Commit b59167ac authored Oct 11, 2018 by Peter Zijlstra Committed by Thomas Gleixner Oct 14, 2018

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: 7c3576d2 ("[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

parent 4907c68a

arch/x86/include/asm/percpu.h

+4 −4

Original line number	Diff line number	Diff line
		@@ -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; \