From: David Laight <David.Laight@ACULAB.COM> To: "'Anton Blanchard'" <anton@samba.org>, "behanw@converseincode.com" <behanw@converseincode.com>, "ying.huang@intel.com" <ying.huang@intel.com>, "a.p.zijlstra@chello.nl" <a.p.zijlstra@chello.nl>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "oleg@redhat.com" <oleg@redhat.com>, Segher Boessenkool <segher@kernel.crashing.org>, "mingo@elte.hu" <mingo@elte.hu> Cc: "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org> Subject: RE: llist code relies on undefined behaviour, upsets llvm/clang Date: Mon, 16 Jan 2017 14:34:43 +0000 [thread overview] Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB0264AA5@AcuExch.aculab.com> (raw) In-Reply-To: <20170116083600.47175073@kryten> From: Anton Blanchard > Sent: 15 January 2017 21:36 > I was debugging a hang on a ppc64le kernel built with clang, and it > looks to be undefined behaviour with pointer wrapping in the llist code. > > A test case is below. llist_for_each_entry() does container_of() on a > NULL pointer, which wraps our pointer negative, then adds the same > offset back in and expects to get back to NULL. Unfortunately clang > decides that this can never be NULL and optimises it into an infinite > loop. ... > #define llist_for_each_entry(pos, node, member) \ > for ((pos) = llist_entry((node), typeof(*(pos)), member); \ > &(pos)->member != NULL; \ > (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member)) Maybe the above could be rewritten as (untested): for ((pos) = NULL; (!(pos) ? (node) : ((pos)->member.next) || (pos) = 0) && \ (((pos) = !(pos) ? llist_entry((node), typeof(*(pos)), member) \ : llist_entry((pos)->member.next, typeof(*(pos)), member)),1); ) Provided the compiler assumes that the loop body is never executed with 'pos == 0' it should generate the same code. David
WARNING: multiple messages have this Message-ID (diff)
From: David Laight <David.Laight@ACULAB.COM> To: 'Anton Blanchard' <anton@samba.org>, "behanw@converseincode.com" <behanw@converseincode.com>, "ying.huang@intel.com" <ying.huang@intel.com>, "a.p.zijlstra@chello.nl" <a.p.zijlstra@chello.nl>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "oleg@redhat.com" <oleg@redhat.com>, Segher Boessenkool <segher@kernel.crashing.org>, "mingo@elte.hu" <mingo@elte.hu> Cc: "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org> Subject: RE: llist code relies on undefined behaviour, upsets llvm/clang Date: Mon, 16 Jan 2017 14:34:43 +0000 [thread overview] Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB0264AA5@AcuExch.aculab.com> (raw) In-Reply-To: <20170116083600.47175073@kryten> From: Anton Blanchard > Sent: 15 January 2017 21:36 > I was debugging a hang on a ppc64le kernel built with clang, and it > looks to be undefined behaviour with pointer wrapping in the llist code. >=20 > A test case is below. llist_for_each_entry() does container_of() on a > NULL pointer, which wraps our pointer negative, then adds the same > offset back in and expects to get back to NULL. Unfortunately clang > decides that this can never be NULL and optimises it into an infinite > loop. ... > #define llist_for_each_entry(pos, node, member) \ > for ((pos) =3D llist_entry((node), typeof(*(pos)), member); = \ > &(pos)->member !=3D NULL; = \ > (pos) =3D llist_entry((pos)->member.next, typeof(*(pos)), me= mber)) Maybe the above could be rewritten as (untested): for ((pos) =3D NULL; (!(pos) ? (node) : ((pos)->member.next) || (pos) =3D= 0) && \ (((pos) =3D !(pos) ? llist_entry((node), typeof(*(pos)), member) \ : llist_entry((pos)->member.next, typeof(*(pos)), member)),1); ) Provided the compiler assumes that the loop body is never executed with 'po= s =3D=3D 0' it should generate the same code. David
next prev parent reply other threads:[~2017-01-16 14:34 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-01-15 21:36 llist code relies on undefined behaviour, upsets llvm/clang Anton Blanchard 2017-01-16 9:05 ` Peter Zijlstra 2017-01-16 11:42 ` Anton Blanchard 2017-01-16 12:53 ` Peter Zijlstra 2017-01-16 13:09 ` Andrey Ryabinin 2017-01-16 14:34 ` David Laight [this message] 2017-01-16 14:34 ` David Laight 2017-01-16 16:25 ` Peter Zijlstra
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