPTODATE, &dev->flags)) {
> /* We can return any write requests */
> struct bio *wbi, *wbi2;
> - int bitmap_end = 0;
> pr_debug("Return write for disc %d\n", i);
> - spin_lock_irq(&sh->stripe_lock);
> wbi = dev->written;
> dev->written = NULL;
> while (wbi && wbi->bi_sector <
> @@ -2728,15 +2721,10 @@ static void handle_stripe_clean_event(st
> }
> wbi = wbi2;
> }
> - if (dev->towrite == NULL)
> - bitmap_end = 1;
> - spin_unlock_irq(&sh->stripe_lock);
> - if (bitmap_end)
> - bitmap_endwrite(conf->mddev->bitmap,
> - sh->sector,
> - STRIPE_SECTORS,
> + bitmap_endwrite(conf->mddev->bitmap, sh->sector,
> + STRIPE_SECTORS,
> !test_bit(STRIPE_DEGRADED, &sh->state),
> - 0);
> + 0);
> }
> }
>
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