Messages in this thread Patch in this message | | | Date | Tue, 3 Nov 2020 22:08:28 +0800 | From | Boqun Feng <> | Subject | Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion") |
| |
Hi Filipe,
On Mon, Oct 26, 2020 at 11:26:49AM +0000, Filipe Manana wrote: > Hello, > > I've recently started to hit a warning followed by tasks hanging after > attempts to freeze a filesystem. A git bisection pointed to the > following commit: > > commit 4d004099a668c41522242aa146a38cc4eb59cb1e > Author: Peter Zijlstra <peterz@infradead.org> > Date: Fri Oct 2 11:04:21 2020 +0200 > > lockdep: Fix lockdep recursion > > This happens very reliably when running all xfstests with lockdep > enabled, and the tested filesystem is btrfs (haven't tried other > filesystems, but it shouldn't matter). The warning and task hangs always > happen at either test generic/068 or test generic/390, and (oddly) > always have to run all tests for it to trigger, running those tests > individually on an infinite loop doesn't seem to trigger it (at least > for a couple hours). > > The warning triggered is at fs/super.c:__sb_start_write() which always > results later in several tasks hanging on a percpu rw_sem: > > https://pastebin.com/qnLvf94E >
In your dmesg, I see line:
[ 9304.920151] INFO: lockdep is turned off.
, that means debug_locks is 0, that usually happens when lockdep find a problem (i.e. a deadlock) and it turns itself off, because a problem is found and it's pointless for lockdep to continue to run.
And I haven't found a lockdep splat in your dmesg, do you have a full dmesg so that I can have a look?
This may be relevant because in commit 4d004099a66, we have
@@ -5056,13 +5081,13 @@ noinstr int lock_is_held_type(const struct lockdep_map *lock, int read) unsigned long flags; int ret = 0;
- if (unlikely(current->lockdep_recursion)) + if (unlikely(!lockdep_enabled())) return 1; /* avoid false negative lockdep_assert_held() */
before this commit lock_is_held_type() and its friends may return false if debug_locks==0, after this commit lock_is_held_type() and its friends will always return true if debug_locks == 0. That could cause the behavior here.
In case I'm correct, the following "fix" may be helpful.
Regards, Boqun
----------8 diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 3e99dfef8408..c0e27fb949ff 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -5471,7 +5464,7 @@ 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);
> What happens is percpu_rwsem_is_held() is apparently returning a false > positive, so this makes __sb_start_write() do a > percpu_down_read_trylock() on a percpu_rw_sem at a higher level, which > is expected to always succeed, because if the calling task is holding a > freeze percpu_rw_sem at level 1, it's supposed to be able to try_lock > the semaphore at level 2, since the freeze semaphores are always > acquired by increasing level order. > > But the try_lock fails, it triggers the warning at __sb_start_write(), > then its caller sb_start_pagefault() ignores the return value and > callers such as btrfs_page_mkwrite() make the assumption the freeze > semaphore was taken, proceed to do their stuff, and later call > sb_end_pagefault(), which which will do an up_read() on the percpu_rwsem > at level 2 despite not having not been able to down_read() the > semaphore. This obviously corrupts the semaphore's read_count state, and > later causes any task trying to down_write() it to hang forever. > > After such a hang I ran a drgn script to confirm it: > > $ cat dump_freeze_sems.py > import sys > import drgn > from drgn import NULL, Object, cast, container_of, execscript, \ > reinterpret, sizeof > from drgn.helpers.linux import * > > mnt_path = b'/home/fdmanana/btrfs-tests/scratch_1' > > mnt = None > for mnt in for_each_mount(prog, dst = mnt_path): > pass > > if mnt is None: > sys.stderr.write(f'Error: mount point {mnt_path} not found\n') > sys.exit(1) > > def dump_sem(level_enum): > level = level_enum.value_() > sem = mnt.mnt.mnt_sb.s_writers.rw_sem[level - 1] > print(f'freeze semaphore at level {level}, {str(level_enum)}') > print(f' block {sem.block.counter.value_()}') > for i in for_each_possible_cpu(prog): > read_count = per_cpu_ptr(sem.read_count, i) > print(f' read_count at cpu {i} = {read_count}') > print() > > # dump semaphore read counts for all freeze levels (fs.h) > dump_sem(prog['SB_FREEZE_WRITE']) > dump_sem(prog['SB_FREEZE_PAGEFAULT']) > dump_sem(prog['SB_FREEZE_FS']) > > > $ drgn dump_freeze_sems.py > freeze semaphore at level 1, (enum <anonymous>)SB_FREEZE_WRITE > block 1 > read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c74 = 3 > read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c74 = 4294967293 > read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c74 = 3 > read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c74 = 4294967293 > > freeze semaphore at level 2, (enum <anonymous>)SB_FREEZE_PAGEFAULT > block 1 > read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c78 = 0 > read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c78 = 4294967295 > read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c78 = 0 > read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c78 = 0 > > freeze semaphore at level 3, (enum <anonymous>)SB_FREEZE_FS > block 0 > read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c7c = 0 > read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c7c = 0 > read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c7c = 0 > read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c7c = 0 > > At levels 1 and 3, read_count sums to 0, so it's fine, but at level 2 it > sums to -1. The system remains like that for hours at least, with no > progress at all. > > Is there a known regression with that lockdep commit? > Anything I can do to help debug it in case it's not obvious? > > Thanks.
| |