[v6, 02/10] riscv: vector: make Vector always available for softirq context

[Andy Chiu]

By disabling bottom halves in active kerne-mode Vector, softirq will not
be able to nest on top of any kernel-mode Vector.

After this patch, Vector context cannot start with irqs disabled.
Otherwise local_bh_enable() may run in a wrong context.

Disabling bh is not enough for RT-kernel to prevent preeemption. So
we must disable preemption, which also implies disabling bh on RT.

Related-to: commit 696207d4258b ("arm64/sve: Make kernel FPU protection RT friendly")
Related-to: commit 66c3ec5a7120 ("arm64: neon: Forbid when irqs are disabled")
Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
---
Changelog v4:
 - new patch since v4
---
 arch/riscv/include/asm/simd.h          |  6 +++++-
 arch/riscv/kernel/kernel_mode_vector.c | 10 ++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/simd.h b/arch/riscv/include/asm/simd.h
index 269752bfa2cc..cd6180fe37c0 100644
--- a/arch/riscv/include/asm/simd.h
+++ b/arch/riscv/include/asm/simd.h
@@ -26,8 +26,12 @@ static __must_check inline bool may_use_simd(void)
 	/*
 	 * RISCV_KERNEL_MODE_V is only set while preemption is disabled,
 	 * and is clear whenever preemption is enabled.
+	 *
+	 * Kernel-mode Vector temperarily disables bh. So we must not return
+	 * true on irq_disabled(). Otherwise we would fail the lockdep check
+	 * calling local_bh_enable()
 	 */
-	return !in_hardirq() && !in_nmi() && !(riscv_v_ctx_cnt() & RISCV_KERNEL_MODE_V_MASK);
+	return !in_hardirq() && !in_nmi() && !irqs_disabled() && !(riscv_v_ctx_cnt() & RISCV_KERNEL_MODE_V_MASK);
 }
 
 #else /* ! CONFIG_RISCV_ISA_V */
diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
index 105147c7d2da..db0cf06f2abf 100644
--- a/arch/riscv/kernel/kernel_mode_vector.c
+++ b/arch/riscv/kernel/kernel_mode_vector.c
@@ -23,7 +23,10 @@
  */
 void get_cpu_vector_context(void)
 {
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_disable();
+	else
+		preempt_disable();
 
 	WARN_ON((riscv_v_ctx_cnt() & RISCV_KERNEL_MODE_V_MASK) != 0);
 	riscv_v_ctx_cnt_add(RISCV_KERNEL_MODE_V);
@@ -41,7 +44,10 @@ void put_cpu_vector_context(void)
 	WARN_ON((riscv_v_ctx_cnt() & RISCV_KERNEL_MODE_V_MASK) != RISCV_KERNEL_MODE_V);
 	riscv_v_ctx_cnt_sub(RISCV_KERNEL_MODE_V);
 
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_enable();
+	else
+		preempt_enable();
 }
 
 /*
-- 
2.17.1