Re: [patch 1/1] genirq: Disable interrupts for force threaded handlers
On 2021-03-17 17:23:39 [+0100], Johan Hovold wrote: > > > thread(irq_A) > > > irq_handler(A) > > > spin_lock(>lock); > > > > > > interrupt(irq_B) > > > irq_handler(B) > > > spin_lock(>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
Re: [patch 1/1] genirq: Disable interrupts for force threaded handlers
On Wed, Mar 17, 2021 at 03:48:06PM +0100, Sebastian Andrzej Siewior wrote: > 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(>lock); > > > > interrupt(irq_B) > > irq_handler(B) > > spin_lock(>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. > It is an issue if > - if >lock is shared between a hrtimer and threaded-IRQ > - if >lock is shared between a non-threaded and thread-IRQ So this is the above case. > - if >lock is shared between a printk() in hardirq context and > thread-IRQ as I learned today. But generally it's any lock taken by a threaded handler that can end up being taken in hard interrupt context. > > 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 Reviewed-by: Johan Hovold > > Fixes: 8d32a307e4fa ("genirq: Provide forced interrupt threading") > > Reported-by: Johan Hovold > > Signed-off-by: Thomas Gleixner > > Cc: Eric Dumazet > > Cc: Sebastian Andrzej Siewior > > Cc: netdev > > Cc: "David S. Miller" > > Cc: Krzysztof Kozlowski > > Cc: Greg Kroah-Hartman > > Cc: Andy Shevchenko > > CC: Peter Zijlstra > > Cc: linux-ser...@vger.kernel.org > > Cc: netdev > > --- > > 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(>threads_handled); > > > > irq_finalize_oneshot(desc, action); > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > > + local_irq_enable(); > > local_bh_enable(); > > return ret; > > } Johan
Re: [patch 1/1] genirq: Disable interrupts for force threaded handlers
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(>lock); >> >> interrupt(irq_B) >> irq_handler(B) >> spin_lock(>lock); > > It will not because both threads will wake_up(thread). It is an issue if > - if >lock is shared between a hrtimer and threaded-IRQ > - if >lock is shared between a non-threaded and thread-IRQ > - if >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.
Re: [patch 1/1] genirq: Disable interrupts for force threaded handlers
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(>lock); > > interrupt(irq_B) > irq_handler(B) > spin_lock(>lock); It will not because both threads will wake_up(thread). It is an issue if - if >lock is shared between a hrtimer and threaded-IRQ - if >lock is shared between a non-threaded and thread-IRQ - if >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 > Fixes: 8d32a307e4fa ("genirq: Provide forced interrupt threading") > Reported-by: Johan Hovold > Signed-off-by: Thomas Gleixner > Cc: Eric Dumazet > Cc: Sebastian Andrzej Siewior > Cc: netdev > Cc: "David S. Miller" > Cc: Krzysztof Kozlowski > Cc: Greg Kroah-Hartman > Cc: Andy Shevchenko > CC: Peter Zijlstra > Cc: linux-ser...@vger.kernel.org > Cc: netdev > --- > 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(>threads_handled); > > irq_finalize_oneshot(desc, action); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > + local_irq_enable(); > local_bh_enable(); > return ret; > } Sebastian
[patch 1/1] genirq: Disable interrupts for force threaded handlers
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(>lock); interrupt(irq_B) irq_handler(B) spin_lock(>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 Signed-off-by: Thomas Gleixner Cc: Eric Dumazet Cc: Sebastian Andrzej Siewior Cc: netdev Cc: "David S. Miller" Cc: Krzysztof Kozlowski Cc: Greg Kroah-Hartman Cc: Andy Shevchenko CC: Peter Zijlstra Cc: linux-ser...@vger.kernel.org Cc: netdev --- 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(>threads_handled); irq_finalize_oneshot(desc, action); + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) + local_irq_enable(); local_bh_enable(); return ret; }