linux-kernel - [PATCH] arm64: neon: Fix function may_use

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Date:   Thu, 12 Jul 2018 11:29:38 +0800
From:   "Yandong.Zhao" <yandong77520@...il.com>
To:     Dave.Martin@....com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ard.biesheuvel@...aro.org, will.deacon@....com,
        catalin.marinas@....com, zhaoyd@...ndersoft.com,
        zhaoxb@...ndersoft.com, fanlc0801@...ndersoft.com,
        Yandong Zhao <yandong77520@...il.com>
Subject: [PATCH] arm64: neon: Fix function may_use_simd() return error status

From: Yandong Zhao <yandong77520@...il.com>

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.

Fixes: cb84d11e1625 ("arm64: neon: Remove support for nested or hardirq kernel-mode NEON")
Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Reviewed-by: Dave Martin <Dave.Martin@....com>
Reviewed-by: Mark Rutland <mark.rutland@.....com>
Signed-off-by: Yandong Zhao <yandong77520@...il.com>
---
 arch/arm64/include/asm/simd.h | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..6495cc5 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,20 +29,15 @@
 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 */
-- 
1.9.1