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LoongArch: Prevent cond_resched() occurring within kernel-fpu

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commit 2468b0e3d5 upstream.

When CONFIG_PREEMPT_COUNT is not configured (i.e. CONFIG_PREEMPT_NONE/
CONFIG_PREEMPT_VOLUNTARY), preempt_disable() / preempt_enable() merely
acts as a barrier(). However, in these cases cond_resched() can still
trigger a context switch and modify the CSR.EUEN, resulting in do_fpu()
exception being activated within the kernel-fpu critical sections, 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 commit d021985504 (x86/fpu: Improve crypto
performance by making kernel-mode FPU reliably usable in softirqs).  It
uses local_bh_disable() instead of preempt_disable() for non-RT kernels
so it can avoid the cond_resched() issue, and also extend the kernel-fpu
application scenarios to the softirq context.

Cc: stable@vger.kernel.org
Signed-off-by: Tianyang Zhang <zhangtianyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Tianyang Zhang
2025-05-14 22:17:43 +08:00
committed by Greg Kroah-Hartman
parent f8544be7e8
commit 111e55db3c
1 changed files with 20 additions and 2 deletions
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arch/loongarch/kernel/kfpu.c
+20 -2
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@@ -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 fpregs_lock(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_disable();
else
local_bh_disable();
}
static inline void fpregs_unlock(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_enable();
else
local_bh_enable();
}
void kernel_fpu_begin(void)
{
unsigned int *euen_curr;
preempt_disable();
if (!irqs_disabled())
fpregs_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())
fpregs_unlock();
}
EXPORT_SYMBOL_GPL(kernel_fpu_end);