Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

[Paul E. McKenney]

On Thu, Sep 28, 2017 at 03:38:16PM +0000, Levin, Alexander (Sasha Levin) wrote:
> On Thu, Sep 28, 2017 at 05:30:55AM -0700, Paul E. McKenney wrote:
> >On Thu, Sep 28, 2017 at 02:37:20AM -0700, Sasha Levin wrote:
> >> On Wed, Apr 19, 2017 at 9:58 AM, Paul E. McKenney
> >> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >> > Currently, a call to schedule() acts as a Tasks RCU quiescent state
> >> > only if a context switch actually takes place.  However, just the
> >> > call to schedule() guarantees that the calling task has moved off of
> >> > whatever tracing trampoline that it might have been one previously.
> >> > This commit therefore plumbs schedule()'s "preempt" parameter into
> >> > rcu_note_context_switch(), which then records the Tasks RCU quiescent
> >> > state, but only if this call to schedule() was -not- due to a preemption.
> >> >
> >> > To avoid adding overhead to the common-case context-switch path,
> >> > this commit hides the rcu_note_context_switch() check under an existing
> >> > non-common-case check.
> >> >
> >> > Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> >> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >>
> >> Hey Paul,
> >>
> >> I'm seeing the following on the latest -next kernel, and suspect it's
> >> caused bit this patch:
> >
> >Hmmm...  kernel/rcu/tree_plugin.h:329 thinks that someone slept (as opposed
> >to was preempted) in an RCU read-side critical section.
> >
> >If this is reproducible, could you please enable lockdep if you are not
> >already doing so?
> 
> lockdep was on, as far as I can tell.
> 
> It happened once in ~3 weeks of fuzzing, so not sure how easily I can reproduce.

Oh, wait...  Looking at the stack trace:

> Call Trace:
>  __dump_stack lib/dump_stack.c:16 [inline]
>  dump_stack+0x11d/0x1e5 lib/dump_stack.c:52
>  panic+0x1bc/0x3b7 kernel/panic.c:181
>  __warn+0x1c4/0x1d9 kernel/panic.c:542
>  report_bug+0x211/0x2d0 lib/bug.c:183
>  fixup_bug+0x3f/0x90 arch/x86/kernel/traps.c:178
>  do_trap_no_signal arch/x86/kernel/traps.c:212 [inline]
>  do_trap+0x260/0x390 arch/x86/kernel/traps.c:261
>  do_error_trap+0x133/0x380 arch/x86/kernel/traps.c:298
>  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:311
>  invalid_op+0x18/0x20 arch/x86/entry/entry_64.S:905
> RIP: 0010:rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
> RIP: 0010:rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
> RSP: 0018:ffff88003b2debc8 EFLAGS: 00010002
> RAX: 0000000000000001 RBX: 1ffff1000765bd85 RCX: 0000000000000000
> RDX: 1ffff100075d7882 RSI: ffffffffb5c7da20 RDI: ffff88003aebc410
> RBP: ffff88003b2def30 R08: dffffc0000000000 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff88003b2def08
> R13: 0000000000000000 R14: ffff88003aebc040 R15: ffff88003aebc040

Up to here is all about the kernel complaining.

>  __schedule+0x201/0x2240 kernel/sched/core.c:3292
>  schedule+0x113/0x460 kernel/sched/core.c:3421
>  kvm_async_pf_task_wait+0x43f/0x940 arch/x86/kernel/kvm.c:158

It is kvm_async_pf_task_wait() that calls schedule(), but it carefully
sets state to make that legal.  Except...

>  do_async_page_fault+0x72/0x90 arch/x86/kernel/kvm.c:271
>  async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069
> RIP: 0010:format_decode+0x240/0x830 lib/vsprintf.c:1996
> RSP: 0018:ffff88003b2df520 EFLAGS: 00010283
> RAX: 000000000000003f RBX: ffffffffb5d1e141 RCX: ffff88003b2df670
> RDX: 0000000000000001 RSI: dffffc0000000000 RDI: ffffffffb5d1e140
> RBP: ffff88003b2df560 R08: dffffc0000000000 R09: 0000000000000000
> R10: ffff88003b2df718 R11: 0000000000000000 R12: ffff88003b2df5d8
> R13: 0000000000000064 R14: ffffffffb5d1e140 R15: 0000000000000000
>  vsnprintf+0x173/0x1700 lib/vsprintf.c:2136

We took a page fault in vsnprintf() while doing link_path_walk(),
which looks to be within an RCU read-side critical section.

Maybe the page fault confused lockdep?

Sigh.  It is going to be a real pain if all printk()s need to be
outside of RCU read-side critical sections due to the possibility of
page faults...

							Thanx, Paul

>  sprintf+0xbe/0xf0 lib/vsprintf.c:2386
>  proc_self_get_link+0xfb/0x1c0 fs/proc/self.c:23
>  get_link fs/namei.c:1047 [inline]
>  link_path_walk+0x1041/0x1490 fs/namei.c:2127
>  path_openat+0x2d0/0x3600 fs/namei.c:3527
>  do_filp_open+0x263/0x3c0 fs/namei.c:3562
>  do_sys_open+0x515/0x6f0 fs/open.c:1059
>  SYSC_openat fs/open.c:1086 [inline]
>  SyS_openat+0x30/0x40 fs/open.c:1080
>  do_syscall_64+0x26a/0x800 arch/x86/entry/common.c:287
>  entry_SYSCALL64_slow_path+0x25/0x25
> RIP: 0033:0x4889da
> RSP: 002b:000000c4261eb8f0 EFLAGS: 00000206 ORIG_RAX: 0000000000000101
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004889da
> RDX: 0000000000000002 RSI: 000000c42338d2c0 RDI: ffffffffffffff9c
> RBP: 000000c4261eb980 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
> R13: 00000000ffffffee R14: 00000000000012c0 R15: 0000000000000100
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: 0x30000000 from 0xffffffff81000000 (relocation range:
> 0xffffffff80000000-0xffffffffbfffffff)
> Rebooting in 86400 seconds..

So kvm_async_pf_task_wait() does a schedule(), and it carefully