On Thu, Jul 3, 2014 at 5:46 AM, Chandan Rajendra
<chan...@linux.vnet.ibm.com> wrote:
> On Monday 23 Jun 2014 11:25:47 Filipe David Borba Manana wrote:
>> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
>> index aeab453..8f1a371 100644
>> --- a/fs/btrfs/ctree.c
>> +++ b/fs/btrfs/ctree.c
>> @@ -2825,12 +2825,12 @@ cow_done:
>> * It is safe to drop the lock on our parent before we
>> * go through the expensive btree search on b.
>> *
>> - * If we're inserting or deleting (ins_len != 0), then we might
>> - * be changing slot zero, which may require changing the
>> parent.
>> - * So, we can't drop the lock until after we know which slot
>> - * we're operating on.
>> + * If we're inserting, deleting or updating a key (cow != 0),
>> + * then we might be changing slot zero, which may require
>> + * changing the parent. So, we can't drop the lock until after
>> + * we know which slot we're operating on.
>> */
>> - if (!ins_len && !p->keep_locks) {
>> + if (!cow && !p->keep_locks) {
>> int u = level + 1;
>
> For the "key update" case (i.e. (ins_len == 0) and (cow == 1)), maybe
> we could optimize by having a variable to hold the return value of
> should_cow_block(), i.e. it keeps track of whether the current
> metadata block was COWed. If the variable indicates that the block was
> *not* COWed, then we could release the lock on the parent block since
> the update operation (even on slot 0) isn't going to change the
> corresponding entry in the parent block.
Hi Chandan,
Just because a node wasn't cowed it doesn't mean it isn't going to be
updated. Further updating the key works bottom-up - we don't know
during btrfs_search_slot() if our caller intends to update a key in
the leaf or just update the item, so we need to return a path here
with all nodes with slot == 0 having a write lock on them. I'm
actually for now basically just undoing this
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=eb653de15987612444b6cde3b0e67b1edd94625f
Anyway right now I have a functional issue to tackle first.
Thanks.
>
> Thanks,
> chandan
>
--
Filipe David Manana,
"Reasonable men adapt themselves to the world.
Unreasonable men adapt the world to themselves.
That's why all progress depends on unreasonable men."
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