x86/fpu: Make kernel FPU protection RT friendly (cba08c5d) · Commits · 戴 / test

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

arch/x86/include/asm/fpu/api.h

+16 −2

Original line number	Diff line number	Diff line
		@@ -32,15 +32,29 @@ extern void fpregs_mark_activate(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)
		{
		if (!IS_ENABLED(CONFIG_PREEMPT_RT))
		local_bh_disable();
		else
		preempt_disable();
		}

		static inline void fpregs_unlock(void)
		{
		if (!IS_ENABLED(CONFIG_PREEMPT_RT))
		local_bh_enable();
		else
		preempt_enable();
		}

		#ifdef CONFIG_X86_DEBUG_FPU

Admin message