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The scenario is: > > A test program creates the 4th child, which means the MM_CID users become It would be clearer to talk in terms of threads, e.g. "creates the 5th thread". AFAIR threads are "siblings", so I'm not sure that the parent/child relationship really applies here. > more than the number of CPUs (four in this example), so it switches to per > CPU ownership mode. > > At this point each live task of the program has a CID associated. Assume > thread creation order assignment for simplicity. > > T0 (main thread) CID0 runs fork() and creates T4 > T1 (1st child) CID1 2nd thread and so on... > T2 (2nd child) CID2 > T3 (3rd child) CID3 > T4 (4th child) --- not visible yet > > T0 sets mm_cid::percpu = true and transfers it's own CID to CPU0 where it its > runs on and then starts the fixup which walks through the threads to > transfer the per task CIDs either to the CPU the task is running on or drop > it back into the pool if the task is not on a CPU. > > During that T1 - T3 are free to schedule in and out before the fixup caught > up with them. Going through all possible permutations with a python script > revealed a few problematic cases. The most trivial one is: > > T1 schedules in on CPU1 and observes percpu == true, so it transfers > it's CID to CPU1 its > > T1 is migrated to CPU1 and schedule in observes percpu == true, but I think you mean "to CPU2" here. > CPU2 does not have a CID associated and T1 transferred it's own to its [...] > + * > + * Aside of that this mechanism also ensures RT compability: compatibility [...] > @@ -10596,11 +10628,13 @@ void sched_mm_cid_fork(struct task_struc > if (!percpu) > mm_cid_transit_to_task(current, pcp); > else > - mm_cid_transfer_to_cpu(current, pcp); > + mm_cid_transit_to_cpu(current, pcp); > } > > if (percpu) { > mm_cid_fixup_tasks_to_cpus(); > + /* Clear the transition bit */ > + WRITE_ONCE(mm->mm_cid.transit, 0); You should move this WRITE_ONCE at the end of mm_cid_fixup_tasks_to_cpus() to keep the same pattern as for mm_cid_fixup_cpus_to_tasks(). Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com