Re: [BUG] Lockdep recursive locking in kmem_cache_free

[Arjan van de Ven <arjan@infradead.org>]

> cache_free_alien could get called, but there is no recursion here:
> 
> 1. reap_alien tries dropping remote objects freed by local node (A) to the 
> remote node (B) shared array cache (choosing a remote node as indicated by the 
> node rotor), to do this, it takes the local alien cache lock (A), and calls 
> __drain_alien_cache. The remote object comes from a slab cache X say.
> 
> 2. __drain_alien_cache. takes the remote node l3 lock (B), transfers as many
> objects as shared array cache of the remote node can hold, and calls
> free_block to free remaining objects that could not be dropped in into the
> shared array cache of remote node (B).  Now free_block is being called from
> (A) to free objects on (B). 
> 
> 3. free_block calls slab_destroy for the slab belonging to B. calls
> kmem_cache_free for the slab management, which calls __cache_free, and 
> hence cache_free_alien().  Now since this is being called from A for a local
> object of B, the check in cache_free_alien fails, and cache_free_alien
> *does* get executed.  Since slab management of a slab from B, local to B is
> freed from A, A tries to write to the local alien cache corresponding to B,
> which comes from a slab cache Y.  There is a recursion if X and Y are the
> same caches.   But that is not a possibility at all, as the off slab management
> for a slab cache cannot come from the same slab cache.  So this looks like a
> false positive from lockdep.  
actually lockdep doesn't see normal slabs and the slabs where off-slab
management comes from as the same lock, so it really shouldn't complain
about this specific case.


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