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Message-ID: <y2p8d20b11a1005041538zc2daea6cg2580e8cc14ccf8ed@mail.gmail.com> Date: Tue, 4 May 2010 15:38:56 -0700 From: Michel Lespinasse <walken@...gle.com> To: David Howells <dhowells@...hat.com> Cc: Andrew Morton <akpm@...gle.com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: x86 rwsem: up_read() does not check active count in fast path On Tue, May 4, 2010 at 5:41 AM, David Howells <dhowells@...hat.com> wrote: > Michel Lespinasse <walken@...gle.com> wrote: > >> Looking at the x86 rwsem code, I have been wondering about the up_read() >> path. __rwsem_do_wake() comment mentions that one should check the active >> count on the way there; however I could not find that check when coming from >> up_read(). > > Look in call_rwsem_wake(), which is implemented in assembly in two places in > the x86 arch: > > arch/x86/lib/rwsem_64.S > arch/x86/lib/semaphore_32.S Gah thanks, I had totally missed that part (I knew about these files but I had taken these as pure ABI wrappers; I did not notice the one nugget of interesting logic hiding in there). BTW, what's the reason for doing this check in call_rwsem_wake ? The check is entirely redundant in the __up_write() case; and in __up_read() things might be less confusing if edx was used locally rather than in a separate function. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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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