On 2021-03-17 15:38:52 [+0100], Thomas Gleixner wrote:
> 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);
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.
> 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.
Acked-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Fixes: 8d32a307e4fa ("genirq: Provide forced interrupt threading")
> Reported-by: Johan Hovold <jo...@kernel.org>
> Signed-off-by: Thomas Gleixner <t...@linutronix.de>
> Cc: Eric Dumazet <eduma...@google.com>
> Cc: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Cc: netdev <netdev@vger.kernel.org>
> Cc: "David S. Miller" <da...@davemloft.net>
> Cc: Krzysztof Kozlowski <krzysztof.kozlow...@canonical.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Andy Shevchenko <andy.shevche...@gmail.com>
> CC: Peter Zijlstra <pet...@infradead.org>
> Cc: linux-ser...@vger.kernel.org
> Cc: netdev <netdev@vger.kernel.org>
> ---
> kernel/irq/manage.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> --- 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;
> }
Sebastian
