Subject: Re: [syzbot] [kvm?] [net?] [virt?] BUG: sleeping function called from invalid context in vhost_get_avail_idx

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 21 Apr 2026 16:54:22 -0400

On Wed, Apr 22, 2026 at 02:11:01AM +0900, Kohei Enju wrote:
> On 04/20 15:09, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    8541d8f725c6 Merge tag 'mtd/for-7.1' of git://git.kernel.o..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=136454ce580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=7e54da1916e8d11f
> > dashboard link: https://syzkaller.appspot.com/bug?extid=6985cb8e543ea90ba8ee
> > compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15d264ce580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=143ec1ba580000
> > 
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-8541d8f7.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/22dfea2c37c2/vmlinux-8541d8f7.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/e2f93ad68fe3/bzImage-8541d8f7.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: [email-masked]
> > 
> > BUG: sleeping function called from invalid context at drivers/vhost/vhost.c:1527
> > in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6110, name: vhost-6109
> > preempt_count: 1, expected: 0
> > RCU nest depth: 0, expected: 0
> > 2 locks held by vhost-6109/6110:
> >  #0: ffff888055624cb0 (&vq->mutex/1){+.+.}-{4:4}, at: handle_tx+0x2d/0x160 drivers/vhost/net.c:971
> >  #1: ffff888055620248 (&vq->mutex){+.+.}-{4:4}, at: vhost_net_busy_poll+0x9c/0x730 drivers/vhost/net.c:554
> > Preemption disabled at:
> > [<ffffffff88f1a006>] vhost_net_busy_poll+0x1c6/0x730 drivers/vhost/net.c:563
> 
> I think the blamed commit may be commit 030881372460 ("vhost_net: basic
> polling support"), since it introduced preempt_{disable,enable}() around
> the busy-poll loop, which calls a sleepable function inside the loop.
> 
> Also, from the changelog of the series,
> 
> https://lore.kernel.org/netdev/[email-masked]/T/#u
> 
>   Changes from RFC V1:
>   ...
>   - Disable preemption during busy looping to make sure local_clock() was
>     correctly used.
> 
> So my understanding is that preempt_disable() was introduced to keep
> local_clock() based timeout accounting on a single CPU, rather than as a
> requirement of busy polling itself.
> 
> If my understanding is correct, migrate_disable() is sufficient here
> instead of preempt_disable(), avoiding sleepable accesses from a
> preempt-disabled context.
> 
> #syz test
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 80965181920c..c6536cad9c4f 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -560,7 +560,7 @@ static void vhost_net_busy_poll(struct vhost_net *net,
>         busyloop_timeout = poll_rx ? rvq->busyloop_timeout:
>                                      tvq->busyloop_timeout;
> 
> -       preempt_disable();
> +       migrate_disable();
>         endtime = busy_clock() + busyloop_timeout;
> 
>         while (vhost_can_busy_poll(endtime)) {
> @@ -577,7 +577,7 @@ static void vhost_net_busy_poll(struct vhost_net *net,
>                 cpu_relax();
>         }
> 
> -       preempt_enable();
> +       migrate_enable();
> 
>         if (poll_rx || sock_has_rx_data(sock))
>                 vhost_net_busy_poll_try_queue(net, vq);

Makes sense but this stipped up the bot. Try again?

> 
> > CPU: 0 UID: 0 PID: 6110 Comm: vhost-6109 Not tainted syzkaller #0 PREEMPT(full) 
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> > Call Trace:
> >  <TASK>
> >  __dump_stack lib/dump_stack.c:94 [inline]
> >  dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
> >  __might_resched.cold+0x1ec/0x232 kernel/sched/core.c:9162
> >  __might_fault+0x8b/0x140 mm/memory.c:7322
> >  vhost_get_avail_idx+0x31c/0x4f0 drivers/vhost/vhost.c:1527
> >  vhost_vq_avail_empty drivers/vhost/vhost.c:3206 [inline]
> >  vhost_vq_avail_empty+0xa9/0xe0 drivers/vhost/vhost.c:3199
> >  vhost_net_busy_poll+0x297/0x730 drivers/vhost/net.c:574
> >  vhost_net_tx_get_vq_desc drivers/vhost/net.c:610 [inline]
> >  get_tx_bufs.constprop.0+0x338/0x600 drivers/vhost/net.c:650
> >  handle_tx_copy+0x28c/0x12e0 drivers/vhost/net.c:778
> >  handle_tx+0x139/0x160 drivers/vhost/net.c:985
> >  vhost_run_work_list+0x183/0x220 drivers/vhost/vhost.c:454
> >  vhost_task_fn+0x156/0x430 kernel/vhost_task.c:49
> >  ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
> >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> >  </TASK>
> > 
> > 
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at [email-masked].
> > 
> > syzbot will keep track of this issue. See:
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> > 
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> > 
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> > 
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> > 
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> > 
> > If you want to undo deduplication, reply with:
> > #syz undup