[PATCH 3/3] arm64/fpsimd: Fix FPSIMD context handling vs PREEMPT_RT

[Valentin Schneider <valentin.schneider@arm.com>]

Running v5.13-rt1 on my arm64 Juno board triggers:

[   11.337654] WARNING: CPU: 4 PID: 1 at arch/arm64/kernel/fpsimd.c:296 task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1))
[   11.337692] Modules linked in:
[   11.337705] CPU: 4 PID: 1 Comm: init Not tainted 5.13.0-rt1 #52
[   11.337719] Hardware name: ARM Juno development board (r0) (DT)
[   11.337727] pstate: 40000005 (nZcv daif -PAN -UAO -TCO BTYPE=--)
[   11.337740] pc : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1))
[   11.337755] lr : task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 3))
[   11.337769] sp : ffff800012f4bdd0
[   11.337775] x29: ffff800012f4bdd0 x28: ffff000800160000 x27: 0000000000000000
[   11.337797] x26: 0000000000000000 x25: 0000000000000000 x24: ffff0008001606f0
[   11.337816] x23: ffff000800160000 x22: ffff000800160700 x21: ffff000800160000
[   11.337837] x20: ffff800012f4beb0 x19: 0000000000000008 x18: 000000000000c9a0
[   11.337857] x17: 00000000ae3495d5 x16: 000000000000c9a0 x15: ffff80001240e128
[   11.337878] x14: ffff8000124b0128 x13: 000000000000000a x12: ffff80001205e5f0
[   11.337898] x11: 0000000000000000 x10: ffff800011a37d28 x9 : 0000000000000050
[   11.337917] x8 : ffff000800160000 x7 : 0000000000000002 x6 : 0000000000000000
[   11.337937] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
[   11.337956] x2 : 0000000000000008 x1 : 0000000000000000 x0 : 0000000000000000
[   11.337975] Call trace:
[   11.337980] task_fpsimd_load (arch/arm64/kernel/fpsimd.c:296 (discriminator 1))
[   11.337996] fpsimd_restore_current_state (arch/arm64/kernel/fpsimd.c:1186)
[   11.338012] do_notify_resume (./arch/arm64/include/asm/daifflags.h:28 arch/arm64/kernel/signal.c:947)
[   11.338032] work_pending (arch/arm64/kernel/entry.S:839)
[   11.338045] irq event stamp: 1228377
[   11.338051] hardirqs last enabled at (1228375): _raw_spin_unlock_irqrestore (./include/linux/spinlock_api_smp.h:160 kernel/locking/spinlock.c:194)
[   11.338076] hardirqs last disabled at (1228377): el1_dbg (arch/arm64/kernel/entry-common.c:144 arch/arm64/kernel/entry-common.c:234)
[   11.338098] softirqs last enabled at (1227024): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:262)
[   11.338121] softirqs last disabled at (1228376): fpsimd_restore_current_state (./include/linux/bottom_half.h:19 arch/arm64/kernel/fpsimd.c:183 arch/arm64/kernel/fpsimd.c:1182)

This is caused by local_bh_disable() not disabling preemption under
CONFIG_PREEMPT_RT, which fails have_cpu_fpsimd_context(). The per-CPU
access safety proved by the CONFIG_PREEMPT_RT version of local_bh_disable()
is not sufficient here, as we end up with both preemption and IRQs enabled
when running do_notify_resume(). This means we can hit:

  el0_sync
  `\
    do_notify_resume()
    `\
      fpsimd_restore_current_state()

  ~~> el1_irq
      `\
	preempt_schedule_irq()
	`\
	  fpsimd_thread_switch()

IOW we *really* need to disable preemption here, even on CONFIG_PREEMPT_RT.

The preempt_{enable, disable}_bh() helpers exist to handle this exact case,
so use them here. This is spiritually close to:

  cba08c5dc6dc ("x86/fpu: Make kernel FPU protection RT friendly")

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 arch/arm64/kernel/fpsimd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 9bf86cd7b605..0c66ea0dd97e 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -177,10 +177,12 @@ static void __get_cpu_fpsimd_context(void)
  *
  * The double-underscore version must only be called if you know the task
  * can't be preempted.
+ *
+ * Disabling preemption prevents nesting via fpsimd_thread_switch().
  */
 static void get_cpu_fpsimd_context(void)
 {
-	local_bh_disable();
+	preempt_disable_bh();
 	__get_cpu_fpsimd_context();
 }
 
@@ -201,7 +203,7 @@ static void __put_cpu_fpsimd_context(void)
 static void put_cpu_fpsimd_context(void)
 {
 	__put_cpu_fpsimd_context();
-	local_bh_enable();
+	preempt_enable_bh();
 }
 
 static bool have_cpu_fpsimd_context(void)
-- 
2.25.1