Re: [PATCH] btrfs: fix lockdep splat in btrfs_alloc_subvolume_writers

[Jeff Mahoney <jeffm@suse.com>]

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On 3/16/18 2:48 PM, Nikolay Borisov wrote:
> 
> 
> On 16.03.2018 20:36, jeffm@suse.com wrote:
>> From: Jeff Mahoney <jeffm@suse.com>
>>
>> While running btrfs/011, I hit the following lockdep splat.
>>
>> This is the important bit:
>>    pcpu_alloc+0x1ac/0x5e0
>>    __percpu_counter_init+0x4e/0xb0
>>    btrfs_init_fs_root+0x99/0x1c0 [btrfs]
>>    btrfs_get_fs_root.part.54+0x5b/0x150 [btrfs]
>>    resolve_indirect_refs+0x130/0x830 [btrfs]
>>    find_parent_nodes+0x69e/0xff0 [btrfs]
>>    btrfs_find_all_roots_safe+0xa0/0x110 [btrfs]
>>    btrfs_find_all_roots+0x50/0x70 [btrfs]
>>    btrfs_qgroup_prepare_account_extents+0x53/0x90 [btrfs]
>>    btrfs_commit_transaction+0x3ce/0x9b0 [btrfs]
>>
>> The percpu_counter_init call in btrfs_alloc_subvolume_writers
>> uses GFP_KERNEL, which we can't do during transaction commit.
>>
>> This switches it to GFP_NOFS.
> 
> Given there is effort underway to actually kill GFP_NOFS and replace it
> with the context annotation routines, shouldn't instead use those
> routines directly ?

I don't think those have landed yet.  When they do, it should obsolete
the gfp flags here in any context since we can also read roots from code
that doesn't need GFP_NOFS.

-Jeff

-- 
Jeff Mahoney
SUSE Labs


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