perf_counter: Provide functions for locking and pinning the context for a task

This abstracts out the code for locking the context associated
with a task.  Because the context might get transferred from
one task to another concurrently, we have to check after
locking the context that it is still the right context for the
task and retry if not.  This was open-coded in
find_get_context() and perf_counter_init_task().

This adds a further function for pinning the context for a
task, i.e. marking it so it can't be transferred to another
task.  This adds a 'pin_count' field to struct
perf_counter_context to indicate that a context is pinned,
instead of the previous method of setting the parent_gen count
to all 1s.  Pinning the context with a pin_count is easier to
undo and doesn't require saving the parent_gen value.  This
also adds a perf_unpin_context() to undo the effect of
perf_pin_task_context() and changes perf_counter_init_task to
use it.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <18979.34748.755674.596386@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -543,6 +543,7 @@ struct perf_counter_context {
 	struct perf_counter_context *parent_ctx;
 	u64			parent_gen;
 	u64			generation;
+	int			pin_count;
 	struct rcu_head		rcu_head;
 };
 

kernel/perf_counter.c

@@ -122,6 +122,69 @@ 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;
 }