Messages in this thread | | | Subject | Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion") | From | Filipe Manana <> | Date | Wed, 4 Nov 2020 09:49:00 +0000 |
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On 04/11/20 03:44, Boqun Feng wrote: > On Wed, Nov 04, 2020 at 10:22:36AM +0800, Boqun Feng wrote: >> On Tue, Nov 03, 2020 at 07:44:29PM +0000, Filipe Manana wrote: >>> >>> >>> On 03/11/20 14:08, Boqun Feng wrote: >>>> 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); >>> >>> Boqun, the patch fixes the problem for me! >>> You can have Tested-by: Filipe Manana <fdmanana@suse.com> >>> >> >> Thanks. Although I think it still means that we have a lock issue when >> running xfstests (because we don't know why debug_locks gets cleared), > > I might find a place where we could turn lockdep off silently: > > in print_circular_bug(), we turn off lockdep via > debug_locks_off_graph_unlock(), and then we try to save the trace for > lockdep splat, however, if we use up the stack_trace buffer (i.e. > nr_stack_trace_entries), save_trace() will return NULL and we return > silently. > > Filipe, in order to check whethter that happens, could you share me your > /proc/lockdep_stats after the full set of xfstests is finished?
Here it is:
$ cat /proc/lockdep_stats lock-classes: 1831 [max: 8192] direct dependencies: 17774 [max: 32768] indirect dependencies: 75662 all direct dependencies: 325284 dependency chains: 34223 [max: 65536] dependency chain hlocks used: 158129 [max: 327680] dependency chain hlocks lost: 0 in-hardirq chains: 57 in-softirq chains: 658 in-process chains: 33508 stack-trace entries: 160748 [max: 524288] number of stack traces: 9237 number of stack hash chains: 7076 combined max dependencies: 1280780998 hardirq-safe locks: 43 hardirq-unsafe locks: 1337 softirq-safe locks: 179 softirq-unsafe locks: 1236 irq-safe locks: 187 irq-unsafe locks: 1337 hardirq-read-safe locks: 2 hardirq-read-unsafe locks: 209 softirq-read-safe locks: 9 softirq-read-unsafe locks: 204 irq-read-safe locks: 9 irq-read-unsafe locks: 209 uncategorized locks: 274 unused locks: 0 max locking depth: 15 max bfs queue depth: 337 debug_locks: 0
zapped classes: 2278 zapped lock chains: 17915 large chain blocks: 1
(That's the result after running all fstests with the previous one line patch you sent.)
My kernel .config: https://pastebin.com/4xEMvLJ9
I'll try the debugging patch and let you know the results. It will take some 4 hours or so to get back with the result.
Thanks!
> > Alternatively, it's also helpful if you can try the following debug > diff, with teh full set of xfstests: > > Thanks! Just trying to understand the real problem. > > Regards, > Boqun > > -------------->8 > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > index b71ad8d9f1c9..9ae3e089e5c0 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(); > >> I guess I will have to reproduce this myself for further analysis, could >> you share you .config? >> >> Anyway, I think this fix still makes a bit sense, I will send a proper >> patch so that the problem won't block fs folks. >> >> Regards, >> Boqun >> >>> Thanks! >>> >>>> >>>> >>>> >>>>> 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. >>>> >
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