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authorYandong Zhao <yandong77520@gmail.com>2018-07-11 19:06:28 +0800
committerWill Deacon <will.deacon@arm.com>2018-07-11 17:02:02 +0100
commit2fd8eb4ad87104c54800ef3cea498c92eb15c78a (patch)
treeba0d6cb6f9dee0872b801d0d021e8ae84382e1b5 /arch
parent96f95a17c1cfe65a002e525114d96616e91a8f2d (diff)
arm64: neon: Fix function may_use_simd() return error status
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. Cc: <stable@vger.kernel.org> Fixes: cb84d11e1625 ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON") Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Dave Martin <Dave.Martin@arm.com> Reviewed-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Yandong Zhao <yandong77520@gmail.com> Signed-off-by: Will Deacon <will.deacon@arm.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/arm64/include/asm/simd.h19
1 files changed, 7 insertions, 12 deletions
diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe932e6..6495cc51246f 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,20 +29,15 @@ DECLARE_PER_CPU(bool, kernel_neon_busy);
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.
+ * kernel_neon_busy is only set while preemption is disabled,
+ * and is clear whenever preemption is enabled. Since
+ * this_cpu_read() is atomic w.r.t. preemption, kernel_neon_busy
+ * cannot change under our feet -- if it's set we cannot be
+ * migrated, and if it's clear we cannot be migrated to a CPU
+ * where it is set.
*/
return !in_irq() && !irqs_disabled() && !in_nmi() &&
- !raw_cpu_read(kernel_neon_busy);
+ !this_cpu_read(kernel_neon_busy);
}
#else /* ! CONFIG_KERNEL_MODE_NEON */