Hello, Paolo. On Thu, Oct 19, 2017 at 08:50:17AM +0200, Paolo Valente wrote: > The blkg obtained through a blkg_lookup, in a rcu_read section, is > protected. But, outside that section, a pointer to that blkg is not > guaranteed to be valid any longer. Stat-update functions seem safe in blkg's destruction is rcu delayed. If you have access to a blkg under rcu, it won't get freed until the rcu read lock is released. > cfq and bfq, just because they are executed within locks that happen > to be taken also before destroying the blkg. They are the > request_queue lock for cfq and the scheduler lock for bfq. Thus, at > least the request_queue lock apparently needs to be taken around > stat-update functions in bfq, if they are moved outside the section > protected by the scheduler lock. So, a blkg stays alive if the queue lock is held, or the cgroup and request_queue stays alive, and won't be freed (different from being alive) as long as RCU read lock is held. Thanks. -- tejun
