From 7926f3eb614245657822bbc34a4ace7d5e1d5cd0 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Thu, 29 Jul 2021 10:36:30 +0200 Subject: [PATCH 143/166] arm64/sve: Make kernel FPU protection RT friendly Non RT kernels need to protect FPU against preemption and bottom half processing. This is achieved by disabling bottom halves 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 halves 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: Sebastian Andrzej Siewior --- arch/arm64/kernel/fpsimd.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) Index: linux-5.15.32-rt39/arch/arm64/kernel/fpsimd.c =================================================================== --- linux-5.15.32-rt39.orig/arch/arm64/kernel/fpsimd.c +++ linux-5.15.32-rt39/arch/arm64/kernel/fpsimd.c @@ -179,10 +179,19 @@ static void __get_cpu_fpsimd_context(voi * * The double-underscore version must only be called if you know the task * can't be preempted. + * + * 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. */ static void get_cpu_fpsimd_context(void) { - local_bh_disable(); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + local_bh_disable(); + else + preempt_disable(); __get_cpu_fpsimd_context(); } @@ -203,7 +212,10 @@ static void __put_cpu_fpsimd_context(voi static void put_cpu_fpsimd_context(void) { __put_cpu_fpsimd_context(); - local_bh_enable(); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + local_bh_enable(); + else + preempt_enable(); } static bool have_cpu_fpsimd_context(void)