Merge tag 'x86-fpu-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 FPU updates from Thomas Gleixner:

 - Simplify the FPU protection for !RT kernels

 - Add the RT variant of FPU protections

* tag 'x86-fpu-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/fpu: Make kernel FPU protection RT friendly
  x86/fpu: Simplify fpregs_[un]lock()

1 files changed, 19 insertions, 4 deletions

diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index dcd9503b1098..a5aba4ab0224 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -29,17 +29,32 @@ extern void fpregs_mark_activate(void);
  * A context switch will (and softirq might) save CPU's FPU registers to
  * fpu->state and set TIF_NEED_FPU_LOAD leaving CPU's FPU registers in
  * a random state.
+ *
+ * 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)
 {
-	preempt_disable();
-	local_bh_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_disable();
+	else
+		preempt_disable();
 }
 
 static inline void fpregs_unlock(void)
 {
-	local_bh_enable();
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_enable();
+	else
+		preempt_enable();
 }
 
 #ifdef CONFIG_X86_DEBUG_FPU