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Subject[PATCH] arm64: neon: Add preemption protection for kernel_neon_busy
Date
From: Yandong Zhao <yandong77520@gmail.com>

may_use_simd() can be called in any case and access kernel_neon_busy,
for example: BUG_ON(!may_use_simd()). This patch ensures that
migration will not occur during program access to kernel_neon_busy.

Signed-off-by: Yandong Zhao <yandong77520@gmail.com>
---
arch/arm64/include/asm/simd.h | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..6580dcd 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,20 +29,12 @@
static __must_check inline bool may_use_simd(void)
{
/*
- * The raw_cpu_read() is racy if called with preemption enabled.
- * 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.
- * So, if we find it clear on some CPU then we're guaranteed to
- * find it clear on any CPU we could migrate to.
- *
- * If we are in between kernel_neon_begin()...kernel_neon_end(),
- * the flag will be set, but preemption is also disabled, so we
- * can't migrate to another CPU and spuriously see it become
- * false.
+ * Operations for contexts where we do not want to do any checks for
+ * preemptions. Unless strictly necessary, always use this_cpu_*()
+ * instead.
*/
return !in_irq() && !irqs_disabled() && !in_nmi() &&
- !raw_cpu_read(kernel_neon_busy);
+ !this_cpu_read(kernel_neon_busy);
}

#else /* ! CONFIG_KERNEL_MODE_NEON */
--
1.9.1
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