1 files changed, 74 insertions, 54 deletions

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 79c3f26541d..da8dfef4b47 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -123,6 +123,69 @@ static void put_ctx(struct perf_counter_context *ctx)
 }
 
 /*
+ * Get the perf_counter_context for a task and lock it.
+ * This has to cope with with the fact that until it is locked,
+ * the context could get moved to another task.
+ */
+static struct perf_counter_context *perf_lock_task_context(
+				struct task_struct *task, unsigned long *flags)
+{
+	struct perf_counter_context *ctx;
+
+	rcu_read_lock();
+ retry:
+	ctx = rcu_dereference(task->perf_counter_ctxp);
+	if (ctx) {
+		/*
+		 * If this context is a clone of another, it might
+		 * get swapped for another underneath us by
+		 * perf_counter_task_sched_out, though the
+		 * rcu_read_lock() protects us from any context
+		 * getting freed.  Lock the context and check if it
+		 * got swapped before we could get the lock, and retry
+		 * if so.  If we locked the right context, then it
+		 * can't get swapped on us any more.
+		 */
+		spin_lock_irqsave(&ctx->lock, *flags);
+		if (ctx != rcu_dereference(task->perf_counter_ctxp)) {
+			spin_unlock_irqrestore(&ctx->lock, *flags);
+			goto retry;
+		}
+	}
+	rcu_read_unlock();
+	return ctx;
+}
+
+/*
+ * Get the context for a task and increment its pin_count so it
+ * can't get swapped to another task.  This also increments its
+ * reference count so that the context can't get freed.
+ */
+static struct perf_counter_context *perf_pin_task_context(struct task_struct *task)
+{
+	struct perf_counter_context *ctx;
+	unsigned long flags;
+
+	ctx = perf_lock_task_context(task, &flags);
+	if (ctx) {
+		++ctx->pin_count;
+		get_ctx(ctx);
+		spin_unlock_irqrestore(&ctx->lock, flags);
+	}
+	return ctx;
+}
+
+static void perf_unpin_context(struct perf_counter_context *ctx)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&ctx->lock, flags);
+	--ctx->pin_count;
+	spin_unlock_irqrestore(&ctx->lock, flags);
+	put_ctx(ctx);
+}
+
+/*
  * Add a counter from the lists for its context.
  * Must be called with ctx->mutex and ctx->lock held.
  */
@@ -916,7 +979,7 @@ static int context_equiv(struct perf_counter_context *ctx1,
 {
 	return ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx
 		&& ctx1->parent_gen == ctx2->parent_gen
-		&& ctx1->parent_gen != ~0ull;
+		&& !ctx1->pin_count && !ctx2->pin_count;
 }
 
 /*
@@ -1271,6 +1334,7 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
 	struct perf_counter_context *ctx;
 	struct perf_counter_context *parent_ctx;
 	struct task_struct *task;
+	unsigned long flags;
 	int err;
 
 	/*
@@ -1323,28 +1387,9 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
 	if (!ptrace_may_access(task, PTRACE_MODE_READ))
 		goto errout;
 
- retry_lock:
-	rcu_read_lock();
  retry:
-	ctx = rcu_dereference(task->perf_counter_ctxp);
+	ctx = perf_lock_task_context(task, &flags);
 	if (ctx) {
-		/*
-		 * If this context is a clone of another, it might
-		 * get swapped for another underneath us by
-		 * perf_counter_task_sched_out, though the
-		 * rcu_read_lock() protects us from any context
-		 * getting freed.  Lock the context and check if it
-		 * got swapped before we could get the lock, and retry
-		 * if so.  If we locked the right context, then it
-		 * can't get swapped on us any more and we can
-		 * unclone it if necessary.
-		 * Once it's not a clone things will be stable.
-		 */
-		spin_lock_irq(&ctx->lock);
-		if (ctx != rcu_dereference(task->perf_counter_ctxp)) {
-			spin_unlock_irq(&ctx->lock);
-			goto retry;
-		}
 		parent_ctx = ctx->parent_ctx;
 		if (parent_ctx) {
 			put_ctx(parent_ctx);
@@ -1355,9 +1400,8 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
 		 * this context won't get freed if the task exits.
 		 */
 		get_ctx(ctx);
-		spin_unlock_irq(&ctx->lock);
+		spin_unlock_irqrestore(&ctx->lock, flags);
 	}
-	rcu_read_unlock();
 
 	if (!ctx) {
 		ctx = kmalloc(sizeof(struct perf_counter_context), GFP_KERNEL);
@@ -1372,7 +1416,7 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
 			 * the context they set.
 			 */
 			kfree(ctx);
-			goto retry_lock;
+			goto retry;
 		}
 		get_task_struct(task);
 	}
@@ -3667,7 +3711,6 @@ int perf_counter_init_task(struct task_struct *child)
 	struct perf_counter *counter;
 	struct task_struct *parent = current;
 	int inherited_all = 1;
-	u64 cloned_gen;
 	int ret = 0;
 
 	child->perf_counter_ctxp = NULL;
@@ -3693,32 +3736,17 @@ int perf_counter_init_task(struct task_struct *child)
 	get_task_struct(child);
 
 	/*
-	 * If the parent's context is a clone, temporarily set its
-	 * parent_gen to an impossible value (all 1s) so it won't get
-	 * swapped under us.  The rcu_read_lock makes sure that
-	 * parent_ctx continues to exist even if it gets swapped to
-	 * another process and then freed while we are trying to get
-	 * its lock.
+	 * If the parent's context is a clone, pin it so it won't get
+	 * swapped under us.
 	 */
-	rcu_read_lock();
- retry:
-	parent_ctx = rcu_dereference(parent->perf_counter_ctxp);
+	parent_ctx = perf_pin_task_context(parent);
+
 	/*
 	 * No need to check if parent_ctx != NULL here; since we saw
 	 * it non-NULL earlier, the only reason for it to become NULL
 	 * is if we exit, and since we're currently in the middle of
 	 * a fork we can't be exiting at the same time.
 	 */
-	spin_lock_irq(&parent_ctx->lock);
-	if (parent_ctx != rcu_dereference(parent->perf_counter_ctxp)) {
-		spin_unlock_irq(&parent_ctx->lock);
-		goto retry;
-	}
-	cloned_gen = parent_ctx->parent_gen;
-	if (parent_ctx->parent_ctx)
-		parent_ctx->parent_gen = ~0ull;
-	spin_unlock_irq(&parent_ctx->lock);
-	rcu_read_unlock();
 
 	/*
 	 * Lock the parent list. No need to lock the child - not PID
@@ -3759,7 +3787,7 @@ int perf_counter_init_task(struct task_struct *child)
 		cloned_ctx = rcu_dereference(parent_ctx->parent_ctx);
 		if (cloned_ctx) {
 			child_ctx->parent_ctx = cloned_ctx;
-			child_ctx->parent_gen = cloned_gen;
+			child_ctx->parent_gen = parent_ctx->parent_gen;
 		} else {
 			child_ctx->parent_ctx = parent_ctx;
 			child_ctx->parent_gen = parent_ctx->generation;
@@ -3769,15 +3797,7 @@ int perf_counter_init_task(struct task_struct *child)
 
 	mutex_unlock(&parent_ctx->mutex);
 
-	/*
-	 * Restore the clone status of the parent.
-	 */
-	if (parent_ctx->parent_ctx) {
-		spin_lock_irq(&parent_ctx->lock);
-		if (parent_ctx->parent_ctx)
-			parent_ctx->parent_gen = cloned_gen;
-		spin_unlock_irq(&parent_ctx->lock);
-	}
+	perf_unpin_context(parent_ctx);
 
 	return ret;
 }