Commit e9f8a790 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

slab: Explain why SLAB_TYPESAFE_BY_RCU reference before locking 



It is not obvious to the casual user why it is absolutely necessary to
acquire a reference to a SLAB_TYPESAFE_BY_RCU structure before acquiring
a lock in that structure.  Therefore, add a comment explaining this point.

[ paulmck: Apply Vlastimil Babka feedback. ]

Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: <linux-mm@kvack.org>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
parent aba9645b

include/linux/slab.h

+11 −0
Original line number Diff line number Diff line
@@ -76,6 +76,17 @@ 
 * rcu_read_lock before reading the address, then rcu_read_unlock after

 * taking the spinlock within the structure expected at that address.

 *

 * Note that it is not possible to acquire a lock within a structure

 * allocated with SLAB_TYPESAFE_BY_RCU without first acquiring a reference

 * as described above.  The reason is that SLAB_TYPESAFE_BY_RCU pages

 * are not zeroed before being given to the slab, which means that any

 * locks must be initialized after each and every kmem_struct_alloc().

 * Alternatively, make the ctor passed to kmem_cache_create() initialize

 * the locks at page-allocation time, as is done in __i915_request_ctor(),

 * sighand_ctor(), and anon_vma_ctor().  Such a ctor permits readers

 * to safely acquire those ctor-initialized locks under rcu_read_lock()

 * protection.

 *

 * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.

 */

/* Defer freeing slabs to RCU */