----- On Aug 6, 2020, at 1:07 PM, Peter Oskolkov [email protected] wrote: > On Thu, Aug 6, 2020 at 6:48 AM <[email protected]> wrote: >> >> On Wed, Aug 05, 2020 at 05:08:58PM -0700, Peter Oskolkov wrote: >> >> Thanks for the Cc! > > Always a pleasure! > > (Sorry, included only membarrier maintainers in v1; in v2 included > both membarrier and rseq maintainers). > >> >> > + * @MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU: >> > + * If a thread belonging to the current process >> > + * is currently in an RSEQ critical section on >> > the >> > + * CPU identified by flags parameter, restart it. >> > + * @flags: if @flags >= 0, identifies the CPU to >> > + * restart RSEQ CS on; if == -1, restarts >> > + * RSEQ CSs on all CPUs. >> >> > + } else if (cpu_id == -1) { >> > + on_each_cpu(membarrier_rseq_ipi, >> > + current->group_leader, true); >> >> This is an unpriv IPI the world. That's a big no-no. > > removed in v2.
I don't think the feature must be removed, but its implementation needs adjustment. How about we simply piggy-back on the membarrier schemes we already have, and implement: membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ) membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ) All the logic is there to prevent sending IPIs to runqueues which are not running threads associated with the same mm. Considering that preemption does an rseq abort, running a thread belonging to a different mm should mean that this CPU is not currently executing an rseq critical section, or if it was, it has already been aborted, so it is quiescent. Then you'll probably want to change membarrier_private_expedited so it takes an extra "cpu" argument. If cpu=-1, iterate on all runqueues like we currently do. If cpu >= 0, only IPI that CPU if the thread currently running has the same mm. Also, should this belong to the membarrier or the rseq system call ? It just looks like the membarrier happens to implement very similar things for barriers, but arguably this is really about rseq. I wonder if we should expose this through rseq instead, even if we end up using membarrier code. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
