linux-kernel - Re: [PATCH] raid5: fix incorrectly counter of conf->empty_inactive_list

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Date:	Wed, 3 Aug 2016 13:51:23 +0800
From:	"liuzhengyuan" <liuzhengyuan@...inos.cn>
To:	"NeilBrown" <neilb@...e.com>
Cc:	"shli" <shli@...nel.org>,
	"liuzhengyuang521" <liuzhengyuang521@...il.com>,
	"linux-raid" <linux-raid@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] raid5: fix incorrectly counter of conf->empty_inactive_list_nr

Thanks for you replay.
I think it may be on the temp inactive list. An active sh was handled
and put to temp inactive list firstly, then moved to inactive list.
If sh is on the temp inactive list,  sh->count is zero too.

------------------ Original ------------------
From:  "NeilBrown"<neilb@...e.com>;
Date:  Wed, Aug 3, 2016 08:44 AM
To:  "ZhengYuan Liu"<liuzhengyuan@...inos.cn>;
Cc:  "shli"<shli@...nel.org>; "liuzhengyuang521"<liuzhengyuang521@...il.com>; "linux-raid"<linux-raid@...r.kernel.org>; "linux-kernel"<linux-kernel@...r.kernel.org>; "ZhengYuan Liu"<liuzhengyuan@...inos.cn>;
Subject:  Re: [PATCH] raid5: fix incorrectly counter of conf->empty_inactive_list_nr
 
On Thu, Jul 28 2016, ZhengYuan Liu wrote:

> The counter conf->empty_inactive_list_nr is only used for determine if the
> raid5 is congested which is deal with in function raid5_congested().
> It was increased in get_free_stripe() when conf->inactive_list got to be
> empty and decreased in release_inactive_stripe_list() when splice
> temp_inactive_list to conf->inactive_list. However, this may have a
> problem when raid5_get_active_stripe or stripe_add_to_batch_list was called,
> because these two functions may call list_del_init(&sh->lru) to delete sh from
> "conf->inactive_list + hash" which may cause "conf->inactive_list + hash" to
> be empty when atomic_inc_not_zero(&sh->count) got false. So a check should be
> done at these two point and increase empty_inactive_list_nr accordingly.
> Otherwise the counter may get to be negative number which would influence
> async readahead from VFS.
>
> Signed-off-by: ZhengYuan Liu <liuzhengyuan@...inos.cn>
> ---
>  drivers/md/raid5.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 7c53861..1656c44 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -659,6 +659,7 @@ raid5_get_active_stripe(struct r5conf *conf, sector_t sector,
>  {
>  struct stripe_head *sh;
>  int hash = stripe_hash_locks_hash(sector);
> +	int inc_empty_inactive_list_flag;
>  
>  pr_debug("get_stripe, sector %llu\n", (unsigned long long)sector);
>  
> @@ -703,7 +704,12 @@ raid5_get_active_stripe(struct r5conf *conf, sector_t sector,
>  atomic_inc(&conf->active_stripes);
>  BUG_ON(list_empty(&sh->lru) &&
>         !test_bit(STRIPE_EXPANDING, &sh->state));
> +	inc_empty_inactive_list_flag = 0;
> +	if (!list_empty(conf->inactive_list + hash))
> +	inc_empty_inactive_list_flag = 1;
>  list_del_init(&sh->lru);
> +	if (list_empty(conf->inactive_list + hash) && inc_empty_inactive_list_flag)
> +	atomic_inc(&conf->empty_inactive_list_nr);

Maybe I'm forgetting an important detail, but this seems more
complicated than it needs to be.
The code just just confirmed that sh->count is zero, so sh must be on
the inactive list, mustn't it?
So inc_empty_inactive_list_flag can never be set to 1.

What am I missing?  Could sh not be on the inactive list at this point?

Same for the code below.

NeilBrown


>  if (sh->group) {
>  sh->group->stripes_cnt--;
>  sh->group = NULL;
> @@ -762,6 +768,7 @@ static void stripe_add_to_batch_list(struct r5conf *conf, struct stripe_head *sh
>  sector_t head_sector, tmp_sec;
>  int hash;
>  int dd_idx;
> +	int inc_empty_inactive_list_flag;
>  
>  if (!stripe_can_batch(sh))
>  return;
> @@ -781,7 +788,11 @@ static void stripe_add_to_batch_list(struct r5conf *conf, struct stripe_head *sh
>  atomic_inc(&conf->active_stripes);
>  BUG_ON(list_empty(&head->lru) &&
>         !test_bit(STRIPE_EXPANDING, &head->state));
> +	if (!list_empty(conf->inactive_list + hash))
> +	inc_empty_inactive_list_flag = 1;
>  list_del_init(&head->lru);
> +	if (list_empty(conf->inactive_list + hash) && inc_empty_inactive_list_flag)
> +	atomic_inc(&conf->empty_inactive_list_nr);
>  if (head->group) {
>  head->group->stripes_cnt--;
>  head->group = NULL;
> -- 
> 1.9.1