Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...)

Paul E. McKenney
2020-11-24T15:02:09Z

On Tue, Nov 24, 2020 at 03:03:10PM +0100, Marco Elver wrote:
> On Mon, Nov 23, 2020 at 07:32PM +0000, Mark Rutland wrote:
> > On Fri, Nov 20, 2020 at 03:03:32PM +0100, Marco Elver wrote:
> > > On Fri, Nov 20, 2020 at 10:30AM +0000, Mark Rutland wrote:
> > > > On Thu, Nov 19, 2020 at 10:53:53PM +0000, Will Deacon wrote:
> > > > > FWIW, arm64 is known broken wrt lockdep and irq tracing atm. Mark has been
> > > > > looking at that and I think he is close to having something workable.
> > > > >
> > > > > Mark -- is there anything Marco and Paul can try out?
> > > >
> > > > I initially traced some issues back to commit:
> > > >
> > > > 044d0d6de9f50192 ("lockdep: Only trace IRQ edges")
> > > >
> > > > ... and that change of semantic could cause us to miss edges in some
> > > > cases, but IIUC mostly where we haven't done the right thing in
> > > > exception entry/return.
> > > >
> > > > I don't think my patches address this case yet, but my WIP (currently
> > > > just fixing user<->kernel transitions) is at:
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/irq-fixes
> > > >
> > > > I'm looking into the kernel<->kernel transitions now, and I know that we
> > > > mess up RCU management for a small window around arch_cpu_idle, but it's
> > > > not immediately clear to me if either of those cases could cause this
> > > > report.
> > >
> > > Thank you -- I tried your irq-fixes, however that didn't seem to fix the
> > > problem (still get warnings and then a panic). :-/
> >
> > 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)! I noticed there are a bunch of warnings in the log
> that might be relevant (see attached).
>
> Note, I also reverted
>
>   sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled
>
> and that still works.

This is expected behavior given that there were no RCU CPU stall
warnings. As to the warnings...

[ . . . ]

> [ 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
> [ 91.295524] WARNING: CPU: 0 PID: 0 at kernel/locking/lockdep.c:5279 check_flags.part.0+0x1d4/0x1f8
> [ 91.296302] Modules linked in:
> [ 91.297644] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.10.0-rc4-next-20201119-00002-g51c2bf0ac853 #25
> [ 91.298317] Hardware name: linux,dummy-virt (DT)
> [ 91.298975] pstate: 80000085 (Nzcv daIf -PAN -UAO -TCO BTYPE=--)
> [ 91.299648] pc : check_flags.part.0+0x1d4/0x1f8
> [ 91.300303] lr : check_flags.part.0+0x1d4/0x1f8
> [ 91.300960] sp : ffffdb60f8d73a50
> [ 91.301628] x29: ffffdb60f8d73a50 x28: ffffdb60f8d84000
> [ 91.303527] x27: ffffdb60f869cbb8 x26: ffffdb60f6835930
> [ 91.305431] x25: 0000000000000000 x24: 0000000000000000
> [ 91.307343] x23: ffffdb60f8daf360 x22: 0000000000000001
> [ 91.309242] x21: 0000000000000000 x20: 0000000000000001
> [ 91.311145] x19: ffffdb60f9bcf000 x18: 00000000749f6e65
> [ 91.313044] x17: 00000000dcd3f761 x16: 0000000000000005
> [ 91.314954] x15: 0000000000000000 x14: 0000000000000028
> [ 91.316854] x13: 000000000000067a x12: 0000000000000028
> [ 91.318753] x11: 0101010101010101 x10: ffffdb60f8d73820
> [ 91.320657] x9 : ffffdb60f6960ff8 x8 : 4e5241575f534b43
> [ 91.322582] x7 : 4f4c5f4755424544 x6 : ffff4454fdbd3667
> [ 91.324486] x5 : 00000000ffffffc8 x4 : ffff4454fdbd2c60
> [ 91.326413] x3 : ffffdb60f6800000 x2 : ffffdb60f7c60000
> [ 91.328308] x1 : 1c0af7741e0f0c00 x0 : 0000000000000000
> [ 91.330227] Call trace:
> [ 91.330880] check_flags.part.0+0x1d4/0x1f8
> [ 91.331547] lock_acquire+0x208/0x508
> [ 91.332200] _raw_spin_lock+0x5c/0x80
> [ 91.332849] vprintk_emit+0xb4/0x380
> [ 91.333528] vprintk_default+0x4c/0x60
> [ 91.334189] vprintk_func+0x120/0x330
> [ 91.334863] printk+0x78/0x9c
> [ 91.335523] lockdep_rcu_suspicious+0x2c/0xf8
> [ 91.336195] trace_hardirqs_off+0x214/0x330
> [ 91.336854] trace_graph_return+0x1ac/0x1d8
> [ 91.337518] ftrace_return_to_handler+0xa4/0x170
> [ 91.338190] return_to_handler+0x1c/0x38

This looks like tracing in the idle loop in a place where RCU is not
watching. Historically, this has been addressed by using _rcuidle()
trace events, but the portion of the idle loop that RCU is watching has
recently increased. Last I checked, there were still a few holdouts (that
would splat like this) in x86, though perhaps those have since been fixed.

> [ 91.338841] default_idle_call+0x94/0x38c
> [ 91.339512] do_idle+0x240/0x290
> [ 91.340166] rest_init+0x1e8/0x2dc
> [ 91.340840] arch_call_rest_init+0x1c/0x28
> [ 91.341499] start_kernel+0x638/0x670
> [ 91.342147] irq event stamp: 1727
> [ 91.342832] hardirqs last enabled at (1727): [<ffffdb60f7c33094>] exit_el1_irq_or_nmi+0x24/0x50
> [ 91.343502] hardirqs last disabled at (1724): [<ffffdb60f7c33060>] enter_el1_irq_or_nmi+0x20/0x30
> [ 91.344193] softirqs last enabled at (1726): [<ffffdb60f6835930>] return_to_handler+0x0/0x38
> [ 91.344866] softirqs last disabled at (1725): [<ffffdb60f68c6880>] irq_enter_rcu+0x88/0xa8
> [ 91.345546] ---[ end trace e131d25144579308 ]---

The other warning looked similar.

Thanx, Paul