Re: [PATCH v1 01/10] btrfs: implement a nowait option for tree searches

[Jens Axboe <axboe@kernel.dk>]

On 9/2/22 8:48 AM, Filipe Manana wrote:
> On Fri, Sep 2, 2022 at 12:01 AM Stefan Roesch <shr@fb.com> wrote:
>>
>> From: Josef Bacik <josef@toxicpanda.com>
>>
>> For NOWAIT IOCB's we'll need a way to tell search to not wait on locks
>> or anything.  Accomplish this by adding a path->nowait flag that will
>> use trylocks and skip reading of metadata, returning -EWOULDBLOCK in
>> either of these cases.  For now we only need this for reads, so only the
>> read side is handled.  Add an ASSERT() to catch anybody trying to use
>> this for writes so they know they'll have to implement the write side.
>>
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>>  fs/btrfs/ctree.c   | 39 ++++++++++++++++++++++++++++++++++++---
>>  fs/btrfs/ctree.h   |  1 +
>>  fs/btrfs/locking.c | 23 +++++++++++++++++++++++
>>  fs/btrfs/locking.h |  1 +
>>  4 files changed, 61 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
>> index ebfa35fe1c38..052c768b2297 100644
>> --- a/fs/btrfs/ctree.c
>> +++ b/fs/btrfs/ctree.c
>> @@ -1447,6 +1447,11 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,
>>                         return 0;
>>                 }
>>
>> +               if (p->nowait) {
>> +                       free_extent_buffer(tmp);
>> +                       return -EWOULDBLOCK;
>> +               }
>> +
>>                 if (unlock_up)
>>                         btrfs_unlock_up_safe(p, level + 1);
>>
>> @@ -1467,6 +1472,8 @@ read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,
>>                         ret = -EAGAIN;
>>
>>                 goto out;
>> +       } else if (p->nowait) {
>> +               return -EWOULDBLOCK;
>>         }
>>
>>         if (unlock_up) {
>> @@ -1634,7 +1641,13 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
>>                  * We don't know the level of the root node until we actually
>>                  * have it read locked
>>                  */
>> -               b = btrfs_read_lock_root_node(root);
>> +               if (p->nowait) {
>> +                       b = btrfs_try_read_lock_root_node(root);
>> +                       if (IS_ERR(b))
>> +                               return b;
>> +               } else {
>> +                       b = btrfs_read_lock_root_node(root);
>> +               }
>>                 level = btrfs_header_level(b);
>>                 if (level > write_lock_level)
>>                         goto out;
>> @@ -1910,6 +1923,13 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>>         WARN_ON(p->nodes[0] != NULL);
>>         BUG_ON(!cow && ins_len);
>>
>> +       /*
>> +        * For now only allow nowait for read only operations.  There's no
>> +        * strict reason why we can't, we just only need it for reads so I'm
>> +        * only implementing it for reads right now.
>> +        */
>> +       ASSERT(!p->nowait || !cow);
>> +
>>         if (ins_len < 0) {
>>                 lowest_unlock = 2;
>>
>> @@ -1936,7 +1956,12 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>>
>>         if (p->need_commit_sem) {
>>                 ASSERT(p->search_commit_root);
>> -               down_read(&fs_info->commit_root_sem);
>> +               if (p->nowait) {
>> +                       if (!down_read_trylock(&fs_info->commit_root_sem))
>> +                               return -EAGAIN;
> 
> Why EAGAIN here and everywhere else EWOULDBLOCK? See below.

Is EWOULDBLOCK ever different from EAGAIN? But it should be used
consistently, EAGAIN would be the return of choice for that.

-- 
Jens Axboe