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[PATCH v4 1/1] MIPS: Fix idle VS timer enqueue

Marco Crivellari posted 1 patch 1 week, 6 days ago
There is a newer version of this series
[PATCH v4 1/1] MIPS: Fix idle VS timer enqueue
Posted by Marco Crivellari 1 week, 6 days ago 
MIPS re-enables interrupts on its idle routine and performs
a TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.

The IRQs firing between the check and the 'wait' instruction may set the
TIF_NEED_RESCHED flag. In order to deal with this possible race, IRQs
interrupting __r4k_wait() rollback their return address to the
beginning of __r4k_wait() so that TIF_NEED_RESCHED is checked
again before going back to sleep.

However idle IRQs can also queue timers that may require a tick
reprogramming through a new generic idle loop iteration but those timers
would go unnoticed here because __r4k_wait() only checks
TIF_NEED_RESCHED. It doesn't check for pending timers.

Fix this with fast-forwarding idle IRQs return address to the end of the
idle routine instead of the beginning, so that the generic idle loop
handles both TIF_NEED_RESCHED and pending timers.

CONFIG_CPU_MICROMIPS has been removed along with the nop instructions.
There, NOPs are 2 byte in size, so change the code with 3 _ssnop which are
always 4 byte and remove the ifdef. Added ehb to make sure the hazard
is always cleared.

Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 arch/mips/kernel/genex.S | 42 ++++++++++++++++++++++------------------
 arch/mips/kernel/idle.c  |  1 -
 2 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index a572ce36a24f..5e333cd27fc7 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -104,27 +104,30 @@ handle_vcei:
 
 	__FINIT
 
-	.align	5	/* 32 byte rollback region */
+	.align	5
 LEAF(__r4k_wait)
 	.set	push
 	.set	noreorder
-	/* start of rollback region */
-	LONG_L	t0, TI_FLAGS($28)
-	nop
-	andi	t0, _TIF_NEED_RESCHED
-	bnez	t0, 1f
-	 nop
-	nop
-	nop
-#ifdef CONFIG_CPU_MICROMIPS
-	nop
-	nop
-	nop
-	nop
-#endif
+	/* start of idle interrupt region */
+	MFC0	t0, CP0_STATUS
+	/* Enable Interrput */
+	ori 	t0, 0x1f
+	xori	t0, 0x1e
+	MTC0	t0, CP0_STATUS
+	_ssnop
+	_ssnop
+	_ssnop
+	_ehb
 	.set	MIPS_ISA_ARCH_LEVEL_RAW
+	/*
+	 * If an interrupt lands here, between enabling interrupts above and
+	 * going idle on the next instruction, we must *NOT* go idle since the
+	 * interrupt could have set TIF_NEED_RESCHED or caused a timer to need
+	 * resched. Fall through -- see rollback_handler below -- and have
+	 * the idle loop take care of things.
+	 */
 	wait
-	/* end of rollback region (the region size must be power of two) */
+	/* end of rollback region */
 1:
 	jr	ra
 	 nop
@@ -136,9 +139,10 @@ LEAF(__r4k_wait)
 	.set	push
 	.set	noat
 	MFC0	k0, CP0_EPC
-	PTR_LA	k1, __r4k_wait
-	ori	k0, 0x1f	/* 32 byte rollback region */
-	xori	k0, 0x1f
+	PTR_LA	k1, 1b
+	/* 36 byte idle interrupt region */
+	ori 	k0, 0x1f
+	PTR_ADDIU	k0, 5
 	bne	k0, k1, \handler
 	MTC0	k0, CP0_EPC
 	.set pop
diff --git a/arch/mips/kernel/idle.c b/arch/mips/kernel/idle.c
index 5abc8b7340f8..1f74c0589f7e 100644
--- a/arch/mips/kernel/idle.c
+++ b/arch/mips/kernel/idle.c
@@ -37,7 +37,6 @@ static void __cpuidle r3081_wait(void)
 
 void __cpuidle r4k_wait(void)
 {
-	raw_local_irq_enable();
 	__r4k_wait();
 	raw_local_irq_disable();
 }
-- 
2.48.1
Re: [PATCH v4 1/1] MIPS: Fix idle VS timer enqueue
Posted by Huacai Chen 1 week, 6 days ago 
On Thu, Feb 27, 2025 at 9:45 PM Marco Crivellari
<marco.crivellari@suse.com> wrote:
>
> MIPS re-enables interrupts on its idle routine and performs
> a TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.
>
> The IRQs firing between the check and the 'wait' instruction may set the
> TIF_NEED_RESCHED flag. In order to deal with this possible race, IRQs
> interrupting __r4k_wait() rollback their return address to the
> beginning of __r4k_wait() so that TIF_NEED_RESCHED is checked
> again before going back to sleep.
>
> However idle IRQs can also queue timers that may require a tick
> reprogramming through a new generic idle loop iteration but those timers
> would go unnoticed here because __r4k_wait() only checks
> TIF_NEED_RESCHED. It doesn't check for pending timers.
>
> Fix this with fast-forwarding idle IRQs return address to the end of the
> idle routine instead of the beginning, so that the generic idle loop
> handles both TIF_NEED_RESCHED and pending timers.
>
> CONFIG_CPU_MICROMIPS has been removed along with the nop instructions.
> There, NOPs are 2 byte in size, so change the code with 3 _ssnop which are
> always 4 byte and remove the ifdef. Added ehb to make sure the hazard
> is always cleared.
>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>  arch/mips/kernel/genex.S | 42 ++++++++++++++++++++++------------------
>  arch/mips/kernel/idle.c  |  1 -
>  2 files changed, 23 insertions(+), 20 deletions(-)
>
> diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
> index a572ce36a24f..5e333cd27fc7 100644
> --- a/arch/mips/kernel/genex.S
> +++ b/arch/mips/kernel/genex.S
> @@ -104,27 +104,30 @@ handle_vcei:
>
>         __FINIT
>
> -       .align  5       /* 32 byte rollback region */
> +       .align  5
>  LEAF(__r4k_wait)
>         .set    push
>         .set    noreorder
> -       /* start of rollback region */
> -       LONG_L  t0, TI_FLAGS($28)
> -       nop
> -       andi    t0, _TIF_NEED_RESCHED
> -       bnez    t0, 1f
> -        nop
> -       nop
> -       nop
> -#ifdef CONFIG_CPU_MICROMIPS
> -       nop
> -       nop
> -       nop
> -       nop
> -#endif
> +       /* start of idle interrupt region */
> +       MFC0    t0, CP0_STATUS
> +       /* Enable Interrput */
> +       ori     t0, 0x1f
> +       xori    t0, 0x1e
> +       MTC0    t0, CP0_STATUS
> +       _ssnop
> +       _ssnop
> +       _ssnop
> +       _ehb
>         .set    MIPS_ISA_ARCH_LEVEL_RAW
> +       /*
> +        * If an interrupt lands here, between enabling interrupts above and
> +        * going idle on the next instruction, we must *NOT* go idle since the
> +        * interrupt could have set TIF_NEED_RESCHED or caused a timer to need
> +        * resched. Fall through -- see rollback_handler below -- and have
> +        * the idle loop take care of things.
> +        */
>         wait
> -       /* end of rollback region (the region size must be power of two) */
> +       /* end of rollback region */
Oh, no, still something wrong.... "rollback region" here should be
"idle interrupt region".
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>

Huacai

>  1:
>         jr      ra
>          nop
> @@ -136,9 +139,10 @@ LEAF(__r4k_wait)
>         .set    push
>         .set    noat
>         MFC0    k0, CP0_EPC
> -       PTR_LA  k1, __r4k_wait
> -       ori     k0, 0x1f        /* 32 byte rollback region */
> -       xori    k0, 0x1f
> +       PTR_LA  k1, 1b
> +       /* 36 byte idle interrupt region */
> +       ori     k0, 0x1f
> +       PTR_ADDIU       k0, 5
>         bne     k0, k1, \handler
>         MTC0    k0, CP0_EPC
>         .set pop
> diff --git a/arch/mips/kernel/idle.c b/arch/mips/kernel/idle.c
> index 5abc8b7340f8..1f74c0589f7e 100644
> --- a/arch/mips/kernel/idle.c
> +++ b/arch/mips/kernel/idle.c
> @@ -37,7 +37,6 @@ static void __cpuidle r3081_wait(void)
>
>  void __cpuidle r4k_wait(void)
>  {
> -       raw_local_irq_enable();
>         __r4k_wait();
>         raw_local_irq_disable();
>  }
> --
> 2.48.1
>

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