linux-kernel - Re: [PATCH v2] slub: fix false-positive lockdep warning in free

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Date:	Wed, 5 Feb 2014 00:01:08 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] slub: fix false-positive lockdep warning in
 free_partial()

On Wed, 5 Feb 2014, Vladimir Davydov wrote:

> Commit c65c1877bd68 ("slub: use lockdep_assert_held") requires
> remove_partial() to be called with n->list_lock held, but free_partial()
> called from kmem_cache_close() on cache destruction does not follow this
> rule, leading to a warning:
> 
>   WARNING: CPU: 0 PID: 2787 at mm/slub.c:1536 __kmem_cache_shutdown+0x1b2/0x1f0()
>   Modules linked in:
>   CPU: 0 PID: 2787 Comm: modprobe Tainted: G        W    3.14.0-rc1-mm1+ #1
>   Hardware name:
>    0000000000000600 ffff88003ae1dde8 ffffffff816d9583 0000000000000600
>    0000000000000000 ffff88003ae1de28 ffffffff8107c107 0000000000000000
>    ffff880037ab2b00 ffff88007c240d30 ffffea0001ee5280 ffffea0001ee52a0
>   Call Trace:
>    [<ffffffff816d9583>] dump_stack+0x51/0x6e
>    [<ffffffff8107c107>] warn_slowpath_common+0x87/0xb0
>    [<ffffffff8107c145>] warn_slowpath_null+0x15/0x20
>    [<ffffffff811c7fe2>] __kmem_cache_shutdown+0x1b2/0x1f0
>    [<ffffffff811908d3>] kmem_cache_destroy+0x43/0xf0
>    [<ffffffffa013a123>] xfs_destroy_zones+0x103/0x110 [xfs]
>    [<ffffffffa0192b54>] exit_xfs_fs+0x38/0x4e4 [xfs]
>    [<ffffffff811036fa>] SyS_delete_module+0x19a/0x1f0
>    [<ffffffff816dfcd8>] ? retint_swapgs+0x13/0x1b
>    [<ffffffff810d2125>] ? trace_hardirqs_on_caller+0x105/0x1d0
>    [<ffffffff81359efe>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>    [<ffffffff816e8539>] system_call_fastpath+0x16/0x1b
> 
> Although this cannot actually result in a race, because on cache
> destruction there should not be any concurrent frees or allocations from
> the cache, let's add spin_lock/unlock to free_partial() just to keep
> lockdep happy.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> ---
>  v2: add a comment explaining why we need to take the lock
> 
>  mm/slub.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 0eeea85034c8..24bf05e962ff 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3191,6 +3191,11 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  {
>  	struct page *page, *h;
>  
> +	/*
> +	 * the lock is for lockdep's sake, not for any actual
> +	 * race protection
> +	 */

I think Christoph was referring to altering the comment for this function 
which still says "We must be the last thread using the cache and therefore 
we do not need to lock anymore."

> +	spin_lock_irq(&n->list_lock);
>  	list_for_each_entry_safe(page, h, &n->partial, lru) {
>  		if (!page->inuse) {
>  			remove_partial(n, page);
> @@ -3200,6 +3205,7 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  			"Objects remaining in %s on kmem_cache_close()");
>  		}
>  	}
> +	spin_unlock_irq(&n->list_lock);
>  }
>  
>  /*
--
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