[PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

[PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

[Peng Liu]

handle_nested_irq() is supposed to be running inside the parent thread
handler context. It per se has no dedicated kernel thread, thus shouldn't
touch desc->threads_active. The parent kernel thread has already taken
care of this.

Fixes: e2c12739ccf7 ("genirq: Prevent nested thread vs synchronize_hardirq() deadlock")
Cc: stable@vger.kernel.org
Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
---

Despite of its correctness, I'm afraid the testing on my only PC can't
cover the affected code path. So the patch may be totally -UNTESTED-.

 kernel/irq/chip.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
index dc94e0bf2c94..85d4f29134e9 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -478,7 +478,6 @@ void handle_nested_irq(unsigned int irq)
 	}
 
 	kstat_incr_irqs_this_cpu(desc);
-	atomic_inc(&desc->threads_active);
 	raw_spin_unlock_irq(&desc->lock);
 
 	action_ret = IRQ_NONE;
@@ -487,8 +486,6 @@ void handle_nested_irq(unsigned int irq)
 
 	if (!irq_settings_no_debug(desc))
 		note_interrupt(desc, action_ret);
-
-	wake_threads_waitq(desc);
 }
 EXPORT_SYMBOL_GPL(handle_nested_irq);
 
-- 
2.39.2

Re: [PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

[Thomas Gleixner]

On Mon, Jun 10 2024 at 02:30, Peng Liu wrote:
> handle_nested_irq() is supposed to be running inside the parent thread
> handler context. It per se has no dedicated kernel thread, thus shouldn't
> touch desc->threads_active. The parent kernel thread has already taken
> care of this.

No it has not. The parent thread has marked itself in the parent threads
interrupt descriptor.

How does that help synchronizing the nested interrupt, which has a
separate interrupt descriptor?

> Fixes: e2c12739ccf7 ("genirq: Prevent nested thread vs synchronize_hardirq() deadlock")
> Cc: stable@vger.kernel.org

There is nothing to fix.

> Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
> ---
>
> Despite of its correctness, I'm afraid the testing on my only PC can't
> cover the affected code path. So the patch may be totally -UNTESTED-.

Which correctness?

The change log of the commit you want to "fix" says:

    Remove the incorrect usage in the nested threaded interrupt case and
    instead re-use the threads_active / wait_for_threads mechanism to
    wait for nested threaded interrupts to complete.

It's very clearly spelled out, no?

Thanks,

        tglx

Re: [PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

[Peng Liu]

On Mon, Jun 10, 2024 at 08:23:09PM +0200, Thomas Gleixner wrote:
> On Mon, Jun 10 2024 at 02:30, Peng Liu wrote:
> > handle_nested_irq() is supposed to be running inside the parent thread
> > handler context. It per se has no dedicated kernel thread, thus shouldn't
> > touch desc->threads_active. The parent kernel thread has already taken
> > care of this.
> 
> No it has not. The parent thread has marked itself in the parent threads
> interrupt descriptor.
> 
> How does that help synchronizing the nested interrupt, which has a
> separate interrupt descriptor?

Right, I never thought there would be more than one interrupt
descriptors involved which is quite common.

> 
> > Fixes: e2c12739ccf7 ("genirq: Prevent nested thread vs synchronize_hardirq() deadlock")
> > Cc: stable@vger.kernel.org
> 
> There is nothing to fix.
> 
> > Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
> > ---
> >
> > Despite of its correctness, I'm afraid the testing on my only PC can't
> > cover the affected code path. So the patch may be totally -UNTESTED-.
> 
> Which correctness?
> 
> The change log of the commit you want to "fix" says:
> 
>     Remove the incorrect usage in the nested threaded interrupt case and
>     instead re-use the threads_active / wait_for_threads mechanism to
>     wait for nested threaded interrupts to complete.
> 
> It's very clearly spelled out, no?

Indeed, due to my ignorance, I never thought there might be more
descriptors involved. Now think about it, I never really understood
the meaning of the above change log.

Thanks for your time and concise explanation.

Peng

>
> Thanks,
> 
>         tglx