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x86/resctrl: Fix task CLOSID/RMID update race
When the user moves a running task to a new rdtgroup using the task's file interface or by deleting its rdtgroup, the resulting change in CLOSID/RMID must be immediately propagated to the PQR_ASSOC MSR on the task(s) CPUs. x86 allows reordering loads with prior stores, so if the task starts running between a task_curr() check that the CPU hoisted before the stores in the CLOSID/RMID update then it can start running with the old CLOSID/RMID until it is switched again because __rdtgroup_move_task() failed to determine that it needs to be interrupted to obtain the new CLOSID/RMID. Refer to the diagram below: CPU 0 CPU 1 ----- ----- __rdtgroup_move_task(): curr <- t1->cpu->rq->curr __schedule(): rq->curr <- t1 resctrl_sched_in(): t1->{closid,rmid} -> {1,1} t1->{closid,rmid} <- {2,2} if (curr == t1) // false IPI(t1->cpu) A similar race impacts rdt_move_group_tasks(), which updates tasks in a deleted rdtgroup. In both cases, use smp_mb() to order the task_struct::{closid,rmid} stores before the loads in task_curr(). In particular, in the rdt_move_group_tasks() case, simply execute an smp_mb() on every iteration with a matching task. It is possible to use a single smp_mb() in rdt_move_group_tasks(), but this would require two passes and a means of remembering which task_structs were updated in the first loop. However, benchmarking results below showed too little performance impact in the simple approach to justify implementing the two-pass approach. Times below were collected using `perf stat` to measure the time to remove a group containing a 1600-task, parallel workload. CPU: Intel(R) Xeon(R) Platinum P-8136 CPU @ 2.00GHz (112 threads) # mkdir /sys/fs/resctrl/test # echo $$ > /sys/fs/resctrl/test/tasks # perf bench sched messaging -g 40 -l 100000 task-clock time ranges collected using: # perf stat rmdir /sys/fs/resctrl/test Baseline: 1.54 - 1.60 ms smp_mb() every matching task: 1.57 - 1.67 ms [ bp: Massage commit message. ] Fixes:ae28d1aae4
("x86/resctrl: Use an IPI instead of task_work_add() to update PQR_ASSOC MSR") Fixes:0efc89be94
("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount") Signed-off-by: Peter Newman <peternewman@google.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com> Reviewed-by: Babu Moger <babu.moger@amd.com> Cc: <stable@kernel.org> Link: https://lore.kernel.org/r/20221220161123.432120-1-peternewman@google.com
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1 changed files with 11 additions and 1 deletions
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@ -580,8 +580,10 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
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/*
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* Ensure the task's closid and rmid are written before determining if
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* the task is current that will decide if it will be interrupted.
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* This pairs with the full barrier between the rq->curr update and
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* resctrl_sched_in() during context switch.
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*/
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barrier();
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smp_mb();
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/*
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* By now, the task's closid and rmid are set. If the task is current
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@ -2401,6 +2403,14 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
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WRITE_ONCE(t->closid, to->closid);
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WRITE_ONCE(t->rmid, to->mon.rmid);
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/*
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* Order the closid/rmid stores above before the loads
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* in task_curr(). This pairs with the full barrier
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* between the rq->curr update and resctrl_sched_in()
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* during context switch.
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*/
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smp_mb();
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/*
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* If the task is on a CPU, set the CPU in the mask.
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* The detection is inaccurate as tasks might move or
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