From ebfd3ddaa3c75b0a3e840304bdb3def7321aaeab Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 27 Oct 2020 11:09:51 +0100 Subject: [PATCH 155/323] x86/fpu: Make kernel FPU protection RT friendly Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.215-rt107.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 Link: https://lore.kernel.org/r/20201027101349.588965083@linutronix.de Signed-off-by: Sebastian Andrzej Siewior --- 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 5174c0a640ef3..ad2fee7853108 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 -- 2.44.0