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 <[email protected]> --- arch/x86/include/asm/fpu/api.h | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) --- a/arch/x86/include/asm/fpu/api.h +++ b/arch/x86/include/asm/fpu/api.h @@ -32,15 +32,27 @@ 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. */ 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
