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Date: Fri, 22 Nov 2013 00:18:58 +0100 From: Alexander Holler <holler@...oftware.de> To: Linus Torvalds <torvalds@...ux-foundation.org> CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jakub Jelinek <jakub@...hat.com>, Richard Henderson <rth@...ddle.net>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>, Peter Zijlstra <peterz@...radead.org>, lttng-dev@...ts.lttng.org, Nathan Lynch <Nathan_Lynch@...tor.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Luis Lozano <llozano@...omium.org>, Bhaskar Janakiraman <bjanakiraman@...omium.org>, Han Shen <shenhan@...omium.org> Subject: Re: current_thread_info() not respecting program order with gcc 4.8.x Am 21.11.2013 23:32, schrieb Linus Torvalds: > On Thu, Nov 21, 2013 at 8:02 AM, Alexander Holler <holler@...oftware.de> wrote: > The bug is not that gcc can re-order or combine the accesses to "sp". > WE WANT THAT TO HAPPEN. Sure, and I don't disagree on that. > > The bug is *outside* that "current_thread_info()" macro/inline > function. It's the *dereference* of the pointer that gcc re-orders. > AND THAT IS WRONG. > > Gcc seems to mess up the alias analysis, and decide that the > deferences cannot alias. Which is wrong. They clearly *can* alias, > exactly because the value of "sp & ~(THREAD_SIZE - 1)" ends up having > the same value all the time. Sorry, that I still disagree. I try to describe it more clearly why I still think that the problem might be because of that const declaration. (...) foobar1 = current_thread_info() __attribute_const__ { return sp->somewhere_local; } (...) foobar2 = current_thread_info() __attribute_const__ { return sp->somewhere_local; } So, even if sp is the same in both cases, that const states that wherever sp points to is local to current_thread_info(), so it can't be the same for both cases. Regards, Alexander Holler -- 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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