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Message-ID: <YkOCqJ4WDObmaAcn@codewreck.org> Date: Wed, 30 Mar 2022 07:05:28 +0900 From: asmadeus@...ewreck.org To: syzbot <syzbot+bde0f89deacca7c765b8@...kaller.appspotmail.com> Cc: davem@...emloft.net, ericvh@...il.com, kuba@...nel.org, linux-kernel@...r.kernel.org, linux_oss@...debyte.com, lucho@...kov.net, netdev@...r.kernel.org, pabeni@...hat.com, syzkaller-bugs@...glegroups.com, v9fs-developer@...ts.sourceforge.net Subject: Re: [syzbot] possible deadlock in p9_write_work syzbot wrote on Tue, Mar 29, 2022 at 02:23:17PM -0700: > ====================================================== > WARNING: possible circular locking dependency detected > 5.17.0-next-20220328-syzkaller #0 Not tainted > ------------------------------------------------------ > kworker/1:1/26 is trying to acquire lock: > ffff88807eece460 (sb_writers#3){.+.+}-{0:0}, at: p9_fd_write net/9p/trans_fd.c:428 [inline] > ffff88807eece460 (sb_writers#3){.+.+}-{0:0}, at: p9_write_work+0x25e/0xca0 net/9p/trans_fd.c:479 > > but task is already holding lock: > ffffc90000a1fda8 ((work_completion)(&m->wq)){+.+.}-{0:0}, at: process_one_work+0x8ae/0x1610 kernel/workqueue.c:2264 > > which lock already depends on the new lock. > > > the existing dependency chain (in reverse order) is: > > -> #3 ((work_completion)(&m->wq)){+.+.}-{0:0}: > process_one_work+0x905/0x1610 kernel/workqueue.c:2265 > worker_thread+0x665/0x1080 kernel/workqueue.c:2436 > kthread+0x2e9/0x3a0 kernel/kthread.c:376 > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298 > > -> #2 ((wq_completion)events){+.+.}-{0:0}: > flush_workqueue+0x164/0x1440 kernel/workqueue.c:2831 > flush_scheduled_work include/linux/workqueue.h:583 [inline] > ext4_put_super+0x99/0x1150 fs/ext4/super.c:1202 > generic_shutdown_super+0x14c/0x400 fs/super.c:462 > kill_block_super+0x97/0xf0 fs/super.c:1394 > deactivate_locked_super+0x94/0x160 fs/super.c:332 > deactivate_super+0xad/0xd0 fs/super.c:363 > cleanup_mnt+0x3a2/0x540 fs/namespace.c:1186 > task_work_run+0xdd/0x1a0 kernel/task_work.c:164 > resume_user_mode_work include/linux/resume_user_mode.h:49 [inline] > exit_to_user_mode_loop kernel/entry/common.c:183 [inline] > exit_to_user_mode_prepare+0x23c/0x250 kernel/entry/common.c:215 > __syscall_exit_to_user_mode_work kernel/entry/common.c:297 [inline] > syscall_exit_to_user_mode+0x19/0x60 kernel/entry/common.c:308 > do_syscall_64+0x42/0x80 arch/x86/entry/common.c:86 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > -> #1 (&type->s_umount_key#32){++++}-{3:3}: > down_read+0x98/0x440 kernel/locking/rwsem.c:1461 > iterate_supers+0xdb/0x290 fs/super.c:692 > drop_caches_sysctl_handler+0xdb/0x110 fs/drop_caches.c:62 > proc_sys_call_handler+0x4a1/0x6e0 fs/proc/proc_sysctl.c:604 > call_write_iter include/linux/fs.h:2080 [inline] > do_iter_readv_writev+0x3d1/0x640 fs/read_write.c:726 > do_iter_write+0x182/0x700 fs/read_write.c:852 > vfs_iter_write+0x70/0xa0 fs/read_write.c:893 > iter_file_splice_write+0x723/0xc70 fs/splice.c:689 > do_splice_from fs/splice.c:767 [inline] > direct_splice_actor+0x110/0x180 fs/splice.c:936 > splice_direct_to_actor+0x34b/0x8c0 fs/splice.c:891 > do_splice_direct+0x1a7/0x270 fs/splice.c:979 > do_sendfile+0xae0/0x1240 fs/read_write.c:1246 > __do_sys_sendfile64 fs/read_write.c:1305 [inline] > __se_sys_sendfile64 fs/read_write.c:1297 [inline] > __x64_sys_sendfile64+0x149/0x210 fs/read_write.c:1297 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x35/0x80 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > -> #0 (sb_writers#3){.+.+}-{0:0}: > check_prev_add kernel/locking/lockdep.c:3096 [inline] > check_prevs_add kernel/locking/lockdep.c:3219 [inline] > validate_chain kernel/locking/lockdep.c:3834 [inline] > __lock_acquire+0x2ac6/0x56c0 kernel/locking/lockdep.c:5060 > lock_acquire kernel/locking/lockdep.c:5672 [inline] > lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5637 > percpu_down_read include/linux/percpu-rwsem.h:51 [inline] > __sb_start_write include/linux/fs.h:1728 [inline] > sb_start_write include/linux/fs.h:1798 [inline] > file_start_write include/linux/fs.h:2815 [inline] > kernel_write fs/read_write.c:564 [inline] > kernel_write+0x2ac/0x540 fs/read_write.c:555 > p9_fd_write net/9p/trans_fd.c:428 [inline] > p9_write_work+0x25e/0xca0 net/9p/trans_fd.c:479 > process_one_work+0x996/0x1610 kernel/workqueue.c:2289 > worker_thread+0x665/0x1080 kernel/workqueue.c:2436 > kthread+0x2e9/0x3a0 kernel/kthread.c:376 > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298 So p9_write_work cannot write because there's.. a backing ext4 umount (I assume it's been mounted with trans fd with an ext4 file) and a drop_caches stuck in parallel, and we just got caught in the crossfire ? I'm not sure why it got stuck there but that doesn't look like anything we can do about it, using trans fd with filesystem backed files isn't a usage we care about in the first place, maybe there's a way to refuse these and only keep sockets but I don't really see the point of artificially limiting the interface (unless using a 9p mount with a file could have security implications I don't see) wontfix/dontcare for me, -- Dominique
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