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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Cc: hpa@zytor.com
Cc: eric.dumazet@gmail.com
Cc: bp@alien8.de
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181011104019.748208519@infradead.org

1 files 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;                                  \