Commit 696207d4 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Catalin Marinas
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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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: default avatarMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220505163207.85751-3-bigeasy@linutronix.de


Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent a1259dd8

arch/arm64/kernel/fpsimd.c

+14 −2
Original line number Diff line number Diff line
@@ -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)

{

	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();

	if (!IS_ENABLED(CONFIG_PREEMPT_RT))

		local_bh_enable();

	else

		preempt_enable();

}



static bool have_cpu_fpsimd_context(void)