From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 28 Sep 2018 10:37:26 +0200 From: Martin Schwidefsky Subject: Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test In-Reply-To: References: <74fb6ce22f62e0fb48b91ca9918b74cedbcecaf1.1538096323.git.luto@kernel.org> Message-Id: <20180928103726.07761672@mschwideX1> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-Archive: List-Post: To: Geert Uytterhoeven Cc: Thomas Gleixner , Andy Lutomirski , the arch/x86 maintainers , Peter Zijlstra , Ingo Molnar , Darren Hart , Linux Kernel Mailing List , linux-s390 , Heiko Carstens , Finn Thain List-ID: On Fri, 28 Sep 2018 09:12:10 +0200 Geert Uytterhoeven wrote: > Hi Thomas, > > On Fri, Sep 28, 2018 at 8:21 AM Thomas Gleixner wrote: > > On Thu, 27 Sep 2018, Andy Lutomirski wrote: > > > I have a couple questions here: > > > > > > - Is this actually okay on all architectures? That is, are there > > > cases where we'll screw up if we fail a USER_DS access this early? > > > s390 stands out as the obvious special case (where USER_DS is not > > > than just a subset of KERNEL_DS), but s390 opts out. > > > > > > - Why doesn't x86 set HAVE_FUTEX_CMPXCHG? Or do we still support > > > some 32-bit configurations that don't have cmpxchg and don't know > > > about it at compile time? > > > > I'm not entirely sure. Have to dig into the details. I assume S390 just can > > set it though. > > Not sure. My "[PATCH] futex: Switch to USER_DS for futex test" > (https://www.spinics.net/lists/stable/msg28846.html), which is > basically the same > as this patch, broke s390, so it was never merged. > > See "[BUG -next] "futex: switch to USER_DS for futex test" breaks s390" > (https://www.spinics.net/lists/linux-next/msg27902.html) > > Heiko said: > | Martin and I discussed this today and we will change the s390 code so that > | it will also survive very early USER_DS accesses (without valid current->mm) > | since we also discovered a couple of other oddities in our code. > > I don't know if that has happened, and whether it would work on s390 now. commit 03b8c7b623c80af264c4c8d6111e5c6289933666 Author: Heiko Carstens Date: Sun Mar 2 13:09:47 2014 +0100 futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test If an architecture has futex_atomic_cmpxchg_inatomic() implemented and there is no runtime check necessary, allow to skip the test within futex_init(). This allows to get rid of some code which would always give the same result, and also allows the compiler to optimize a couple of if statements away. Signed-off-by: Heiko Carstens Cc: Finn Thain Cc: Geert Uytterhoeven Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris Signed-off-by: Thomas Gleixner Heiko created the CONFIG_HAVE_FUTEX_CMPXCHG to get around this issue. We just skip the runtime check as well as arc, m68k and sh. Not sure about xtensa, the set it config option only for !MMU. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.