On 2024-11-26 14:49:40 [+0000], David Woodhouse wrote: > On Tue, 2024-11-26 at 06:24 -0800, syzbot wrote: > > syzbot has bisected this issue to: > > > > commit 560af5dc839eef08a273908f390cfefefb82aa04 > > Author: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > > Date: Wed Oct 9 15:45:03 2024 +0000 > > > > lockdep: Enable PROVE_RAW_LOCK_NESTING with PROVE_LOCKING. > > That's not it; this has always been broken with PREEMPT_RT I think. > There was an attempt to fix it in > https://lore.kernel.org/all/20240227115648.3104-8-dwmw2@xxxxxxxxxxxxx/ > > I'll dust that off and try again. Oh thank you. The timer has been made to always expire in hardirq due to HRTIMER_MODE_ABS_HARD, this is why you see the splat. If the hardirq invocation is needed/ possible then the callback needs to be updated. The linked patch has this hunk: |- read_lock_irqsave(&gpc->lock, flags); |+ local_irq_save(flags); |+ if (!read_trylock(&gpc->lock)) { … |+ if (in_interrupt()) |+ goto out; |+ |+ read_lock(&gpc->lock); This does not work. If interrupts are disabled (due to local_irq_save()) then read_lock() must not be used. in_interrupt() does not matter. Side note: Using HRTIMER_MODE_ABS would avoid the splat at the cost that on PREEMPT_RT the timer will be invoked in softirq context (as with HRTIMER_MODE_ABS_SOFT on !PREEMPT_RT). There is no changed behaviour on !PREEMPT_RT. Sebastian
