On Mon, Nov 28, 2016 at 04:40:21PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 28, 2016 at 03:54:11PM -0600, Josh Poimboeuf wrote:
> > On Thu, Nov 24, 2016 at 12:33:48PM -0500, Vince Weaver wrote:
> > >
> > > This is on a skylake machine, linus git as of yesterday after the various
> > > kasan-related fixes went in. Not sure if there were any that hadn't hit
> > > upstream yet.
> > >
> > > Anyway I can't tell from this one what the actual trigger is. After this
> > > mess the fuzzer process was locked, udev started complaining, and it
> > > eventually died completely after a few hours of repeated messages like
> > > this.
> > >
> > > [38898.373183] INFO: rcu_sched self-detected stall on CPU
> > > [38898.378452] 7-...: (5249 ticks this GP) idle=727/140000000000001/0
> > > softirq=3141908/3141908 fqs=2625
> > > [38898.381211] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > [38898.381214] 0-...: (1 GPs behind) idle=05f/140000000000001/2
> > > softirq=3285458/3285459 fqs=2625
> > > [38898.381217] 7-...: (5249 ticks this GP) idle=727/140000000000001/0
> > > softirq=3141908/3141908 fqs=2625
> > > [38898.381218] (detected by 1, t=5252 jiffies, g=3685053, c=3685052,
> > > q=32)
> > > [38898.381244]
> > > ==================================================================
> > > [38898.381247] BUG: KASAN: stack-out-of-bounds in
> > > __unwind_start+0x1a2/0x1c0 at addr ffff8801e9727c28
> > > [38898.381248] Read of size 8 by task swapper/1/0
> > > [38898.381250] page:ffffea0007a5c9c0 count:0 mapcount:0 mapping:
> > > (null) index:0x0
> > > [38898.381251] flags: 0x2ffff8000000000()
> > > [38898.381251] page dumped because: kasan: bad access detected
> > > [38898.381328] Memory state around the buggy address:
> > > [38898.381330] ffff8801e9727b00: 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 00 00 00
> > > [38898.381331] ffff8801e9727b80: 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 00 00 00
> > > [38898.381332] >ffff8801e9727c00: 00 00 00 00 f1 f1 f1 f1 00 00 00 00 f3
> > > f3 f3 f3
> > > [38898.381333] ^
> > > [38898.381334] ffff8801e9727c80: 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > 00 00 f1
> > > [38898.381335] ffff8801e9727d00: f1 f1 f1 00 00 00 f4 f2 f2 f2 f2 00 00
> > > 00 00 f3
> > > [38898.381335]
> > > ==================================================================
> > > [38898.510702] (t=5284 jiffies g=3685053 c=3685052 q=32)
> > >
> > > (That's all, the report above repeats but no useful things like a
> > > backtrace are ever printed)
> >
> > After looking at the RCU stall detection code, I think the KASAN error
> > and missing stack dump aren't very surprising. RCU calls the scheduler
> > dump_cpu_task() function, which seems inherently problematic: it tries
> > to dump the stack of a task while it's running on another CPU.
> >
> > There are some issues with that:
> >
> > 1) There's no way to find the starting frame of a currently running task
> > from another CPU.
> >
> > In fact, I'm wondering how dump_cpu_task() ever worked at all? It
> > seems like you'd have to get lucky that the sp/bp registers stored by
> > the last call to schedule() happen to point to a currently valid
> > stack frame.
> >
> > 2) Even if there were a way to find the starting frame, it's racy
> > because the target task could be overwriting the stack while we're
> > reading it.
> >
> > 3) IRQ/exception stack dumps would be missing anyway because the stack
> > dump code only looks at the current CPU's interrupt stacks.
> >
> > Maybe dump_cpu_task() should instead run the stack dump directly from
> > the target CPU, e.g. with trigger_single_cpu_backtrace() or
> > smp_call_function_single()?
> >
> > Paul, Peter, Ingo, any thoughts?
>
> We used to do that, but the resulting NMIs were problematic on some
> platforms. Perhaps things have gotten better?
Did a little digging on git blame and found the following commit (which
seems to be the cause of the KASAN warning and missing stack dump):
bc1dce514e9b ("rcu: Don't use NMIs to dump other CPUs' stacks")
I presume this commit is still needed because of the NMI printk deadlock
issues which were discussed at Kernel Summit. I guess those issues need
to be sorted out before the above commit can be reverted.
--
Josh
