On 11.04.2018 10:59, Nikolay Borisov wrote:
> When the delayed refs for a head are all run, eventually
> cleanup_ref_head is called which (in case of deletion) obtains a
> reference for the relevant btrfs_space_info struct by querying the bg
> for the range. This is problematic because when the last extent of a
> bg is deleted a race window emerges between removal of that bg and the
> subsequent invocation of cleanup_ref_head. This can result in cache being null
> and either a null pointer dereference or assertion failure.
>
> task: ffff8d04d31ed080 task.stack: ffff9e5dc10cc000
> RIP: 0010:assfail.constprop.78+0x18/0x1a [btrfs]
> RSP: 0018:ffff9e5dc10cfbe8 EFLAGS: 00010292
> RAX: 0000000000000044 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff8d04ffc1f868 RSI: ffff8d04ffc178c8 RDI: ffff8d04ffc178c8
> RBP: ffff8d04d29e5ea0 R08: 00000000000001f0 R09: 0000000000000001
> R10: ffff9e5dc0507d58 R11: 0000000000000001 R12: ffff8d04d29e5ea0
> R13: ffff8d04d29e5f08 R14: ffff8d04efe29b40 R15: ffff8d04efe203e0
> FS: 00007fbf58ead500(0000) GS:ffff8d04ffc00000(0000)
> knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fe6c6975648 CR3: 0000000013b2a000 CR4: 00000000000006f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> __btrfs_run_delayed_refs+0x10e7/0x12c0 [btrfs]
> btrfs_run_delayed_refs+0x68/0x250 [btrfs]
> btrfs_should_end_transaction+0x42/0x60 [btrfs]
> btrfs_truncate_inode_items+0xaac/0xfc0 [btrfs]
> btrfs_evict_inode+0x4c6/0x5c0 [btrfs]
> evict+0xc6/0x190
> do_unlinkat+0x19c/0x300
> do_syscall_64+0x74/0x140
> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> RIP: 0033:0x7fbf589c57a7
>
> To fix this, introduce a new flag "is_system" to head_ref structs,
> which is populated at insertion time. This allows to decouple the
> querying for the spaceinfo from querying the possibly deleted bg.
>
> Fixes: d7eae3403f46 ("Btrfs: rework delayed ref total_bytes_pinned
> accounting")
> Suggested-by: Omar Sandoval <osan...@osandov.com>
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
> ---
>
> v2:
> * Collapsed the if/else logic in cleanup_ref_head
So I had a bit written up about how I tested this code:
I did a full xfstest run with both the new code and old code and the
following assert:
ASSERT(space_info == cache->space_info);
It didn't trigger, so I'm confident this shouldn't introduce any
regressions.
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