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SubjectRe: possible deadlock in brd_probe
I think this is a false positive due to the cycling through different
bd_mutex instances. I think my next big series aimed at 5.11 will
fix this as a side effect, so I plan to ignore this for now, but will
test the reproducer with the follow on work applied eventually.

On Tue, Nov 17, 2020 at 02:30:20AM -0800, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 03430750 Add linux-next specific files for 20201116
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1642f486500000
> kernel config: https://syzkaller.appspot.com/x/.config?x=a1c4c3f27041fdb8
> dashboard link: https://syzkaller.appspot.com/bug?extid=fd01c5d29a476390728d
> compiler: gcc (GCC) 10.1.0-syz 20200507
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14646461500000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17dc946a500000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+fd01c5d29a476390728d@syzkaller.appspotmail.com
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.10.0-rc3-next-20201116-syzkaller #0 Not tainted
> ------------------------------------------------------
> syz-executor541/8491 is trying to acquire lock:
> ffffffff8bd213a8 (brd_devices_mutex){+.+.}-{3:3}, at: brd_probe+0x31/0x250 drivers/block/brd.c:434
>
> but task is already holding lock:
> ffffffff8bac3508 (major_names_lock){+.+.}-{3:3}, at: request_gendisk_module block/genhd.c:976 [inline]
> ffffffff8bac3508 (major_names_lock){+.+.}-{3:3}, at: get_gendisk+0x3c8/0x6a0 block/genhd.c:1029
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #3 (major_names_lock){+.+.}-{3:3}:
> __mutex_lock_common kernel/locking/mutex.c:956 [inline]
> __mutex_lock+0x134/0x1110 kernel/locking/mutex.c:1103
> __register_blkdev+0x2b/0x390 block/genhd.c:449
> register_mtd_blktrans+0x85/0x3c0 drivers/mtd/mtd_blkdevs.c:534
> do_one_initcall+0x103/0x650 init/main.c:1222
> do_initcall_level init/main.c:1295 [inline]
> do_initcalls init/main.c:1311 [inline]
> do_basic_setup init/main.c:1331 [inline]
> kernel_init_freeable+0x600/0x684 init/main.c:1531
> kernel_init+0xd/0x1b8 init/main.c:1420
> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
>
> -> #2 (mtd_table_mutex){+.+.}-{3:3}:
> __mutex_lock_common kernel/locking/mutex.c:956 [inline]
> __mutex_lock+0x134/0x1110 kernel/locking/mutex.c:1103
> blktrans_open+0x69/0x600 drivers/mtd/mtd_blkdevs.c:212
> __blkdev_get+0x80f/0x1870 fs/block_dev.c:1494
> blkdev_get+0xd1/0x240 fs/block_dev.c:1634
> blkdev_open+0x21d/0x2b0 fs/block_dev.c:1751
> do_dentry_open+0x4b9/0x11b0 fs/open.c:817
> do_open fs/namei.c:3252 [inline]
> path_openat+0x1b9a/0x2730 fs/namei.c:3369
> do_filp_open+0x17e/0x3c0 fs/namei.c:3396
> do_sys_openat2+0x16d/0x420 fs/open.c:1168
> do_sys_open fs/open.c:1184 [inline]
> __do_sys_open fs/open.c:1192 [inline]
> __se_sys_open fs/open.c:1188 [inline]
> __x64_sys_open+0x119/0x1c0 fs/open.c:1188
> do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> -> #1 (&bdev->bd_mutex){+.+.}-{3:3}:
> __mutex_lock_common kernel/locking/mutex.c:956 [inline]
> __mutex_lock+0x134/0x1110 kernel/locking/mutex.c:1103
> __blkdev_get+0x457/0x1870 fs/block_dev.c:1479
> blkdev_get+0xd1/0x240 fs/block_dev.c:1634
> blkdev_get_by_dev+0x3b/0x70 fs/block_dev.c:1715
> disk_scan_partitions block/genhd.c:677 [inline]
> register_disk block/genhd.c:727 [inline]
> __device_add_disk+0xe02/0x1250 block/genhd.c:814
> add_disk include/linux/genhd.h:295 [inline]
> brd_init+0x2af/0x475 drivers/block/brd.c:518
> do_one_initcall+0x103/0x650 init/main.c:1222
> do_initcall_level init/main.c:1295 [inline]
> do_initcalls init/main.c:1311 [inline]
> do_basic_setup init/main.c:1331 [inline]
> kernel_init_freeable+0x600/0x684 init/main.c:1531
> kernel_init+0xd/0x1b8 init/main.c:1420
> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
>
> -> #0 (brd_devices_mutex){+.+.}-{3:3}:
> check_prev_add kernel/locking/lockdep.c:2866 [inline]
> check_prevs_add kernel/locking/lockdep.c:2991 [inline]
> validate_chain kernel/locking/lockdep.c:3606 [inline]
> __lock_acquire+0x2ca6/0x5c00 kernel/locking/lockdep.c:4830
> lock_acquire kernel/locking/lockdep.c:5435 [inline]
> lock_acquire+0x2a3/0x8c0 kernel/locking/lockdep.c:5400
> __mutex_lock_common kernel/locking/mutex.c:956 [inline]
> __mutex_lock+0x134/0x1110 kernel/locking/mutex.c:1103
> brd_probe+0x31/0x250 drivers/block/brd.c:434
> request_gendisk_module block/genhd.c:979 [inline]
> get_gendisk+0x4c3/0x6a0 block/genhd.c:1029
> bdev_get_gendisk fs/block_dev.c:1094 [inline]
> __blkdev_get+0x344/0x1870 fs/block_dev.c:1455
> blkdev_get+0xd1/0x240 fs/block_dev.c:1634
> blkdev_open+0x21d/0x2b0 fs/block_dev.c:1751
> do_dentry_open+0x4b9/0x11b0 fs/open.c:817
> do_open fs/namei.c:3252 [inline]
> path_openat+0x1b9a/0x2730 fs/namei.c:3369
> do_filp_open+0x17e/0x3c0 fs/namei.c:3396
> do_sys_openat2+0x16d/0x420 fs/open.c:1168
> do_sys_open fs/open.c:1184 [inline]
> __do_sys_openat fs/open.c:1200 [inline]
> __se_sys_openat fs/open.c:1195 [inline]
> __x64_sys_openat+0x13f/0x1f0 fs/open.c:1195
> do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> other info that might help us debug this:
>
> Chain exists of:
> brd_devices_mutex --> mtd_table_mutex --> major_names_lock
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(major_names_lock);
> lock(mtd_table_mutex);
> lock(major_names_lock);
> lock(brd_devices_mutex);
>
> *** DEADLOCK ***
>
> 1 lock held by syz-executor541/8491:
> #0: ffffffff8bac3508 (major_names_lock){+.+.}-{3:3}, at: request_gendisk_module block/genhd.c:976 [inline]
> #0: ffffffff8bac3508 (major_names_lock){+.+.}-{3:3}, at: get_gendisk+0x3c8/0x6a0 block/genhd.c:1029
>
> stack backtrace:
> CPU: 0 PID: 8491 Comm: syz-executor541 Not tainted 5.10.0-rc3-next-20201116-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:79 [inline]
> dump_stack+0x107/0x163 lib/dump_stack.c:120
> check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2115
> check_prev_add kernel/locking/lockdep.c:2866 [inline]
> check_prevs_add kernel/locking/lockdep.c:2991 [inline]
> validate_chain kernel/locking/lockdep.c:3606 [inline]
> __lock_acquire+0x2ca6/0x5c00 kernel/locking/lockdep.c:4830
> lock_acquire kernel/locking/lockdep.c:5435 [inline]
> lock_acquire+0x2a3/0x8c0 kernel/locking/lockdep.c:5400
> __mutex_lock_common kernel/locking/mutex.c:956 [inline]
> __mutex_lock+0x134/0x1110 kernel/locking/mutex.c:1103
> brd_probe+0x31/0x250 drivers/block/brd.c:434
> request_gendisk_module block/genhd.c:979 [inline]
> get_gendisk+0x4c3/0x6a0 block/genhd.c:1029
> bdev_get_gendisk fs/block_dev.c:1094 [inline]
> __blkdev_get+0x344/0x1870 fs/block_dev.c:1455
> blkdev_get+0xd1/0x240 fs/block_dev.c:1634
> blkdev_open+0x21d/0x2b0 fs/block_dev.c:1751
> do_dentry_open+0x4b9/0x11b0 fs/open.c:817
> do_open fs/namei.c:3252 [inline]
> path_openat+0x1b9a/0x2730 fs/namei.c:3369
> do_filp_open+0x17e/0x3c0 fs/namei.c:3396
> do_sys_openat2+0x16d/0x420 fs/open.c:1168
> do_sys_open fs/open.c:1184 [inline]
> __do_sys_openat fs/open.c:1200 [inline]
> __se_sys_openat fs/open.c:1195 [inline]
> __x64_sys_openat+0x13f/0x1f0 fs/open.c:1195
> do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x445ec9
> Code: e8 fc b8 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 0b 12 fc ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f6bca748d98 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
> RAX: ffffffffffffffda RBX: 00000000006ddc38 RCX: 0000000000445ec9
> RDX: 0000000000000000 RSI: 0000000020002040 RDI: 00000000ffffff9c
>
>
> ---
> 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.
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches
---end quoted text---

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