From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jonathan Nieder Date: Mon, 05 Mar 2012 23:42:40 +0000 Subject: Re: [regression] Re: [PATCH 2/3] futex: Sanitize cmpxchg_futex_value_locked API Message-Id: <20120305234240.GB1360@burratino> List-Id: References: <20110307021127.GB31188@google.com> <20110309112550.GA3050@google.com> <20110311021654.GA26122@google.com> <20110311024851.GC26122@google.com> <20120305000112.GA27512@burratino> <3908561D78D1C84285E8C5FCA982C28F041A65@ORSMSX104.amr.corp.intel.com> In-Reply-To: <3908561D78D1C84285E8C5FCA982C28F041A65@ORSMSX104.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: quoted-printable To: "Luck, Tony" Cc: "linux-ia64@vger.kernel.org" , Michel Lespinasse , =?utf-8?Q?=C3=89meric?= Maschino , Patrick Baggett , Jakub Jelinek , "linux-kernel@vger.kernel.org" Luck, Tony wrote: >> It doesn't look like the return value (r8) is actually being set beyond >> initialized to 0. If there is some ia64 instruction that modifies it, GCC >> doesn't know about it from the inline assembly (r8 doesn't appear in the >> inputs/outputs list). From looking at the x86 version (agh, inline asm is >> hard to parse), it does modify the return value based on whether the >> comparison was a success or not, and the return value is certainly used = by >> the callers. > > The commit comment for the change makes it sound like the return value > is an error code (-ENOSYS if the function isn't implemented/configured; > -EFAULT if the user address is bogus) - or zero if nothing bad happened. Yes, that's right. > Not "the comparison was a success or not". > > What's the real answer? The ia64 code is returning 0 regardless of whethe= r the > compare/exchange found the old value or not. Is this a bad assumption? No, I think something else is wrong, though I don't know what it would be. =C3=89meric, was the bisection result reproducible? E.g., if you try building 37a9d912b24f and 37a9d912b24f^ again, does the former consistently produce and the latter consistently not produce a crashy system?