linux-kernel - [PATCH] Loongarch:Prevent cond_resched occurring within kernel-fpu

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Message-Id: <20250507100624.8925-1-zhangtianyang@loongson.cn>
Date: Wed,  7 May 2025 18:06:24 +0800
From: Tianyang Zhang <zhangtianyang@...ngson.cn>
To: chenhuacai@...nel.org,
	kernel@...0n.name,
	bigeasy@...utronix.de,
	clrkwllms@...nel.org,
	rostedt@...dmis.org
Cc: loongarch@...ts.linux.dev,
	linux-rt-devel@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Tianyang Zhang <zhangtianyang@...ngson.cn>
Subject: [PATCH] Loongarch:Prevent cond_resched occurring within kernel-fpu

When CONFIG_PREEMPT_COUNT is not configured, preempt_disable/preempt_enable
merely acts as a barrier. However, cond_resched can still trigger a
context switch and modify the EUEN, resulting in do_fpu being activated
within kernel-fpu critical section, as demonstrated in the following path:

dcn32_calculate_wm_and_dlg
    DC_FP_START
	dcn32_calculate_wm_and_dlg_fpu
	    dcn32_find_dummy_latency_index_for_fw_based_mclk_switch
		dcn32_internal_validate_bw
		    dcn32_enable_phantom_stream
			dc_create_stream_for_sink
			   kzalloc(GFP_KERNEL)
				__kmem_cache_alloc_node
				    __cond_resched
    DC_FP_END

This patch is similar to d02198550423a0b695e7a24ec77153209ad45b09
(x86/fpu: Improve crypto performance by making kernel-mode FPU reliably
usablein softirqs),to avoids the issue, and extends kernel-fpu application
scenarios to softirq.

Signed-off-by: Tianyang Zhang <zhangtianyang@...ngson.cn>
---
 arch/loongarch/kernel/kfpu.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/loongarch/kernel/kfpu.c b/arch/loongarch/kernel/kfpu.c
index ec5b28e570c9..e60bbaca357a 100644
--- a/arch/loongarch/kernel/kfpu.c
+++ b/arch/loongarch/kernel/kfpu.c
@@ -18,11 +18,28 @@ static unsigned int euen_mask = CSR_EUEN_FPEN;
 static DEFINE_PER_CPU(bool, in_kernel_fpu);
 static DEFINE_PER_CPU(unsigned int, euen_current);
 
+static inline void fpu_lock(void)
+{
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_disable();
+	else
+		preempt_disable();
+}
+
+static inline void fpu_unlock(void)
+{
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_bh_enable();
+	else
+		preempt_enable();
+}
+
 void kernel_fpu_begin(void)
 {
 	unsigned int *euen_curr;
 
-	preempt_disable();
+	if (!irqs_disabled())
+		fpu_lock();
 
 	WARN_ON(this_cpu_read(in_kernel_fpu));
 
@@ -73,7 +90,8 @@ void kernel_fpu_end(void)
 
 	this_cpu_write(in_kernel_fpu, false);
 
-	preempt_enable();
+	if (!irqs_disabled())
+		fpu_unlock();
 }
 EXPORT_SYMBOL_GPL(kernel_fpu_end);
 
-- 
2.20.1