> Il giorno 17 mag 2017, alle ore 21:12, Tejun Heo <tj@xxxxxxxxxx> ha scritto: > > Hello, > > On Mon, May 15, 2017 at 09:49:13PM +0200, Paolo Valente wrote: >> So, unless you tell me that there are other races I haven't seen, or, >> even worse, that I'm just talking nonsense, I have thought of a simple >> solution to address this issue without resorting to the request_queue >> lock: further caching, on blkg lookups, the only policy or blkg data >> the scheduler may use, and access this data directly when needed. By >> doing so, the issue is reduced to the occasional use of stale data. >> And apparently this already happens, e.g., in cfq when it uses the >> weight of a cfq_queue associated with a process whose group has just >> been changed (and for which a blkg_lookup has not yet been invoked). >> The same should happen when cfq invokes cfq_log_cfqq for such a >> cfq_queue, as this function prints the path of the group the bfq_queue >> belongs to. > > I haven't studied the code but the problem sounds correct to me. All > of blkcg code assumes the use of rq lock. And, yeah, none of the hot > paths requires strong synchornization. All the actual management > operations can be synchronized separately and the hot lookup path can > be protected with rcu and maybe percpu reference counters. > Great, thanks for this ack. User reports do confirm the problem, and, so far, the effectiveness of a solution I have implemented. I'm finalizing the patch for submission. Thanks, Paolo > Thanks. > > -- > tejun
