linux-kernel - RE: [PATCH] locking/atomic: Make test_and_*

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Date:   Wed, 17 Aug 2022 08:20:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Hector Martin' <marcan@...can.st>, Will Deacon <will@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>
CC:     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" <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-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Asahi Linux <asahi@...ts.linux.dev>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] locking/atomic: Make test_and_*_bit() ordered on failure

...
>  	p += BIT_WORD(nr);
> -	if (READ_ONCE(*p) & mask)
> -		return 1;
> -
>  	old = arch_atomic_long_fetch_or(mask, (atomic_long_t *)p);
>  	return !!(old & mask);
>  }

This looks like the same pattern (attempting to avoid a
locked bus cycle) that caused the qdisc code to sit on
transmit packets (even on x86).
That had some barriers in it (possibly nops on x86) that
didn't help - although the comments suggested otherwise.

I wonder if the pattern has been used anywhere else?

	David

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