On 8/2/21 8:13 AM, Paolo Valente wrote: > The function bfq_setup_merge prepares the merging between two > bfq_queues, say bfqq and new_bfqq. To this goal, it assigns > bfqq->new_bfqq = new_bfqq. Then, each time some I/O for bfqq arrives, > the process that generated that I/O is disassociated from bfqq and > associated with new_bfqq (merging is actually a redirection). In this > respect, bfq_setup_merge increases new_bfqq->ref in advance, adding > the number of processes that are expected to be associated with > new_bfqq. > > Unfortunately, the stable-merging mechanism interferes with this > setup. After bfqq->new_bfqq has been set by bfq_setup_merge, and > before all the expected processes have been associated with > bfqq->new_bfqq, bfqq may happen to be stably merged with a different > queue than the current bfqq->new_bfqq. In this case, bfqq->new_bfqq > gets changed. So, some of the processes that have been already > accounted for in the ref counter of the previous new_bfqq will not be > associated with that queue. This creates an unbalance, because those > references will never be decremented. > > This commit fixes this issue by reestablishing the previous, natural > behaviour: once bfqq->new_bfqq has been set, it will not be changed > until all expected redirections have occurred. Applied, thanks. -- Jens Axboe
