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Subject[PATCH] arm64: neon: Fix function may_use_simd() return error status
From: Yandong Zhao <>

It does not matter if the caller of may_use_simd() migrates to
another cpu after the call, but it is still important that the
kernel_neon_busy percpu instance that is read matches the cpu the
task is running on at the time of the read.

This means that raw_cpu_read() is not sufficient. kernel_neon_busy
may appear true if the caller migrates during the execution of
raw_cpu_read() and the next task to be scheduled in on the initial
cpu calls kernel_neon_begin().

This patch replaces raw_cpu_read() with this_cpu_read() to protect
against this race.

Fixes: cb84d11e1625 ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON")
Reviewed-by: Dave Martin <>
Signed-off-by: Yandong Zhao <>
arch/arm64/include/asm/simd.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..784a8c2 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,7 +29,8 @@
static __must_check inline bool may_use_simd(void)
- * The raw_cpu_read() is racy if called with preemption enabled.
+ * The this_cpu_read() is racy if called with preemption enabled,
+ * since the task may subsequently migrate to another CPU.
* This is not a bug: kernel_neon_busy is only set when
* preemption is disabled, so we cannot migrate to another CPU
* while it is set, nor can we migrate to a CPU where it is set.
@@ -42,7 +43,7 @@ static __must_check inline bool may_use_simd(void)
* false.
return !in_irq() && !irqs_disabled() && !in_nmi() &&
- !raw_cpu_read(kernel_neon_busy);
+ !this_cpu_read(kernel_neon_busy);

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