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Date: Mon, 2 Jun 2008 20:54:33 +0200 From: Ingo Molnar <mingo@...e.hu> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Ulrich Drepper <drepper@...hat.com>, linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, mtk.manpages@...il.com Subject: Re: [PATCH 0/3] 64-bit futexes: Intro * Linus Torvalds <torvalds@...ux-foundation.org> wrote: > IOW, I'm faking it, but I'm making a point. Namely that you can > efficiently do read-write lock using *only* 32-bit ops, and without > any real kind of limitations on the number of readers and writers. > > So here goes the explanation and the pseudo-code. > > - have two levels of locking: the contended case, and the uncontended > case i suspect _any_ abstract locking functionality around a data structure can be implemented via atomic control over just a single user-space bit. That bit can be used as a lock and if all access to the state of that atomic variable uses it, arbitrary higher-order atomic state transitions can be derived from it. The cost would be a bit more instructions in the fastpath, but there would still only be a single atomic op (the acquire op), as the unlock would be a natural barrier (on x86 at least). Concurrency (and scheduling) of that lock would still be exactly the same as with genuine 64-bit (or even larger) atomic ops, and the fastpath would be very close as well. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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