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From 4918a84000ae19a1db7790609037f15839e6b6fd Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 27 Oct 2020 11:09:51 +0100
Subject: [PATCH 158/296] x86/fpu: Make kernel FPU protection RT friendly
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.35-rt39.tar.xz
Non RT kernels need to protect FPU against preemption and bottom half
processing. This is achieved by disabling bottom halfs via
local_bh_disable() which implictly disables preemption.
On RT kernels this protection mechanism is not sufficient because
local_bh_disable() does not disable preemption. It serializes bottom half
related processing via a CPU local lock.
As bottom halfs are running always in thread context on RT kernels
disabling preemption is the proper choice as it implicitly prevents bottom
half processing.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201027101349.588965083@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
arch/x86/include/asm/fpu/api.h | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 78b2c9cbd94d..67a4f1cb2aac 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -43,15 +43,29 @@ static inline void kernel_fpu_begin(void)
*
* local_bh_disable() protects against both preemption and soft interrupts
* on !RT kernels.
+ *
+ * On RT kernels local_bh_disable() is not sufficient because it only
+ * serializes soft interrupt related sections via a local lock, but stays
+ * preemptible. Disabling preemption is the right choice here as bottom
+ * half processing is always in thread context on RT kernels so it
+ * implicitly prevents bottom half processing as well.
+ *
+ * Disabling preemption also serializes against kernel_fpu_begin().
*/
static inline void fpregs_lock(void)
{
- local_bh_disable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_bh_disable();
+ else
+ preempt_disable();
}
static inline void fpregs_unlock(void)
{
- local_bh_enable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_bh_enable();
+ else
+ preempt_enable();
}
#ifdef CONFIG_X86_DEBUG_FPU
--
2.30.2
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