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Date: Tue, 16 Aug 2022 10:16:04 +0200 From: Arnd Bergmann <arnd@...db.de> To: Hector Martin <marcan@...can.st> Cc: Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>, Alan Stern <stern@...land.harvard.edu>, Andrea Parri <parri.andrea@...il.com>, Boqun Feng <boqun.feng@...il.com>, Nicholas Piggin <npiggin@...il.com>, David Howells <dhowells@...hat.com>, Jade Alglave <j.alglave@....ac.uk>, Luc Maranget <luc.maranget@...ia.fr>, "Paul E. McKenney" <paulmck@...nel.org>, Akira Yokosawa <akiyks@...il.com>, Daniel Lustig <dlustig@...dia.com>, Joel Fernandes <joel@...lfernandes.org>, Mark Rutland <mark.rutland@....com>, Jonathan Corbet <corbet@....net>, Tejun Heo <tj@...nel.org>, jirislaby@...nel.org, Marc Zyngier <maz@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Oliver Neukum <oneukum@...e.com>, Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, Asahi Linux <asahi@...ts.linux.dev>, stable@...r.kernel.org Subject: Re: [PATCH] locking/atomic: Make test_and_*_bit() ordered on failure On Tue, Aug 16, 2022 at 9:03 AM Hector Martin <marcan@...can.st> wrote: > > 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@...ux-foundation.org> > Cc: stable@...r.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@...can.st> > --- > Documentation/atomic_bitops.txt | 2 +- > include/asm-generic/bitops/atomic.h | 6 ------ I double-checked all the architecture specific implementations to ensure that the asm-generic one is the only one that needs the fix. I assume this gets merged through the locking tree or that Linus picks it up directly, not through my asm-generic tree. Reviewed-by: Arnd Bergmann <arnd@...db.de>
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