3.16.51-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Boqun Feng <boqun.feng@xxxxxxxxx> commit b862789aa5186d5ea3a024b7cfe0f80c3a38b980 upstream. Sasha Levin reported a WARNING: | 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 ... | 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: ... | 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 ... This happened when the host hit a page fault, and delivered it as in an async page fault, while the guest was in an RCU read-side critical section. The guest then tries to reschedule in kvm_async_pf_task_wait(), but rcu_preempt_note_context_switch() would treat the reschedule as a sleep in RCU read-side critical section, which is not allowed (even in preemptible RCU). Thus the WARN. To cure this, make kvm_async_pf_task_wait() go to the halt path if the PF happens in a RCU read-side critical section. Reported-by: Sasha Levin <levinsasha928@xxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- arch/x86/kernel/kvm.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -139,7 +139,8 @@ void kvm_async_pf_task_wait(u32 token) n.token = token; n.cpu = smp_processor_id(); - n.halted = is_idle_task(current) || preempt_count() > 1; + n.halted = is_idle_task(current) || preempt_count() > 1 || + rcu_preempt_depth(); init_waitqueue_head(&n.wq); hlist_add_head(&n.link, &b->list); spin_unlock(&b->lock);