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arm64/sve: 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 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 <bigeasy@linutronix.de>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220505163207.85751-3-bigeasy@linutronix.de
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Sebastian Andrzej Siewior 2022-05-05 18:32:06 +02:00 • committed by Catalin Marinas
parent a1259dd807
commit 696207d425
1 changed files with 14 additions and 2 deletions
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16
arch/arm64/kernel/fpsimd.c
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@ -237,10 +237,19 @@ static void __get_cpu_fpsimd_context(void)
*
* 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();
}
@ -261,7 +270,10 @@ static void __put_cpu_fpsimd_context(void)
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)