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>

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 */