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SubjectRe: [PATCH] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim
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On 1/3/20 9:36 PM, Dave Chinner wrote:
> On Thu, Jan 02, 2020 at 10:52:08AM -0500, Waiman Long wrote:
>> Depending on the workloads, the following circular locking dependency
>> warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo
>> lock) may show up:
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 5.0.0-rc1+ #60 Tainted: G W
>> ------------------------------------------------------
>> fsfreeze/4346 is trying to acquire lock:
>> 0000000026f1d784 (fs_reclaim){+.+.}, at:
>> fs_reclaim_acquire.part.19+0x5/0x30
>>
>> but task is already holding lock:
>> 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>
>> which lock already depends on the new lock.
>> :
>> Possible unsafe locking scenario:
>>
>> CPU0 CPU1
>> ---- ----
>> lock(sb_internal);
>> lock(fs_reclaim);
>> lock(sb_internal);
>> lock(fs_reclaim);
>>
>> *** DEADLOCK ***
>>
>> 4 locks held by fsfreeze/4346:
>> #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650
>> #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at: freeze_super+0xda/0x290
>> #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650
>> #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650
>>
>> stack backtrace:
>> Call Trace:
>> dump_stack+0xe0/0x19a
>> print_circular_bug.isra.10.cold.34+0x2f4/0x435
>> check_prev_add.constprop.19+0xca1/0x15f0
>> validate_chain.isra.14+0x11af/0x3b50
>> __lock_acquire+0x728/0x1200
>> lock_acquire+0x269/0x5a0
>> fs_reclaim_acquire.part.19+0x29/0x30
>> fs_reclaim_acquire+0x19/0x20
>> kmem_cache_alloc+0x3e/0x3f0
>> kmem_zone_alloc+0x79/0x150
>> xfs_trans_alloc+0xfa/0x9d0
>> xfs_sync_sb+0x86/0x170
>> xfs_log_sbcount+0x10f/0x140
>> xfs_quiesce_attr+0x134/0x270
>> xfs_fs_freeze+0x4a/0x70
>> freeze_super+0x1af/0x290
>> do_vfs_ioctl+0xedc/0x16c0
>> ksys_ioctl+0x41/0x80
>> __x64_sys_ioctl+0x73/0xa9
>> do_syscall_64+0x18f/0xd23
>> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> According to Dave Chinner:
>>
>> Freezing the filesystem, after all the data has been cleaned. IOWs
>> memory reclaim will never run the above writeback path when
>> the freeze process is trying to allocate a transaction here because
>> there are no dirty data pages in the filesystem at this point.
>>
>> Indeed, this xfs_sync_sb() path sets XFS_TRANS_NO_WRITECOUNT so that
>> it /doesn't deadlock/ by taking freeze references for the
>> transaction. We've just drained all the transactions
>> in progress and written back all the dirty metadata, too, and so the
>> filesystem is completely clean and only needs the superblock to be
>> updated to complete the freeze process. And to do that, it does not
>> take a freeze reference because calling sb_start_intwrite() here
>> would deadlock.
>>
>> IOWs, this is a false positive, caused by the fact that
>> xfs_trans_alloc() is called from both above and below memory reclaim
>> as well as within /every level/ of freeze processing. Lockdep is
>> unable to describe the staged flush logic in the freeze process that
>> prevents deadlocks from occurring, and hence we will pretty much
>> always see false positives in the freeze path....
>>
>> Perhaps breaking the fs_reclaim pseudo lock into a per filesystem lock
>> may fix the issue. However, that will greatly complicate the logic and
>> may not be worth it.
> ANd it won't work, because now we'll just get lockedp warnings on
> the per-fs reclaim lockdep context.

It may or may not work depending on how we are going to break it up. I
haven't thought through that alternative yet as I am expecting that it
will be a bigger change if we decide to go this route.


>
>> Another way to fix it is to disable the taking of the fs_reclaim
>> pseudo lock when in the freezing code path as a reclaim on the freezed
>> filesystem is not possible as stated above. This patch takes this
>> approach by setting the __GFP_NOLOCKDEP flag in the slab memory
>> allocation calls when the filesystem has been freezed.
> IOWs, "fix" it by stating that "lockdep can't track freeze
> dependencies correctly"?
The lockdep code has a singular focus on spotting possible deadlock
scenarios from a locking point of view. The freeze dependency has to be
properly translated into appropriate locking sequences to make lockdep
work correctly. I would say that the use of a global fs_reclaim pseudo
lock is not a perfect translation and so it has exception cases we need
to handle.
>
> In the past we have just used KM_NOFS for that, because
> __GFP_NOLOCKDEP didn't exist. But that has just been a nasty hack
> because lockdep isn't capable of understanding allocation context
> constraints because allocation contexts are much more complex than a
> "lock"....
>
>
>> --- a/fs/xfs/kmem.h
>> +++ b/fs/xfs/kmem.h
>> @@ -20,6 +20,12 @@ typedef unsigned __bitwise xfs_km_flags_t;
>> #define KM_MAYFAIL ((__force xfs_km_flags_t)0x0008u)
>> #define KM_ZERO ((__force xfs_km_flags_t)0x0010u)
>>
>> +#ifdef CONFIG_LOCKDEP
>> +#define KM_NOLOCKDEP ((__force xfs_km_flags_t)0x0020u)
>> +#else
>> +#define KM_NOLOCKDEP ((__force xfs_km_flags_t)0)
>> +#endif
> Nope. We are getting rid of kmem_alloc wrappers and all the
> associated flags, not adding new ones. Part of that process is
> identifying all the places we currently use KM_NOFS to "shut up
> lockdep" and converting them to explicit __GFP_NOLOCKDEP flags.
>
> So right now, this change needs to be queued up behind the API
> changes that are currently in progress...

That is fine. I can wait and post a revised patch after that. Who is
going to make these changes?


>> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
>> index f6006d94a581..b1997649ecd8 100644
>> --- a/fs/xfs/xfs_log.c
>> +++ b/fs/xfs/xfs_log.c
>> @@ -454,7 +454,8 @@ xfs_log_reserve(
>> XFS_STATS_INC(mp, xs_try_logspace);
>>
>> ASSERT(*ticp == NULL);
>> - tic = xlog_ticket_alloc(log, unit_bytes, cnt, client, permanent, 0);
>> + tic = xlog_ticket_alloc(log, unit_bytes, cnt, client, permanent,
>> + mp->m_super->s_writers.frozen ? KM_NOLOCKDEP : 0);
> This is pretty nasty. Having to spew conditional code like this
> across every allocation that could be done in freeze conditions is
> a non-starter.
>
> We already have a flag to tell us we are doing a transaction in a
> freeze state, so use that to turn off lockdep. That is:

OK.


>> *ticp = tic;
>>
>> xlog_grant_push_ail(log, tic->t_cnt ? tic->t_unit_res * tic->t_cnt
>> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
>> index 3b208f9a865c..c0e42e4f5b77 100644
>> --- a/fs/xfs/xfs_trans.c
>> +++ b/fs/xfs/xfs_trans.c
>> @@ -262,8 +262,14 @@ xfs_trans_alloc(
>> * Allocate the handle before we do our freeze accounting and setting up
>> * GFP_NOFS allocation context so that we avoid lockdep false positives
>> * by doing GFP_KERNEL allocations inside sb_start_intwrite().
>> + *
>> + * To prevent false positive lockdep warning of circular locking
>> + * dependency between sb_internal and fs_reclaim, disable the
>> + * acquisition of the fs_reclaim pseudo-lock when the superblock
>> + * has been frozen or in the process of being frozen.
>> */
>> - tp = kmem_zone_zalloc(xfs_trans_zone, 0);
>> + tp = kmem_zone_zalloc(xfs_trans_zone,
>> + mp->m_super->s_writers.frozen ? KM_NOLOCKDEP : 0);
>> if (!(flags & XFS_TRANS_NO_WRITECOUNT))
>> sb_start_intwrite(mp->m_super);
> This code here should be setting PF_GFP_NOLOCKDEP state to turn off
> lockdep for all allocations in this context, similar to the way we
> use memalloc_nofs_save/restore so that all nested allocations
> inherit GFP_NOFS state...

Sure.

Thanks for the comments.

Cheers,
Longman

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