]> git.ipfire.org Git - thirdparty/kernel/stable.git/commit
locking/atomic: Make test_and_*_bit() ordered on failure
authorHector Martin <marcan@marcan.st>
Tue, 16 Aug 2022 07:03:11 +0000 (16:03 +0900)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 25 Aug 2022 09:38:10 +0000 (11:38 +0200)
commit823280a8fba3e5de2b2d8708546c807c3bce525d
treee32e7527e6f81e8dc5255a0d187caebd50476a44
parent0bd35968bcd028658f86418e8a5e64c89bd18d45
locking/atomic: Make test_and_*_bit() ordered on failure

commit 415d832497098030241605c52ea83d4e2cfa7879 upstream.

These operations are documented as always ordered in
include/asm-generic/bitops/instrumented-atomic.h, and producer-consumer
type use cases where one side needs to ensure a flag is left pending
after some shared data was updated rely on this ordering, even in the
failure case.

This is the case with the workqueue code, which currently suffers from a
reproducible ordering violation on Apple M1 platforms (which are
notoriously out-of-order) that ends up causing the TTY layer to fail to
deliver data to userspace properly under the right conditions.  This
change fixes that bug.

Change the documentation to restrict the "no order on failure" story to
the _lock() variant (for which it makes sense), and remove the
early-exit from the generic implementation, which is what causes the
missing barrier semantics in that case.  Without this, the remaining
atomic op is fully ordered (including on ARM64 LSE, as of recent
versions of the architecture spec).

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable@vger.kernel.org
Fixes: e986a0d6cb36 ("locking/atomics, asm-generic/bitops/atomic.h: Rewrite using atomic_*() APIs")
Fixes: 61e02392d3c7 ("locking/atomic/bitops: Document and clarify ordering semantics for failed test_and_{}_bit()")
Signed-off-by: Hector Martin <marcan@marcan.st>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Documentation/atomic_bitops.txt
include/asm-generic/bitops/atomic.h