Path: csiph.com!aioe.org!bofh.it!news.nic.it!robomod From: Greg Kroah-Hartman Newsgroups: linux.kernel Subject: [PATCH 4.0 24/58] ARC: add compiler barrier to LLSC based cmpxchg Date: Sun, 19 Jul 2015 21:20:02 +0200 Message-ID: References: X-Original-To: linux-kernel@vger.kernel.org X-Mailer: git-send-email 2.4.6 User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: robomod@news.nic.it List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Approved: robomod@news.nic.it Lines: 66 Organization: linux.* mail to news gateway X-Original-Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Chuck Jordan , "Peter Zijlstra (Intel)" , Vineet Gupta X-Original-Date: Sun, 19 Jul 2015 12:11:02 -0700 X-Original-Message-ID: <20150719190812.129273748@linuxfoundation.org> X-Original-References: <20150719190811.308546345@linuxfoundation.org> X-Original-Sender: linux-kernel-owner@vger.kernel.org Xref: aioe.org linux.kernel:1187759 4.0-stable review patch. If anyone has any objections, please let me know. ------------------ From: Vineet Gupta commit d57f727264f1425a94689bafc7e99e502cb135b5 upstream. When auditing cmpxchg call sites, Chuck noted that gcc was optimizing away some of the desired LDs. | do { | new = old = *ipi_data_ptr; | new |= 1U << msg; | } while (cmpxchg(ipi_data_ptr, old, new) != old); was generating to below | 8015cef8: ld r2,[r4,0] <-- First LD | 8015cefc: bset r1,r2,r1 | | 8015cf00: llock r3,[r4] <-- atomic op | 8015cf04: brne r3,r2,8015cf10 | 8015cf08: scond r1,[r4] | 8015cf0c: bnz 8015cf00 | | 8015cf10: brne r3,r2,8015cf00 <-- Branch doesn't go to orig LD Although this was fixed by adding a ACCESS_ONCE in this call site, it seems safer (for now at least) to add compiler barrier to LLSC based cmpxchg Reported-by: Chuck Jordan Acked-by: Peter Zijlstra (Intel) Signed-off-by: Vineet Gupta Signed-off-by: Greg Kroah-Hartman --- arch/arc/include/asm/cmpxchg.h | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) --- a/arch/arc/include/asm/cmpxchg.h +++ b/arch/arc/include/asm/cmpxchg.h @@ -33,10 +33,11 @@ __cmpxchg(volatile void *ptr, unsigned l " scond %3, [%1] \n" " bnz 1b \n" "2: \n" - : "=&r"(prev) - : "r"(ptr), "ir"(expected), - "r"(new) /* can't be "ir". scond can't take limm for "b" */ - : "cc"); + : "=&r"(prev) /* Early clobber, to prevent reg reuse */ + : "r"(ptr), /* Not "m": llock only supports reg direct addr mode */ + "ir"(expected), + "r"(new) /* can't be "ir". scond can't take LIMM for "b" */ + : "cc", "memory"); /* so that gcc knows memory is being written here */ smp_mb(); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/