[PATCH 2/6] perf: Avoid RCU vs preemption assumptions

[Peter Zijlstra]

The per-pmu per-cpu context patch converted things from get_cpu_var() to
this_cpu_ptr(), but that only works if rcu_read_lock() actually disables
preemption, and since there is no such guarantee, we need to fix that.

Use the newly introduced {get,put}_cpu_ptr().

Cc: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 kernel/perf_event.c |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

Index: linux-2.6/kernel/perf_event.c
===================================================================
--- linux-2.6.orig/kernel/perf_event.c
+++ linux-2.6/kernel/perf_event.c
@@ -3814,18 +3814,20 @@ static void perf_event_task_event(struct
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(pmu, &pmus, entry) {
-		cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
+		cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
 		perf_event_task_ctx(&cpuctx->ctx, task_event);
 
 		ctx = task_event->task_ctx;
 		if (!ctx) {
 			ctxn = pmu->task_ctx_nr;
 			if (ctxn < 0)
-				continue;
+				goto next;
 			ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
 		}
 		if (ctx)
 			perf_event_task_ctx(ctx, task_event);
+next:
+		put_cpu_ptr(pmu->pmu_cpu_context);
 	}
 	rcu_read_unlock();
 }
@@ -3947,16 +3949,18 @@ static void perf_event_comm_event(struct
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(pmu, &pmus, entry) {
-		cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
+		cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
 		perf_event_comm_ctx(&cpuctx->ctx, comm_event);
 
 		ctxn = pmu->task_ctx_nr;
 		if (ctxn < 0)
-			continue;
+			goto next;
 
 		ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
 		if (ctx)
 			perf_event_comm_ctx(ctx, comm_event);
+next:
+		put_cpu_ptr(pmu->pmu_cpu_context);
 	}
 	rcu_read_unlock();
 }
@@ -4130,19 +4134,21 @@ static void perf_event_mmap_event(struct
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(pmu, &pmus, entry) {
-		cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
+		cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
 		perf_event_mmap_ctx(&cpuctx->ctx, mmap_event,
 					vma->vm_flags & VM_EXEC);
 
 		ctxn = pmu->task_ctx_nr;
 		if (ctxn < 0)
-			continue;
+			goto next;
 
 		ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
 		if (ctx) {
 			perf_event_mmap_ctx(ctx, mmap_event,
 					vma->vm_flags & VM_EXEC);
 		}
+next:
+		put_cpu_ptr(pmu->pmu_cpu_context);
 	}
 	rcu_read_unlock();
 


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