On Tue, Nov 24, 2020 at 07:30PM +0000, Mark Rutland wrote:
[...]
> > > I've just updated that branch with a new version which I hope covers
> > > kernel<->kernel transitions too. If you get a chance, would you mind
> > > giving that a spin?
> > > 
> > > The HEAD commit should be:
> > > 
> > >   a51334f033f8ee88 ("HACK: check IRQ tracing has RCU watching")
> > 
> > Thank you! Your series appears to work and fixes the stalls and
> > deadlocks (3 trials)! 
> 
> Thanks for testing! I'm glad that appears to work, as it suggests
> there's not another massive problem lurking in this area.
> 
> While cleaning/splitting that up today, I spotted a couple of new
> problems I introduced, and I'm part-way through sorting that out, but
> it's not quite ready today after all. :/
> 
> Fingers crossed for tomorrow...
> 
> > I noticed there are a bunch of warnings in the log
> > that might be relevant (see attached).
> 
> > [   91.184432] =============================
> > [   91.188301] WARNING: suspicious RCU usage
> > [   91.192316] 5.10.0-rc4-next-20201119-00002-g51c2bf0ac853 #25 Tainted: G        W        
> > [   91.197536] -----------------------------
> > [   91.201431] kernel/trace/trace_preemptirq.c:78 RCU not watching trace_hardirqs_off()!
> > [   91.206546] 
> > [   91.206546] other info that might help us debug this:
> > [   91.206546] 
> > [   91.211790] 
> > [   91.211790] rcu_scheduler_active = 2, debug_locks = 0
> > [   91.216454] RCU used illegally from extended quiescent state!
> > [   91.220890] no locks held by swapper/0/0.
> > [   91.224712] 
> > [   91.224712] stack backtrace:
> > [   91.228794] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W         5.10.0-rc4-next-20201119-00002-g51c2bf0ac853 #25
> > [   91.234877] Hardware name: linux,dummy-virt (DT)
> > [   91.239032] Call trace:
> > [   91.242587]  dump_backtrace+0x0/0x240
> > [   91.246500]  show_stack+0x34/0x88
> > [   91.250295]  dump_stack+0x140/0x1bc
> > [   91.254159]  lockdep_rcu_suspicious+0xe4/0xf8
> > [   91.258332]  trace_hardirqs_off+0x214/0x330
> > [   91.262462]  trace_graph_return+0x1ac/0x1d8
> > [   91.266564]  ftrace_return_to_handler+0xa4/0x170
> > [   91.270809]  return_to_handler+0x1c/0x38
> > [   91.274826]  default_idle_call+0x94/0x38c
> > [   91.278869]  do_idle+0x240/0x290
> > [   91.282633]  rest_init+0x1e8/0x2dc
> > [   91.286529]  arch_call_rest_init+0x1c/0x28
> > [   91.290585]  start_kernel+0x638/0x670
> 
> Hmm... I suspect that arch_cpu_idle() is being traced here, and I reckon
> we have to mark that and its callees as noinstr, since it doesn't seem
> sane to have ftrace check whether RCU is watching for every function
> call. Maybe Paul or Steve can correct me. ;)

Yes, it's arch_cpu_idle().

> If you still have the binary lying around, can you check whether
> default_idle_call+0x94/0x38c is just after the call to arch_cpu_idle()?
> If you could dump the asm around that, along with whatever faddr2line
> tells you, that'd be a great help. 

I reran to be sure, with similar results. I've attached a
syz-symbolize'd version of the warnings.

Thanks,
-- Marco