/dev/mem: Add missing memory barriers for devmem_inode

WRITE_ONCE() isn't the correct way to publish a pointer to a data
structure, since it doesn't include a write memory barrier.  Therefore
other tasks may see that the pointer has been set but not see that the
pointed-to memory has finished being initialized yet.  Instead a
primitive with "release" semantics is needed.

Use smp_store_release() for this.

The use of READ_ONCE() on the read side is still potentially correct if
there's no control dependency, i.e. if all memory being "published" is
transitively reachable via the pointer itself.  But this pairing is
somewhat confusing and error-prone.  So just upgrade the read side to
smp_load_acquire() so that it clearly pairs with smp_store_release().

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 3234ac66

 ("/dev/mem: Revoke mappings when a driver claims the region")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20200716060553.24618-1-ebiggers@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>