blkcg: update blkg_lookup_create to do locking
To know when to create a blkg, the general pattern is to do a blkg_lookup and if that fails, lock and then do a lookup again and if that fails finally create. It doesn't make much sense for everyone who wants to do creation to write this themselves. This changes blkg_lookup_create to do locking and implement this pattern. The old blkg_lookup_create is renamed to __blkg_lookup_create. If a call site wants to do its own error handling or already owns the queue lock, they can use __blkg_lookup_create. This will be used in upcoming patches. Signed-off-by: NDennis Zhou <dennisszhou@gmail.com> Reviewed-by: NJosef Bacik <josef@toxicpanda.com> Acked-by: NTejun Heo <tj@kernel.org> Reviewed-by: NLiu Bo <bo.liu@linux.alibaba.com> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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