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

[Sasha Levin]

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:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 6974 Comm: syz-fuzzer Not tainted 4.13.0-next-20170908+ #246
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.10.1-1ubuntu1 04/01/2014
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
 __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
 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
 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..