On Mon, 04 Jun 2012 16:01:56 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote:
> release_stripe() is a place conf->device_lock is heavily contended. We take the
> lock even stripe count isn't 1, which isn't required. On the on the other hand,
> decreasing count first and taking lock if count is 0 can expose races:
> 1. bewteen dec count and taking lock, another thread hits the stripe in cache,
> so increase count. The stripe will be deleted from any list. In this case
> stripe count isn't 0.
> 2. between dec count and taking lock, another thread hits the stripe in cache
> and release it. In this case the stripe is already in specific list. We do
> list_move to adjust its position.
> So both cases are fixable to me.
1/ Please keep this as two different entry points, one which is called with
the lock held, which which already holds the lock. i.e. don't add a
'locking' flag.
2/ Use "atomic_dec_and_lock" to avoid taking the lock when not needed.
So one entry point does:
if atomic_dec_and_lock
common code
unlock
while the other does
if atomic_test_and lock
common code
Thanks,
NeilBrown
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> ---
> drivers/md/raid5.c | 43 +++++++++++++++++++++++++++++--------------
> 1 file changed, 29 insertions(+), 14 deletions(-)
>
> Index: linux/drivers/md/raid5.c
> ===================================================================
> --- linux.orig/drivers/md/raid5.c 2012-06-01 13:50:56.336138112 +0800
> +++ linux/drivers/md/raid5.c 2012-06-01 14:03:17.062826938 +0800
> @@ -201,20 +201,39 @@ static int stripe_operations_active(stru
> test_bit(STRIPE_COMPUTE_RUN, &sh->state);
> }
>
> -static void __release_stripe(struct r5conf *conf, struct stripe_head *sh)
> +static void __release_stripe(struct r5conf *conf, struct stripe_head *sh,
> + int locking)
> {
> + unsigned long uninitialized_var(flags);
> +
> if (atomic_dec_and_test(&sh->count)) {
> - BUG_ON(!list_empty(&sh->lru));
> + /*
> + * Before we hold device_lock, other thread can hit this stripe
> + * in cache. It could do:
> + * 1. just get_active_stripe(). The stripe count isn't 0 then.
> + * 2. do get_active_stripe() and follow release_stripe(). So the
> + * stripe might be already released and already in specific
> + * list. we do list_move to adjust its position in the list.
> + */
> + if (locking) {
> + spin_lock_irqsave(&conf->device_lock, flags);
> + if (atomic_read(&sh->count) != 0) {
> + spin_unlock_irqrestore(&conf->device_lock,
> + flags);
> + return;
> + }
> + }
> +
> BUG_ON(atomic_read(&conf->active_stripes)==0);
> if (test_bit(STRIPE_HANDLE, &sh->state)) {
> if (test_bit(STRIPE_DELAYED, &sh->state))
> - list_add_tail(&sh->lru, &conf->delayed_list);
> + list_move_tail(&sh->lru, &conf->delayed_list);
> else if (test_bit(STRIPE_BIT_DELAY, &sh->state) &&
> sh->bm_seq - conf->seq_write > 0)
> - list_add_tail(&sh->lru, &conf->bitmap_list);
> + list_move_tail(&sh->lru, &conf->bitmap_list);
> else {
> clear_bit(STRIPE_BIT_DELAY, &sh->state);
> - list_add_tail(&sh->lru, &conf->handle_list);
> + list_move_tail(&sh->lru, &conf->handle_list);
> }
> md_wakeup_thread(conf->mddev->thread);
> } else {
> @@ -225,23 +244,22 @@ static void __release_stripe(struct r5co
> md_wakeup_thread(conf->mddev->thread);
> atomic_dec(&conf->active_stripes);
> if (!test_bit(STRIPE_EXPANDING, &sh->state)) {
> - list_add_tail(&sh->lru, &conf->inactive_list);
> + list_move_tail(&sh->lru, &conf->inactive_list);
> wake_up(&conf->wait_for_stripe);
> if (conf->retry_read_aligned)
> md_wakeup_thread(conf->mddev->thread);
> }
> }
> + if (locking)
> + spin_unlock_irqrestore(&conf->device_lock, flags);
> }
> }
>
> static void release_stripe(struct stripe_head *sh)
> {
> struct r5conf *conf = sh->raid_conf;
> - unsigned long flags;
>
> - spin_lock_irqsave(&conf->device_lock, flags);
> - __release_stripe(conf, sh);
> - spin_unlock_irqrestore(&conf->device_lock, flags);
> + __release_stripe(conf, sh, 1);
> }
>
> static inline void remove_hash(struct stripe_head *sh)
> @@ -484,9 +502,6 @@ get_active_stripe(struct r5conf *conf, s
> } else {
> if (!test_bit(STRIPE_HANDLE, &sh->state))
> atomic_inc(&conf->active_stripes);
> - if (list_empty(&sh->lru) &&
> - !test_bit(STRIPE_EXPANDING, &sh->state))
> - BUG();
> list_del_init(&sh->lru);
> }
> }
> @@ -3672,7 +3687,7 @@ static void activate_bit_delay(struct r5
> struct stripe_head *sh = list_entry(head.next, struct stripe_head, lru);
> list_del_init(&sh->lru);
> atomic_inc(&sh->count);
> - __release_stripe(conf, sh);
> + __release_stripe(conf, sh, 0);
> }
> }
>
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