On Sat, Sep 30, 2017 at 07:41:56AM +0800, Boqun Feng wrote: > On Fri, Sep 29, 2017 at 04:43:39PM +0000, Paul E. McKenney wrote: > > On Fri, Sep 29, 2017 at 04:53:57PM +0200, Paolo Bonzini wrote: > > > On 29/09/2017 13:01, Boqun Feng wrote: > > > > 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 > > > > ... > > > > > > > > And this happened when we hit a page fault in an RCU read-side critical > > > > section and then we tried to reschedule in kvm_async_pf_task_wait(), > > > > this reschedule would hit the WARN in rcu_preempt_note_context_switch(), > > > > and be treated as a sleep in RCU read-side critical section, which is > > > > not allowed(even in preemptible RCU). > > > > > > Just a small fix to the commit message: > > > > > > 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. > > > > > > Queued with that change, thanks. > > > > Not to be repetitive, but if the schedule() is on the guest, this change > > really does silently break up an RCU read-side critical section on > > guests built with PREEMPT=n. (Yes, they were already being broken, > > but it would be good to avoid this breakage in PREEMPT=n as well as > > in PREEMPT=y.) > > > > Then probably adding !IS_ENABLED(CONFIG_PREEMPT) as one of the reason we > choose the halt path? Like: > > n.halted = is_idle_task(current) || preempt_count() > 1 || > !IS_ENABLED(CONFIG_PREEMPT) || rcu_preempt_depth(); > > > But I think async PF could also happen while a user program is running? > Then maybe add a second parameter @user for kvm_async_pf_task_wait(), > like: > > kvm_async_pf_task_wait((u32)read_cr2(), user_mode(regs)); > > and the halt condition becomes: > > n.halted = is_idle_task(current) || preempt_count() > 1 || > (!IS_ENABLED(CONFIG_PREEMPT) && !user) || rcu_preempt_depth(); > > Thoughts? This looks to me like it would cover it. If !PREEMPT interrupt from kernel, we halt, which would prevent the sleep. I take it that we get unhalted when the host gets things patched up? > A side thing is being broken already for PREEMPT=n means we maybe fail > to detect this in rcutorture? Then should we add a config with > KVM_GUEST=y and try to run some memory consuming things(e.g. stress > --vm) in the rcutorture kvm script simultaneously? Paolo, do you have > any test workload that could trigger async PF quickly? I do not believe that have seen this in rcutorture, but I always run in a guest OS on a large-memory system (well, by my old-fashioned standards, anyway) that would be quite unlikely to evict a guest OS's pages. Plus I tend to run on shared systems, and deliberately running them out of memory would not be particularly friendly to others using those systems. I -do- run background scripts that are intended to force the host OS to preempt the guest OSes frequently, but I don't believe that this would cause that bug. But it seems like it would make more sense to add this sort of thing to whatever KVM tests there are for host-side eviction of guest pages. Thanx, Paul
