Message ID | 1531023561-24195-1-git-send-email-yandong77520@gmail.com |
---|---|
State | New |
Headers | show |
Series | arm64: neon: This is a bug | expand |
On Sun, Jul 08, 2018 at 12:19:21PM +0800, Yandong.Zhao wrote: > From: Yandong Zhao <yandong77520@gmail.com> > > Dear, Dave.Martin > > This is a bug that calls may_use_busy() when preemption is enabled. > > We encountered a ‘sysdump’ problem on Kernel 4.14 code, calling > may_use_simd() in the function kernel_neon_begin() return null. > Our analysis concludes that the thread is migrating while executing > raw_cpu_read(kernel_neon_busy). > > Signed-off-by: Yandong Zhao <yandong77520@gmail.com> (Duplicate patch, see [1] for discussion.) [...] Cheers ---Dave [1] [PATCH] arm64: neon: Do not access kernel_neon_busy with preemption enabled http://lists.infradead.org/pipermail/linux-arm-kernel/2018-July/589259.html
diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h index fa8b3fe..0d91084 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. The 'kernel_neon_busy' here must be the current cpu. */ return !in_irq() && !irqs_disabled() && !in_nmi() && - !raw_cpu_read(kernel_neon_busy); + !this_cpu_read(kernel_neon_busy); } #else /* ! CONFIG_KERNEL_MODE_NEON */