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f3db0bed45
Normally smp_store_release() or cmpxchg_release() is paired with
smp_load_acquire(). Sometimes smp_load_acquire() can be replaced with
the more lightweight READ_ONCE(). However, for this to be safe, all the
published memory must only be accessed in a way that involves the
pointer itself. This may not be the case if allocating the object also
involves initializing a static or global variable, for example.
fsverity_info::tree_params.hash_alg->tfm is a crypto_ahash object that's
internal to and is allocated by the crypto subsystem. So by using
READ_ONCE() for ->i_verity_info, we're relying on internal
implementation details of the crypto subsystem.
Remove this fragile assumption by using smp_load_acquire() instead.
Also fix the cmpxchg logic to correctly execute an ACQUIRE barrier when
losing the cmpxchg race, since cmpxchg doesn't guarantee a memory
barrier on failure.
(Note: I haven't seen any real-world problems here. This change is just
fixing the code to be guaranteed correct and less fragile.)
Fixes: fd2d1acfcadf ("fs-verity: add the hook for file ->open()")
Link: https://lore.kernel.org/r/20200721225920.114347-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>