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x86/fpu: Make kernel FPU protection RT friendly

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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
This commit is contained in:
Thomas Gleixner 2020-10-27 11:09:51 +01:00
parent 5f0c71278d
commit cba08c5dc6
1 changed files with 16 additions and 2 deletions
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18
arch/x86/include/asm/fpu/api.h
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@ -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)
{
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