linux-kernel - Re: [PATCH v4 2/2] fs/dcache.c: fix spin lockup issue on nlru->lock

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Date:   Thu, 29 Jun 2017 15:48:28 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sahitya Tummala <stummala@...eaurora.org>
Cc:     Alexander Polakov <apolyakov@...et.ru>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Jan Kara <jack@...e.cz>, viro@...iv.linux.org.uk,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] fs/dcache.c: fix spin lockup issue on nlru->lock

On Thu, 29 Jun 2017 09:09:35 +0530 Sahitya Tummala <stummala@...eaurora.org> wrote:

> __list_lru_walk_one() acquires nlru spin lock (nlru->lock) for
> longer duration if there are more number of items in the lru list.
> As per the current code, it can hold the spin lock for upto maximum
> UINT_MAX entries at a time. So if there are more number of items in
> the lru list, then "BUG: spinlock lockup suspected" is observed in
> the below path -
> 
> ...
>
> Fix this lockup by reducing the number of entries to be shrinked
> from the lru list to 1024 at once. Also, add cond_resched() before
> processing the lru list again.
> 
> ...
>
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1133,11 +1133,12 @@ void shrink_dcache_sb(struct super_block *sb)
>  		LIST_HEAD(dispose);
>  
>  		freed = list_lru_walk(&sb->s_dentry_lru,
> -			dentry_lru_isolate_shrink, &dispose, UINT_MAX);
> +			dentry_lru_isolate_shrink, &dispose, 1024);
>  
>  		this_cpu_sub(nr_dentry_unused, freed);
>  		shrink_dentry_list(&dispose);
> -	} while (freed > 0);
> +		cond_resched();
> +	} while (list_lru_count(&sb->s_dentry_lru) > 0);
>  }
>  EXPORT_SYMBOL(shrink_dcache_sb);

I'll add a cc:stable to this one - a large dentry list is a relatively
common thing.

I'm assumng that [1/2] does not need to be backported, OK?