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doc: SLAB_TYPESAFE_BY_RCU uses cannot rely on spinlocks

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Because the SLAB_TYPESAFE_BY_RCU code does not zero pages that are
to be broken up into slabs, the memory returned by kmem_cache_alloc()
must be fully initialized, including any spinlocks included in the newly
allocated structure.  This means that readers attempting to look up an
SLAB_TYPESAFE_BY_RCU object must use a reference-counting approach.
A spinlock may be acquired only after a reference is obtained, which
prevents that object from being passed to kmem_struct_free(), but only
while that reference continues to be held.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-08-09 10:24:36 -07:00
parent 022d1b356c
commit 99cf092058
1 changed files with 12 additions and 7 deletions
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19
Documentation/RCU/whatisRCU.rst
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@ -915,13 +915,18 @@ which an RCU reference is held include:
The understanding that RCU provides a reference that only prevents a
change of type is particularly visible with objects allocated from a
slab cache marked ``SLAB_TYPESAFE_BY_RCU``. RCU operations may yield a
reference to an object from such a cache that has been concurrently
freed and the memory reallocated to a completely different object,
though of the same type. In this case RCU doesn't even protect the
identity of the object from changing, only its type. So the object
found may not be the one expected, but it will be one where it is safe
to take a reference or spinlock and then confirm that the identity
matches the expectations.
reference to an object from such a cache that has been concurrently freed
and the memory reallocated to a completely different object, though of
the same type. In this case RCU doesn't even protect the identity of the
object from changing, only its type. So the object found may not be the
one expected, but it will be one where it is safe to take a reference
(and then potentially acquiring a spinlock), allowing subsequent code
to check whether the identity matches expectations. It is tempting
to simply acquire the spinlock without first taking the reference, but
unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be
initialized after each and every call to kmem_cache_alloc(), which renders
reference-free spinlock acquisition completely unsafe. Therefore, when
using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter.
With traditional reference counting -- such as that implemented by the
kref library in Linux -- there is typically code that runs when the last