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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: cb84d11e16 ("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>
hifive-unleashed-5.1
Yandong Zhao 2018-07-11 19:06:28 +08:00 committed by Will Deacon
parent 96f95a17c1
commit 2fd8eb4ad8
1 changed files with 7 additions and 12 deletions
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19
arch/arm64/include/asm/simd.h
View File

@ -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 */