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[80.230.25.21]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-488fb7bf696sm122094795e9.32.2026.04.21.13.54.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Apr 2026 13:54:24 -0700 (PDT) Date: Tue, 21 Apr 2026 16:54:22 -0400 From: "Michael S. Tsirkin" To: Kohei Enju Cc: syzbot , jasowang@redhat.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com Subject: Re: [syzbot] [kvm?] [net?] [virt?] BUG: sleeping function called from invalid context in vhost_get_avail_idx Message-ID: <20260421165358-mutt-send-email-mst@kernel.org> References: <69e6a414.050a0220.24bfd3.002d.GAE@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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: syzbot+6985cb8e543ea90ba8ee@syzkaller.appspotmail.com > > > > 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: > > [] 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/1448435489-5949-4-git-send-email-jasowang@redhat.com/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: > > > > __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 > > > > > > > > --- > > 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 syzkaller@googlegroups.com. > > > > syzbot will keep track of this issue. See: > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. > > > > 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