On 2020/1/4 下午5:37, Qu Wenruo wrote:
>
>
> On 2020/1/4 上午12:15, David Sterba wrote:
>> On Fri, Jan 03, 2020 at 04:52:59PM +0100, David Sterba wrote:
>>> So it's one bit vs refcount and a lock. For the backports I'd go with
>>> the bit, but this needs the barriers as mentioned in my previous reply.
>>> Can you please update the patches?
>>
>> The idea is in the diff below (compile tested only). I found one more
>> case that was not addressed by your patches, it's in
>> btrfs_update_reloc_root.
>
> But fix in btrfs_update_reloc_root() is already included in commit
> d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after
> merge_reloc_roots").
>
> Or would you mind to share more details about the missing check?
>>
>> Given that the type of the fix is the same, I'd rather do that in one
>> patch. The reported stack traces are more or less the same.
>
> To merge them into patch set is no problem, and should make backports a
> little easier.
>
> But I still didn't understand the barrier part.
> If we're relying on that bit operation before accessing reloc_root, it
> should be safe enough, even without memory barrier.
My bad, set_bit() and test_bit() themselves doesn't imply memory
barrier. (I always thought the opposite)
I'll put memory barriers in next version.
Thanks,
Qu
>
> Would you please explain a little more?
>
> Thanks,
> Qu
>>
>> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
>> index af4dd49a71c7..aeba3a7506e1 100644
>> --- a/fs/btrfs/relocation.c
>> +++ b/fs/btrfs/relocation.c
>> @@ -517,6 +517,15 @@ static int update_backref_cache(struct btrfs_trans_handle *trans,
>> return 1;
>> }
>>
>> +static bool have_reloc_root(struct btrfs_root *root)
>> +{
>> + smp_mb__before_atomic();
>> + if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
>> + return false;
>> + if (!root->reloc_root)
>> + return false;
>> + return true;
>> +}
>>
>> static int should_ignore_root(struct btrfs_root *root)
>> {
>> @@ -525,9 +534,9 @@ static int should_ignore_root(struct btrfs_root *root)
>> if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
>> return 0;
>>
>> - reloc_root = root->reloc_root;
>> - if (!reloc_root)
>> + if (!have_reloc_root(root))
>> return 0;
>> + reloc_root = root->reloc_root;
>>
>> if (btrfs_root_last_snapshot(&reloc_root->root_item) ==
>> root->fs_info->running_transaction->transid - 1)
>> @@ -1439,6 +1448,7 @@ int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
>> * The subvolume has reloc tree but the swap is finished, no need to
>> * create/update the dead reloc tree
>> */
>> + smp_mb__before_atomic();
>> if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
>> return 0;
>>
>> @@ -1478,8 +1488,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
>> struct btrfs_root_item *root_item;
>> int ret;
>>
>> - if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state) ||
>> - !root->reloc_root)
>> + if (!have_reloc_root(root))
>> goto out;
>>
>> reloc_root = root->reloc_root;
>> @@ -1489,6 +1498,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
>> if (fs_info->reloc_ctl->merge_reloc_tree &&
>> btrfs_root_refs(root_item) == 0) {
>> set_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
>> + smp_mb__after_atomic();
>> __del_reloc_root(reloc_root);
>> }
>>
>> @@ -2201,6 +2211,7 @@ static int clean_dirty_subvols(struct reloc_control *rc)
>> if (ret2 < 0 && !ret)
>> ret = ret2;
>> }
>> + smp_mb__before_atomic();
>> clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
>> btrfs_put_fs_root(root);
>> } else {
>> @@ -4730,7 +4741,7 @@ void btrfs_reloc_pre_snapshot(struct btrfs_pending_snapshot *pending,
>> struct btrfs_root *root = pending->root;
>> struct reloc_control *rc = root->fs_info->reloc_ctl;
>>
>> - if (!root->reloc_root || !rc)
>> + if (!rc || !have_reloc_root(root))
>> return;
>>
>> if (!rc->merge_reloc_tree)
>> @@ -4764,7 +4775,7 @@ int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
>> struct reloc_control *rc = root->fs_info->reloc_ctl;
>> int ret;
>>
>> - if (!root->reloc_root || !rc)
>> + if (!rc || !have_reloc_root(root))
>> return 0;
>>
>> rc = root->fs_info->reloc_ctl;
>>
>
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