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slab: Explain why SLAB_TYPESAFE_BY_RCU reference before locking

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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: Paul 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: Vlastimil Babka <vbabka@suse.cz>
This commit is contained in:
Paul E. McKenney
2022-09-26 08:57:56 -07:00
parent aba9645bd1
commit e9f8a790bf
1 changed files with 11 additions and 0 deletions
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11
include/linux/slab.h
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@@ -76,6 +76,17 @@
* rcu_read_lock before reading the address, then rcu_read_unlock after * rcu_read_lock before reading the address, then rcu_read_unlock after
* taking the spinlock within the structure expected at that address. * 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. * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
*/ */
/* Defer freeing slabs to RCU */ /* Defer freeing slabs to RCU */