Re: [patch 1/2] mm, slub: fix false-positive lockdep warning in free_partial()

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jul 22, 2014 at 03:57:58PM -0700, David Rientjes wrote:
> From: Vladimir Davydov <vdavydov@parallels.com>
> 
> 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.

Please never add needless locking just to please lockdep.

> Acked-by: Christoph Lameter <cl@linux.com>
> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> Signed-off-by: Pekka Enberg <penberg@kernel.org>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  mm/slub.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3195,12 +3195,13 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
>  /*
>   * Attempt to free all partial slabs on a node.
>   * This is called from kmem_cache_close(). We must be the last thread
> - * using the cache and therefore we do not need to lock anymore.
> + * using the cache, but we still have to lock for lockdep's sake.
>   */
>  static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  {
>  	struct page *page, *h;
>  
> +	spin_lock_irq(&n->list_lock);
>  	list_for_each_entry_safe(page, h, &n->partial, lru) {
>  		if (!page->inuse) {
>  			__remove_partial(n, page);

This already uses __remove_partial(), which does not have the lockdep
assertion.  You even acked the patch that made this change, why add
the spinlock now?

commit 1e4dd9461fabfbc780cdfaf103cec790f3a53325
Author: Steven Rostedt <rostedt@goodmis.org>
Date:   Mon Feb 10 14:25:46 2014 -0800

    slub: do not assert not having lock in removing freed partial
    
    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:
        __kmem_cache_shutdown+0x1b2/0x1f0
        kmem_cache_destroy+0x43/0xf0
        xfs_destroy_zones+0x103/0x110 [xfs]
        exit_xfs_fs+0x38/0x4e4 [xfs]
        SyS_delete_module+0x19a/0x1f0
        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_partial() function that does not test if the
    list_lock is held, as no one should have it due to it being freed.
    
    Also added a __add_partial() function that does not do the lock
    validation either, as it is not needed for the creation of the cache.
    
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Reported-by: Vladimir Davydov <vdavydov@parallels.com>
    Suggested-by: David Rientjes <rientjes@google.com>
    Acked-by: David Rientjes <rientjes@google.com>
    Acked-by: Vladimir Davydov <vdavydov@parallels.com>
    Acked-by: Christoph Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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