Messages in this thread | | | Date | Wed, 9 Dec 2020 11:11:02 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware |
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On Fri, Dec 04, 2020 at 06:01:55PM +0100, Thomas Gleixner wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > Provide a local lock based serialization for soft interrupts on RT which > allows the local_bh_disabled() sections and servicing soft interrupts to be > preemptible. > > Provide the necessary inline helpers which allow to reuse the bulk of the > softirq processing code.
> +struct softirq_ctrl { > + local_lock_t lock; > + int cnt; > +}; > + > +static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = { > + .lock = INIT_LOCAL_LOCK(softirq_ctrl.lock), > +}; > + > +void __local_bh_disable_ip(unsigned long ip, unsigned int cnt) > +{ > + unsigned long flags; > + int newcnt; > + > + WARN_ON_ONCE(in_hardirq()); > + > + /* First entry of a task into a BH disabled section? */ > + if (!current->softirq_disable_cnt) { > + if (preemptible()) { > + local_lock(&softirq_ctrl.lock);
AFAICT this significantly changes the locking rules.
Where previously we could do:
spin_lock(&ponies) spin_lock_bh(&foo);
vs
spin_lock_bh(&bar); spin_lock(&ponies)
provided the rest of the code observed: bar -> ponies -> foo and never takes ponies from in-softirq.
This is now a genuine deadlock on RT.
Also see:
https://lkml.kernel.org/r/X9CheYjuXWc75Spa@hirez.programming.kicks-ass.net
> + /* Required to meet the RCU bottomhalf requirements. */ > + rcu_read_lock(); > + } else { > + DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt)); > + } > + } > + > + /* > + * Track the per CPU softirq disabled state. On RT this is per CPU > + * state to allow preemption of bottom half disabled sections. > + */ > + newcnt = __this_cpu_add_return(softirq_ctrl.cnt, cnt); > + /* > + * Reflect the result in the task state to prevent recursion on the > + * local lock and to make softirq_count() & al work. > + */ > + current->softirq_disable_cnt = newcnt; > + > + if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && newcnt == cnt) { > + raw_local_irq_save(flags); > + lockdep_softirqs_off(ip); > + raw_local_irq_restore(flags); > + } > +}
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