Re: [PATCH] slub: Do not assert not having lock in removing freed partial

[David Rientjes <rientjes@google.com>]

On Wed, 5 Feb 2014, Steven Rostedt wrote:

> Vladimir reported the following issue:
> 
> 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
> 
> 
> His solution was to add a spinlock in order to quiet lockdep. Although
> there would be no contention to adding the lock, that lock also
> requires disabling of interrupts which will have a larger impact on the
> system.
> 
> Instead of adding a spinlock to a location where it is not needed for
> lockdep, make a remove_freed_partial() function that does not test if
> the list_lock is held, as no one should have it due to it being freed.
> 
> Reported-by: Vladimir Davydov <vdavydov@parallels.com>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> 
> Index: linux-trace.git/mm/slub.c
> ===================================================================
> --- linux-trace.git.orig/mm/slub.c
> +++ linux-trace.git/mm/slub.c
> @@ -1530,13 +1530,30 @@ static inline void add_partial(struct km
>  		list_add(&page->lru, &n->partial);
>  }
>  
> +static __always_inline void
> +__remove_partial(struct kmem_cache_node *n, struct page *page)
> +{
> +	list_del(&page->lru);
> +	n->nr_partial--;
> +}
> +
>  static inline void remove_partial(struct kmem_cache_node *n,
>  					struct page *page)
>  {
>  	lockdep_assert_held(&n->list_lock);
> +	__remove_partial(n, page);
> +}
>  
> -	list_del(&page->lru);
> -	n->nr_partial--;
> +/*
> + * The difference between remove_partial and remove_freed_partial
> + * is that remove_freed_partial happens only on a a freed slab

Duplicate "a" there.

> + * that should not have anyone accessing it, and thus does not
> + * require the n->list_lock.
> + */
> +static inline void remove_freed_partial(struct kmem_cache_node *n,
> +					struct page *page)
> +{
> +	__remove_partial(n, page);
>  }
>  
>  /*
> @@ -3195,7 +3212,7 @@ static void free_partial(struct kmem_cac
>  
>  	list_for_each_entry_safe(page, h, &n->partial, lru) {
>  		if (!page->inuse) {
> -			remove_partial(n, page);
> +			remove_freed_partial(n, page);
>  			discard_slab(s, page);
>  		} else {
>  			list_slab_objects(s, page,

We'll want to do something similiar for the add_partial() called from
early_kmem_cache_node_alloc(), right?  It had the added n->list_lock for 
the same reason and is done during early init where nobody else can be 
referencing a kmem_cache_node.

It would probably be better to define these in terms of "partial slabs 
that cannot have anyone else accessing it" rather than "freed slabs".