linux-kernel - [tip:x86/urgent] x86/percpu: Fix this_cpu

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Message-ID: <tip-b59167ac7bafd804c91e49ad53c6d33a7394d4c8@git.kernel.org>
Date:   Sun, 14 Oct 2018 02:16:33 -0700
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, edumazet@...gle.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...nel.org
Subject: [tip:x86/urgent] x86/percpu: Fix this_cpu_read()

Commit-ID:  b59167ac7bafd804c91e49ad53c6d33a7394d4c8
Gitweb:     https://git.kernel.org/tip/b59167ac7bafd804c91e49ad53c6d33a7394d4c8
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Thu, 11 Oct 2018 12:38:27 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Sun, 14 Oct 2018 11:11:22 +0200

x86/percpu: Fix this_cpu_read()

Eric reported that a sequence count loop using this_cpu_read() got
optimized out. This is wrong, this_cpu_read() must imply READ_ONCE()
because the interface is IRQ-safe, therefore an interrupt can have
changed the per-cpu value.

Fixes: 7c3576d261ce ("[PATCH] i386: Convert PDA into the percpu section")
Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Cc: hpa@...or.com
Cc: eric.dumazet@...il.com
Cc: bp@...en8.de
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20181011104019.748208519@infradead.org

---
 arch/x86/include/asm/percpu.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index e9202a0de8f0..1a19d11cfbbd 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -185,22 +185,22 @@ do {									\
 	typeof(var) pfo_ret__;				\
 	switch (sizeof(var)) {				\
 	case 1:						\
-		asm(op "b "__percpu_arg(1)",%0"		\
+		asm volatile(op "b "__percpu_arg(1)",%0"\
 		    : "=q" (pfo_ret__)			\
 		    : "m" (var));			\
 		break;					\
 	case 2:						\
-		asm(op "w "__percpu_arg(1)",%0"		\
+		asm volatile(op "w "__percpu_arg(1)",%0"\
 		    : "=r" (pfo_ret__)			\
 		    : "m" (var));			\
 		break;					\
 	case 4:						\
-		asm(op "l "__percpu_arg(1)",%0"		\
+		asm volatile(op "l "__percpu_arg(1)",%0"\
 		    : "=r" (pfo_ret__)			\
 		    : "m" (var));			\
 		break;					\
 	case 8:						\
-		asm(op "q "__percpu_arg(1)",%0"		\
+		asm volatile(op "q "__percpu_arg(1)",%0"\
 		    : "=r" (pfo_ret__)			\
 		    : "m" (var));			\
 		break;					\