linux-kernel - Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to

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Date:	Wed, 21 Jul 2010 09:00:07 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Wang Sheng-Hui <crosslonelyover@...il.com>
CC:	agruen@...e.de, hch@...radead.org,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	kernel-janitors <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan
 > 0

Wang Sheng-Hui wrote:
> Sorry. regerated the patch, please check it.
> I wrapped most code in single pair of spinlock ops for 2 reasons:
> 1) get spinlock 2 times seems time consuming
> 2) use single pair of spinlock ops can keep "count"
>   consistent for the shrink operation. 2 pairs may
>   get some new ces created by other processes.
> 

Sorry, this patch appears to have whitespace cut & paste mangling.

More comments below.

> Signed-off-by: Wang Sheng-Hui <crosslonelyover@...il.com>
> ---
> fs/mbcache.c |   24 ++++++++++++------------
> 1 files changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index ec88ff3..ee57aa3 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -201,21 +201,15 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
> {
>     LIST_HEAD(free_list);
>     struct list_head *l, *ltmp;
> +    struct mb_cache *cache;
>     int count = 0;
> 
> -    spin_lock(&mb_cache_spinlock);
> -    list_for_each(l, &mb_cache_list) {
> -        struct mb_cache *cache =
> -            list_entry(l, struct mb_cache, c_cache_list);
> -        mb_debug("cache %s (%d)", cache->c_name,
> -              atomic_read(&cache->c_entry_count));
> -        count += atomic_read(&cache->c_entry_count);
> -    }
>     mb_debug("trying to free %d entries", nr_to_scan);
> -    if (nr_to_scan == 0) {
> -        spin_unlock(&mb_cache_spinlock);
> +
> +    spin_lock(&mb_cache_spinlock);
> +    if (nr_to_scan == 0)
>         goto out;
> -    }
> +
>     while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
>         struct mb_cache_entry *ce =
>             list_entry(mb_cache_lru_list.next,
> @@ -223,12 +217,18 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
>         list_move_tail(&ce->e_lru_list, &free_list);
>         __mb_cache_entry_unhash(ce);
>     }
> -    spin_unlock(&mb_cache_spinlock);

you can't do this because

>     list_for_each_safe(l, ltmp, &free_list) {
>         __mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,

this takes the spinlock too and you'll deadlock.

Did you test this patch?

-Eric

>                            e_lru_list), gfp_mask);
>     }
> out:
> +    list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
> +        mb_debug("cache %s (%d)", cache->c_name,
> +              atomic_read(&cache->c_entry_count));
> +        count += atomic_read(&cache->c_entry_count);
> +    }
> +    spin_unlock(&mb_cache_spinlock);
> +
>     return (count / 100) * sysctl_vfs_cache_pressure;
> }
> 

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