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Re: [vfio-users] [PATCH] vgaarb: fix signal handling in vga_get()



Hi

On Mon, Dec 14, 2015 at 9:19 AM, Kirill A. Shutemov
<kirill shutemov name> wrote:
> On Thu, Dec 10, 2015 at 11:28:58AM +0100, David Herrmann wrote:
>> Hi
>>
>> On Mon, Nov 30, 2015 at 3:17 AM, Kirill A. Shutemov
>> <kirill shutemov name> wrote:
>> > There are few defects in vga_get() related to signal hadning:
>> >
>> >   - we shouldn't check for pending signals for TASK_UNINTERRUPTIBLE
>> >     case;
>> >
>> >   - if we found pending signal we must remove ourself from wait queue
>> >     and change task state back to running;
>> >
>> >   - -ERESTARTSYS is more appropriate, I guess.
>> >
>> > Signed-off-by: Kirill A. Shutemov <kirill shutemov name>
>> > ---
>> >
>> > Alex, I try to get KVM with VGA passthrough working properly. I have i915
>> > (HD 4600) on the host and GTX 580 for the guest. The guest GPU is not
>> > capabale of EFI, so I have to use x-vga=on. It's kinda work with your
>> > patch for i915.enable_hd_vgaarb=1. But guest refuse to initialize the GPU
>> > after KVM was not shut down correctly, resulting in host crash like this:
>> >
>> > BUG: unable to handle kernel paging request at ffff880870187ed8
>> > IP: [<ffff880870187ed8>] 0xffff880870187ed8
>> > PGD 2129067 PUD 80000008400001e3
>> > Oops: 0011 [#1] PREEMPT SMP
>> > Modules linked in: iwlmvm iwlwifi
>> > CPU: 6 PID: 3983 Comm: qemu-system-x86 Not tainted 4.3.0-gentoo #6
>> > Hardware name: Gigabyte Technology Co., Ltd. Z87X-UD7 TH/Z87X-UD7 TH-CF, BIOS F5a 06/12/2014
>> > task: ffff88087a910000 ti: ffff8808632c0000 task.ti: ffff8808632c0000
>> > RIP: 0010:[<ffff880870187ed8>]  [<ffff880870187ed8>] 0xffff880870187ed8
>> > RSP: 0018:ffff8808632c3d08  EFLAGS: 00010006
>> > RAX: ffff880870187db0 RBX: 0000000070187f58 RCX: 0000000000000000
>> > RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffff880870187db0
>> > RBP: ffff8808632c3d48 R08: 0000000000000000 R09: 0000000000000000
>> > R10: 00000000000103c0 R11: 0000000000000293 R12: ffffffff81ea03c8
>> > R13: ffffffff8104c7cb R14: 0000000000000000 R15: 0000000000000003
>> > FS:  00007f984f9b2700(0000) GS:ffff88089f380000(0000) knlGS:0000000000000000
>> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> > CR2: ffff880870187ed8 CR3: 00000008645f8000 CR4: 00000000001426e0
>> > Stack:
>> >  ffffffff810cc83d 00000000632c3d28 0000000000000000 ffffffff81ea03c0
>> >  0000000000000046 0000000000000003 0000000000000000 0000000000000000
>> >  ffff8808632c3d80 ffffffff810cca44 ffff88087af63800 0000000000000286
>> > Call Trace:
>> >  [<ffffffff810cc83d>] ? __wake_up_common+0x4d/0x80
>> >  [<ffffffff810cca44>] __wake_up+0x34/0x50
>> >  [<ffffffff815d99e3>] __vga_put+0x73/0xd0
>> >  [<ffffffff815d9db4>] vga_put+0x54/0x80
>> >  [<ffffffff8169d042>] vfio_pci_vga_rw+0x1d2/0x220
>> >  [<ffffffff8169a7f3>] vfio_pci_rw+0x33/0x60
>> >  [<ffffffff8169abf7>] vfio_pci_write+0x17/0x20
>> >  [<ffffffff816966a6>] vfio_device_fops_write+0x26/0x30
>> >  [<ffffffff811a4b23>] __vfs_write+0x23/0xe0
>> >  [<ffffffff811a4a53>] ? __vfs_read+0x23/0xd0
>> >  [<ffffffff811b6e35>] ? do_vfs_ioctl+0x2b5/0x490
>> >  [<ffffffff811a5194>] vfs_write+0xa4/0x190
>> >  [<ffffffff811a5fa6>] SyS_pwrite64+0x66/0xa0
>> >  [<ffffffff819a17d7>] entry_SYSCALL_64_fastpath+0x12/0x6a
>> > Code: 88 ff ff e0 7e 18 70 08 88 ff ff 00 8c 57 76 08 88 ff ff 20 7f 18 70 08 88 ff ff 08 7f 18 70 08 88 ff ff 94 51 1a 81 ff ff ff ff <09> 00 00 00 00 00 00 00 01 8c 57 76 08 88 ff ff 00 8c 57 76 08
>> > RIP  [<ffff880870187ed8>] 0xffff880870187ed8
>> >  RSP <ffff8808632c3d08>
>> > CR2: ffff880870187ed8
>> >
>> > The patch fixes the crash, but doesn't help with getting GPU in guest
>> > working again.
>> >
>> > Any ideas?
>> >
>> > ---
>> >  drivers/gpu/vga/vgaarb.c | 6 ++++--
>> >  1 file changed, 4 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/gpu/vga/vgaarb.c b/drivers/gpu/vga/vgaarb.c
>> > index 3166e4bc4eb6..9abcaa53bd25 100644
>> > --- a/drivers/gpu/vga/vgaarb.c
>> > +++ b/drivers/gpu/vga/vgaarb.c
>> > @@ -395,8 +395,10 @@ int vga_get(struct pci_dev *pdev, unsigned int rsrc, int interruptible)
>> >                 set_current_state(interruptible ?
>> >                                   TASK_INTERRUPTIBLE :
>> >                                   TASK_UNINTERRUPTIBLE);
>> > -               if (signal_pending(current)) {
>> > -                       rc = -EINTR;
>> > +               if (interruptible && signal_pending(current)) {
>> > +                       __set_current_state(TASK_RUNNING);
>> > +                       remove_wait_queue(&vga_wait_queue, &wait);
>> > +                       rc = -ERESTARTSYS;
>> >                         break;
>>
>> All 3 points are valid, and the patch looks good to me:
>>
>> Reviewed-by: David Herrmann <dh herrmann gmail com>
>>
>> However, there seems to be a race between vga_lock and putting the
>> thread asleep.
>
> I'm not sure I understand the race you're talking about.
> Could you elaborate?

Normal wake-up race. You have to check the condition you wait for
*after* linking your wait-queue and setting your thread-state. In the
current code, if the wake-up happens between spin_unlock_irqrestore()
and add_wait_queue(), you will never get woken up.

Thanks
David


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