On Mon, Nov 09, 2020 at 09:57:05AM +0000, Filipe Manana wrote: > > > On 09/11/20 08:44, Boqun Feng wrote: > > Hi Filipe, > > > > On Thu, Nov 05, 2020 at 09:10:12AM +0800, Boqun Feng wrote: > >> On Wed, Nov 04, 2020 at 07:54:40PM +0000, Filipe Manana wrote: > >> [...] > >>> > >>> Ok, so I ran 5.10-rc2 plus your two patches (the fix and the debug one): > >>> > >>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > >>> index b71ad8d9f1c9..b31d4ad482c7 100644 > >>> --- a/kernel/locking/lockdep.c > >>> +++ b/kernel/locking/lockdep.c > >>> @@ -539,8 +539,10 @@ static struct lock_trace *save_trace(void) > >>> LOCK_TRACE_SIZE_IN_LONGS; > >>> > >>> if (max_entries <= 0) { > >>> - if (!debug_locks_off_graph_unlock()) > >>> + if (!debug_locks_off_graph_unlock()) { > >>> + WARN_ON_ONCE(1); > >>> return NULL; > >>> + } > >>> > >>> print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too > >>> low!"); > >>> dump_stack(); > >>> @@ -5465,7 +5467,7 @@ noinstr int lock_is_held_type(const struct > >>> lockdep_map *lock, int read) > >>> unsigned long flags; > >>> int ret = 0; > >>> > >>> - if (unlikely(!lockdep_enabled())) > >>> + if (unlikely(debug_locks && !lockdep_enabled())) > >>> return 1; /* avoid false negative lockdep_assert_held() */ > >>> > >>> raw_local_irq_save(flags); > >>> > >>> With 3 runs of all fstests, the WARN_ON_ONCE(1) wasn't triggered. > >>> Unexpected, right? > >>> > >> > >> Kinda, that means we still don't know why lockdep was turned off. > >> > >>> Should I try something else? > >>> > >> > >> Thanks for trying this. Let me set up the reproducer on my side, and see > >> if I could get more information. > >> > > > > I could hit this with btrfs/187, and when we hit it, lockdep will report > > the deadlock and turn off, and I think this is the root cause for your > > hitting the original problem, I will add some analysis after the lockdep > > splat. > > > > [12295.973309] ============================================ > > [12295.974770] WARNING: possible recursive locking detected > > [12295.974770] 5.10.0-rc2-btrfs-next-71 #20 Not tainted > > [12295.974770] -------------------------------------------- > > [12295.974770] zsh/701247 is trying to acquire lock: > > [12295.974770] ffff92cef43480b8 (&eb->lock){++++}-{2:2}, at: > > btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] > > but task is already holding lock: > > [12295.974770] ffff92cef434a038 (&eb->lock){++++}-{2:2}, at: > > btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] > > other info that might help us debug this: > > [12295.974770] Possible unsafe locking scenario: > > > > [12295.974770] CPU0 > > [12295.974770] ---- > > [12295.974770] lock(&eb->lock); > > [12295.974770] lock(&eb->lock); > > [12295.974770] > > *** DEADLOCK *** > > > > [12295.974770] May be due to missing lock nesting notation > > > > [12295.974770] 2 locks held by zsh/701247: > > [12295.974770] #0: ffff92cef3d315b0 (&sig->cred_guard_mutex){+.+.}-{3:3}, > > at: bprm_execve+0xaa/0x920 > > [12295.974770] #1: ffff92cef434a038 (&eb->lock){++++}-{2:2}, at: > > btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] > > stack backtrace: > > [12295.974770] CPU: 6 PID: 701247 Comm: zsh Not tainted > > 5.10.0-rc2-btrfs-next-71 #20 > > [12295.974770] Hardware name: Microsoft Corporation Virtual Machine/Virtual > > Machine, BIOS Hyper-V UEFI Release v4.0 12/17/2019 > > [12295.974770] Call Trace: > > [12295.974770] dump_stack+0x8b/0xb0 > > [12295.974770] __lock_acquire.cold+0x175/0x2e9 > > [12295.974770] lock_acquire+0x15b/0x490 > > [12295.974770] ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] ? read_block_for_search+0xf4/0x350 [btrfs] > > [12295.974770] _raw_read_lock+0x40/0xa0 > > [12295.974770] ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs] > > [12295.974770] btrfs_search_slot+0x6ac/0xca0 [btrfs] > > [12295.974770] btrfs_lookup_xattr+0x7d/0xd0 [btrfs] > > [12295.974770] btrfs_getxattr+0x67/0x130 [btrfs] > > [12295.974770] __vfs_getxattr+0x53/0x70 > > [12295.974770] get_vfs_caps_from_disk+0x68/0x1a0 > > [12295.974770] ? sched_clock_cpu+0x114/0x180 > > [12295.974770] cap_bprm_creds_from_file+0x181/0x6c0 > > [12295.974770] security_bprm_creds_from_file+0x2a/0x40 > > [12295.974770] begin_new_exec+0xf4/0xc40 > > [12295.974770] ? load_elf_phdrs+0x6b/0xb0 > > [12295.974770] load_elf_binary+0x66b/0x1620 > > [12295.974770] ? read_hv_sched_clock_tsc+0x5/0x20 > > [12295.974770] ? sched_clock+0x5/0x10 > > [12295.974770] ? sched_clock_local+0x12/0x80 > > [12295.974770] ? sched_clock_cpu+0x114/0x180 > > [12295.974770] bprm_execve+0x3ce/0x920 > > [12295.974770] do_execveat_common+0x1b0/0x1f0 > > [12295.974770] __x64_sys_execve+0x39/0x50 > > [12295.974770] do_syscall_64+0x33/0x80 > > [12295.974770] entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > [12295.974770] RIP: 0033:0x7f6aaefc13cb > > [12295.974770] Code: 48 3d 00 f0 ff ff 76 e7 f7 d8 64 41 89 00 eb df 0f 1f > > 80 00 00 00 00 f7 d8 64 41 89 00 eb dc f3 0f 1e fa b8 3b 00 00 00 0f 05 > > <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 75 4a 0f 00 f7 d8 64 89 01 48 > > [12295.974770] RSP: 002b:00007ffd33b54d58 EFLAGS: 00000207 ORIG_RAX: > > 000000000000003b > > [12295.974770] RAX: ffffffffffffffda RBX: 00007f6aaf28bc88 RCX: > > 00007f6aaefc13cb > > [12295.974770] RDX: 00007ffd33b5fd98 RSI: 00007f6aaf28bc88 RDI: > > 00007ffd33b55280 > > [12295.974770] RBP: 00007ffd33b54d80 R08: 00007ffd33b54ce0 R09: > > 0000000000000001 > > [12295.974770] R10: 0000000000000008 R11: 0000000000000207 R12: > > 00007ffd33b55280 > > > > The deadlock senario reported by this splat is as follow: > > > > CPU 0 CPU 2 > > ===== ===== > > btrfs_search_slot(): > > btrfs_tree_read_lock_atomic(): > > read_lock(&eb->lock); <a random writer> > > write_lock(&eb->lock); > > // waiting for the lock > > ... > > btrfs_tree_read_lock_atomic(): > > read_lock(&eb->lock); // blocked because the fairness. > > > > In short, you can not use nested read_lock() on the same lock. However, > > I'm not sure whether we have the real problem here, because I don't > > think btrfs_search_slot() can pick the same extent buffer twice in > > btrfs_search_slot(). Also copy the author of the code for more > > information. > > Ah yes. We have several lockdep splats for which the fixes are not in > 5.10-rcs yet. Some may be already in the integration branch [1] and > others not yet in any tree, but can be found in the btrfs mailing list. > > For that particular one, it's fixed by the following patch: > > https://lore.kernel.org/linux-btrfs/1cee2922a32c305056a9559ccf7aede49777beae.1604591048.git.jo...@toxicpanda.com/ > > (It belongs to a series) > > You'll likely see more different lockdep splats, this is due to a > transition from custom btree locks to rw semaphores that is in progress, > and some preparatory work for that is already in 5.10-rcs. >
Thanks for the information! > Btw, I could hit the fs freeze deadlock even when lockdep didn't produce > any previous splat. > And this is the weird part, which means lockdep get turned off silently. That probably means there is a bug (either in fs code or lockdep) that we don't know, and my previous "fix" although works as per your testing, but it may hide the real problem... So I just send out another fix, which is similar to my previous one, but leave a warning so that once we hit the problem we can be warned and not miss the silent lockdep turn-off. Regards, Boqun > Jan was hitting it too with ext4, and Darrick with xfs - though I can't > tell if they hit any lockdep splat before hitting the freeze deadlock. > > Thanks for reporting it. > > [1] https://github.com/kdave/btrfs-devel/commits/misc-next > > > > > Regards, > > Boqun > > > >> Regards, > >> Boqun > >> > >>> Thanks. > >>> > >>> > >