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[1/1] genirq: Disable interrupts for force threaded handlers

Message ID 20210317143859.513307808@linutronix.de
State New
Headers show
Series [1/1] genirq: Disable interrupts for force threaded handlers | expand

Commit Message

Thomas Gleixner March 17, 2021, 2:38 p.m. UTC
With interrupt force threading all device interrupt handlers are invoked
from kernel threads. Contrary to hard interrupt context the invocation only
disables bottom halfs, but not interrupts. This was an oversight back then
because any code like this will have an issue:

thread(irq_A)
  irq_handler(A)
    spin_lock(&foo->lock);

interrupt(irq_B)
  irq_handler(B)
    spin_lock(&foo->lock);

This has been triggered with networking (NAPI vs. hrtimers) and console
drivers where printk() happens from an interrupt which interrupted the
force threaded handler.

Now people noticed and started to change the spin_lock() in the handler to
spin_lock_irqsave() which affects performance or add IRQF_NOTHREAD to the
interrupt request which in turn breaks RT.

Fix the root cause and not the symptom and disable interrupts before
invoking the force threaded handler which preserves the regular semantics
and the usefulness of the interrupt force threading as a general debugging
tool.

For not RT this is not changing much, except that during the execution of
the threaded handler interrupts are delayed until the handler
returns. Vs. scheduling and softirq processing there is no difference.

For RT kernels there is no issue.

Fixes: 8d32a307e4fa ("genirq: Provide forced interrupt threading")
Reported-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: netdev <netdev@vger.kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
CC: Peter Zijlstra <peterz@infradead.org>
Cc: linux-serial@vger.kernel.org
Cc: netdev <netdev@vger.kernel.org>
---
 kernel/irq/manage.c |    4 ++++
 1 file changed, 4 insertions(+)

Comments

Thomas Gleixner March 17, 2021, 4:09 p.m. UTC | #1
On Wed, Mar 17 2021 at 15:48, Sebastian Andrzej Siewior wrote:
> On 2021-03-17 15:38:52 [+0100], Thomas Gleixner wrote:
>> thread(irq_A)
>>   irq_handler(A)
>>     spin_lock(&foo->lock);
>> 
>> interrupt(irq_B)
>>   irq_handler(B)
>>     spin_lock(&foo->lock);
>
> It will not because both threads will wake_up(thread). It is an issue if
> - if &foo->lock is shared between a hrtimer and threaded-IRQ
> - if &foo->lock is shared between a non-threaded and thread-IRQ
> - if &foo->lock is shared between a printk() in hardirq context and
>   thread-IRQ as I learned today.

That's the point and it's entirely clear from the above: A is thread
context and B is hard interrupt context and if the lock is shared then
it's busted. Otherwise we would not have this discussion at all.
Sebastian Andrzej Siewior March 18, 2021, 8:51 a.m. UTC | #2
On 2021-03-17 17:23:39 [+0100], Johan Hovold wrote:
> > > thread(irq_A)
> > >   irq_handler(A)
> > >     spin_lock(&foo->lock);
> > > 
> > > interrupt(irq_B)
> > >   irq_handler(B)
> > >     spin_lock(&foo->lock);
> > 
> > It will not because both threads will wake_up(thread).
> 
> Note that the above says "interrupt(irq_B)" suggesting it's a
> non-threaded interrupt unlike irq_A.

I missed that bit, thanks.

Sebastian
diff mbox series

Patch

--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -1142,11 +1142,15 @@  irq_forced_thread_fn(struct irq_desc *de
 	irqreturn_t ret;
 
 	local_bh_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_irq_disable();
 	ret = action->thread_fn(action->irq, action->dev_id);
 	if (ret == IRQ_HANDLED)
 		atomic_inc(&desc->threads_handled);
 
 	irq_finalize_oneshot(desc, action);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		local_irq_enable();
 	local_bh_enable();
 	return ret;
 }