Re: [PATCH] KVM: async_pf: avoid recursive flushing of work items

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-11-17 15:55+0100, Paolo Bonzini:
> This was reported by syzkaller:
> 
>     [ INFO: possible recursive locking detected ]
>     4.9.0-rc4+ #49 Not tainted
>     ---------------------------------------------
>     kworker/2:1/5658 is trying to acquire lock:
>      ([ 1644.769018] (&work->work)
>     [<     inline     >] list_empty include/linux/compiler.h:243
>     [<ffffffff8128dd60>] flush_work+0x0/0x660 kernel/workqueue.c:1511
> 
>     but task is already holding lock:
>      ([ 1644.769018] (&work->work)
>     [<ffffffff812916ab>] process_one_work+0x94b/0x1900 kernel/workqueue.c:2093
> 
>     stack backtrace:
>     CPU: 2 PID: 5658 Comm: kworker/2:1 Not tainted 4.9.0-rc4+ #49
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>     Workqueue: events async_pf_execute
>      ffff8800676ff630 ffffffff81c2e46b ffffffff8485b930 ffff88006b1fc480
>      0000000000000000 ffffffff8485b930 ffff8800676ff7e0 ffffffff81339b27
>      ffff8800676ff7e8 0000000000000046 ffff88006b1fcce8 ffff88006b1fccf0
>     Call Trace:
>     ...
>     [<ffffffff8128ddf3>] flush_work+0x93/0x660 kernel/workqueue.c:2846
>     [<ffffffff812954ea>] __cancel_work_timer+0x17a/0x410 kernel/workqueue.c:2916
>     [<ffffffff81295797>] cancel_work_sync+0x17/0x20 kernel/workqueue.c:2951
>     [<ffffffff81073037>] kvm_clear_async_pf_completion_queue+0xd7/0x400 virt/kvm/async_pf.c:126
>     [<     inline     >] kvm_free_vcpus arch/x86/kvm/x86.c:7841
>     [<ffffffff810b728d>] kvm_arch_destroy_vm+0x23d/0x620 arch/x86/kvm/x86.c:7946
>     [<     inline     >] kvm_destroy_vm virt/kvm/kvm_main.c:731
>     [<ffffffff8105914e>] kvm_put_kvm+0x40e/0x790 virt/kvm/kvm_main.c:752
>     [<ffffffff81072b3d>] async_pf_execute+0x23d/0x4f0 virt/kvm/async_pf.c:111
>     [<ffffffff8129175c>] process_one_work+0x9fc/0x1900 kernel/workqueue.c:2096
>     [<ffffffff8129274f>] worker_thread+0xef/0x1480 kernel/workqueue.c:2230
>     [<ffffffff812a5a94>] kthread+0x244/0x2d0 kernel/kthread.c:209
>     [<ffffffff831f102a>] ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:433
> 
> The reason is that kvm_put_kvm is causing the destruction of the VM, but
> the page fault is still on the ->queue list.  The ->queue list is owned
> by the VCPU, not by the work items, so we cannot just add list_del to
> the work item.
> 
> Instead, use work->vcpu to note async page faults that have been resolved
> and will be processed through the done list.  There is no need to flush
> those.
> 
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Applied to kvm/master, thanks.