perf: Fix event->ctx locking

There have been a few reported issues wrt. the lack of locking around
changing event->ctx. This patch tries to address those.

It avoids the whole rwsem thing; and while it appears to work, please
give it some thought in review.

What I did fail at is sensible runtime checks on the use of
event->ctx, the RCU use makes it very hard.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150123125834.209535886@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 207 insertions, 37 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b358cb38e4a5..417a96bf3d41 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -907,6 +907,77 @@ static void put_ctx(struct perf_event_context *ctx)
 }
 
 /*
+ * Because of perf_event::ctx migration in sys_perf_event_open::move_group and
+ * perf_pmu_migrate_context() we need some magic.
+ *
+ * Those places that change perf_event::ctx will hold both
+ * perf_event_ctx::mutex of the 'old' and 'new' ctx value.
+ *
+ * Lock ordering is by mutex address. There is one other site where
+ * perf_event_context::mutex nests and that is put_event(). But remember that
+ * that is a parent<->child context relation, and migration does not affect
+ * children, therefore these two orderings should not interact.
+ *
+ * The change in perf_event::ctx does not affect children (as claimed above)
+ * because the sys_perf_event_open() case will install a new event and break
+ * the ctx parent<->child relation, and perf_pmu_migrate_context() is only
+ * concerned with cpuctx and that doesn't have children.
+ *
+ * The places that change perf_event::ctx will issue:
+ *
+ *   perf_remove_from_context();
+ *   synchronize_rcu();
+ *   perf_install_in_context();
+ *
+ * to affect the change. The remove_from_context() + synchronize_rcu() should
+ * quiesce the event, after which we can install it in the new location. This
+ * means that only external vectors (perf_fops, prctl) can perturb the event
+ * while in transit. Therefore all such accessors should also acquire
+ * perf_event_context::mutex to serialize against this.
+ *
+ * However; because event->ctx can change while we're waiting to acquire
+ * ctx->mutex we must be careful and use the below perf_event_ctx_lock()
+ * function.
+ *
+ * Lock order:
+ *      task_struct::perf_event_mutex
+ *        perf_event_context::mutex
+ *          perf_event_context::lock
+ *          perf_event::child_mutex;
+ *          perf_event::mmap_mutex
+ *          mmap_sem
+ */
+static struct perf_event_context *perf_event_ctx_lock(struct perf_event *event)
+{
+        struct perf_event_context *ctx;
+
+again:
+        rcu_read_lock();
+        ctx = ACCESS_ONCE(event->ctx);
+        if (!atomic_inc_not_zero(&ctx->refcount)) {
+                rcu_read_unlock();
+                goto again;
+        }
+        rcu_read_unlock();
+
+        mutex_lock(&ctx->mutex);
+        if (event->ctx != ctx) {
+                mutex_unlock(&ctx->mutex);
+                put_ctx(ctx);
+                goto again;
+        }
+
+        return ctx;
+}
+
+static void perf_event_ctx_unlock(struct perf_event *event,
+                                  struct perf_event_context *ctx)
+{
+        mutex_unlock(&ctx->mutex);
+        put_ctx(ctx);
+}
+
+/*
  * This must be done under the ctx->lock, such as to serialize against
  * context_equiv(), therefore we cannot call put_ctx() since that might end up
  * calling scheduler related locks and ctx->lock nests inside those.
@@ -1666,7 +1737,7 @@ int __perf_event_disable(void *info)
  * is the current context on this CPU and preemption is disabled,
  * hence we can't get into perf_event_task_sched_out for this context.
  */
-void perf_event_disable(struct perf_event *event)
+static void _perf_event_disable(struct perf_event *event)
 {
         struct perf_event_context *ctx = event->ctx;
         struct task_struct *task = ctx->task;
@@ -1707,6 +1778,19 @@ retry:
         }
         raw_spin_unlock_irq(&ctx->lock);
 }
+
+/*
+ * Strictly speaking kernel users cannot create groups and therefore this
+ * interface does not need the perf_event_ctx_lock() magic.
+ */
+void perf_event_disable(struct perf_event *event)
+{
+        struct perf_event_context *ctx;
+
+        ctx = perf_event_ctx_lock(event);
+        _perf_event_disable(event);
+        perf_event_ctx_unlock(event, ctx);
+}
 EXPORT_SYMBOL_GPL(perf_event_disable);
 
 static void perf_set_shadow_time(struct perf_event *event,
@@ -2170,7 +2254,7 @@ unlock:
  * perf_event_for_each_child or perf_event_for_each as described
  * for perf_event_disable.
  */
-void perf_event_enable(struct perf_event *event)
+static void _perf_event_enable(struct perf_event *event)
 {
         struct perf_event_context *ctx = event->ctx;
         struct task_struct *task = ctx->task;
@@ -2226,9 +2310,21 @@ retry:
 out:
         raw_spin_unlock_irq(&ctx->lock);
 }
+
+/*
+ * See perf_event_disable();
+ */
+void perf_event_enable(struct perf_event *event)
+{
+        struct perf_event_context *ctx;
+
+        ctx = perf_event_ctx_lock(event);
+        _perf_event_enable(event);
+        perf_event_ctx_unlock(event, ctx);
+}
 EXPORT_SYMBOL_GPL(perf_event_enable);
 
-int perf_event_refresh(struct perf_event *event, int refresh)
+static int _perf_event_refresh(struct perf_event *event, int refresh)
 {
         /*
          * not supported on inherited events
@@ -2237,10 +2333,25 @@ int perf_event_refresh(struct perf_event *event, int refresh)
                 return -EINVAL;
 
         atomic_add(refresh, &event->event_limit);
-        perf_event_enable(event);
+        _perf_event_enable(event);
 
         return 0;
 }
+
+/*
+ * See perf_event_disable()
+ */
+int perf_event_refresh(struct perf_event *event, int refresh)
+{
+        struct perf_event_context *ctx;
+        int ret;
+
+        ctx = perf_event_ctx_lock(event);
+        ret = _perf_event_refresh(event, refresh);
+        perf_event_ctx_unlock(event, ctx);
+
+        return ret;
+}
 EXPORT_SYMBOL_GPL(perf_event_refresh);
 
 static void ctx_sched_out(struct perf_event_context *ctx,
@@ -3433,7 +3544,16 @@ static void perf_remove_from_owner(struct perf_event *event)
         rcu_read_unlock();
 
         if (owner) {
-                mutex_lock(&owner->perf_event_mutex);
+                /*
+                 * If we're here through perf_event_exit_task() we're already
+                 * holding ctx->mutex which would be an inversion wrt. the
+                 * normal lock order.
+                 *
+                 * However we can safely take this lock because its the child
+                 * ctx->mutex.
+                 */
+                mutex_lock_nested(&owner->perf_event_mutex, SINGLE_DEPTH_NESTING);
+
                 /*
                  * We have to re-check the event->owner field, if it is cleared
                  * we raced with perf_event_exit_task(), acquiring the mutex
@@ -3559,12 +3679,13 @@ static int perf_event_read_group(struct perf_event *event,
                                    u64 read_format, char __user *buf)
 {
         struct perf_event *leader = event->group_leader, *sub;
-        int n = 0, size = 0, ret = -EFAULT;
         struct perf_event_context *ctx = leader->ctx;
-        u64 values[5];
+        int n = 0, size = 0, ret;
         u64 count, enabled, running;
+        u64 values[5];
+
+        lockdep_assert_held(&ctx->mutex);
 
-        mutex_lock(&ctx->mutex);
         count = perf_event_read_value(leader, &enabled, &running);
 
         values[n++] = 1 + leader->nr_siblings;
@@ -3579,7 +3700,7 @@ static int perf_event_read_group(struct perf_event *event,
         size = n * sizeof(u64);
 
         if (copy_to_user(buf, values, size))
-                goto unlock;
+                return -EFAULT;
 
         ret = size;
 
@@ -3593,14 +3714,11 @@ static int perf_event_read_group(struct perf_event *event,
                 size = n * sizeof(u64);
 
                 if (copy_to_user(buf + ret, values, size)) {
-                        ret = -EFAULT;
-                        goto unlock;
+                        return -EFAULT;
                 }
 
                 ret += size;
         }
-unlock:
-        mutex_unlock(&ctx->mutex);
 
         return ret;
 }
@@ -3672,8 +3790,14 @@ static ssize_t
 perf_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 {
         struct perf_event *event = file->private_data;
+        struct perf_event_context *ctx;
+        int ret;
 
-        return perf_read_hw(event, buf, count);
+        ctx = perf_event_ctx_lock(event);
+        ret = perf_read_hw(event, buf, count);
+        perf_event_ctx_unlock(event, ctx);
+
+        return ret;
 }
 
 static unsigned int perf_poll(struct file *file, poll_table *wait)
@@ -3699,7 +3823,7 @@ static unsigned int perf_poll(struct file *file, poll_table *wait)
         return events;
 }
 
-static void perf_event_reset(struct perf_event *event)
+static void _perf_event_reset(struct perf_event *event)
 {
         (void)perf_event_read(event);
         local64_set(&event->count, 0);
@@ -3718,6 +3842,7 @@ static void perf_event_for_each_child(struct perf_event *event,
         struct perf_event *child;
 
         WARN_ON_ONCE(event->ctx->parent_ctx);
+
         mutex_lock(&event->child_mutex);
         func(event);
         list_for_each_entry(child, &event->child_list, child_list)
@@ -3731,14 +3856,13 @@ static void perf_event_for_each(struct perf_event *event,
         struct perf_event_context *ctx = event->ctx;
         struct perf_event *sibling;
 
-        WARN_ON_ONCE(ctx->parent_ctx);
-        mutex_lock(&ctx->mutex);
+        lockdep_assert_held(&ctx->mutex);
+
         event = event->group_leader;
 
         perf_event_for_each_child(event, func);
         list_for_each_entry(sibling, &event->sibling_list, group_entry)
                 perf_event_for_each_child(sibling, func);
-        mutex_unlock(&ctx->mutex);
 }
 
 static int perf_event_period(struct perf_event *event, u64 __user *arg)
@@ -3808,25 +3932,24 @@ static int perf_event_set_output(struct perf_event *event,
                                  struct perf_event *output_event);
 static int perf_event_set_filter(struct perf_event *event, void __user *arg);
 
-static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned long arg)
 {
-        struct perf_event *event = file->private_data;
         void (*func)(struct perf_event *);
         u32 flags = arg;
 
         switch (cmd) {
         case PERF_EVENT_IOC_ENABLE:
-                func = perf_event_enable;
+                func = _perf_event_enable;
                 break;
         case PERF_EVENT_IOC_DISABLE:
-                func = perf_event_disable;
+                func = _perf_event_disable;
                 break;
         case PERF_EVENT_IOC_RESET:
-                func = perf_event_reset;
+                func = _perf_event_reset;
                 break;
 
         case PERF_EVENT_IOC_REFRESH:
-                return perf_event_refresh(event, arg);
+                return _perf_event_refresh(event, arg);
 
         case PERF_EVENT_IOC_PERIOD:
                 return perf_event_period(event, (u64 __user *)arg);
@@ -3873,6 +3996,19 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
         return 0;
 }
 
+static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+        struct perf_event *event = file->private_data;
+        struct perf_event_context *ctx;
+        long ret;
+
+        ctx = perf_event_ctx_lock(event);
+        ret = _perf_ioctl(event, cmd, arg);
+        perf_event_ctx_unlock(event, ctx);
+
+        return ret;
+}
+
 #ifdef CONFIG_COMPAT
 static long perf_compat_ioctl(struct file *file, unsigned int cmd,
                                 unsigned long arg)
@@ -3895,11 +4031,15 @@ static long perf_compat_ioctl(struct file *file, unsigned int cmd,
 
 int perf_event_task_enable(void)
 {
+        struct perf_event_context *ctx;
         struct perf_event *event;
 
         mutex_lock(&current->perf_event_mutex);
-        list_for_each_entry(event, &current->perf_event_list, owner_entry)
-                perf_event_for_each_child(event, perf_event_enable);
+        list_for_each_entry(event, &current->perf_event_list, owner_entry) {
+                ctx = perf_event_ctx_lock(event);
+                perf_event_for_each_child(event, _perf_event_enable);
+                perf_event_ctx_unlock(event, ctx);
+        }
         mutex_unlock(&current->perf_event_mutex);
 
         return 0;
@@ -3907,11 +4047,15 @@ int perf_event_task_enable(void)
 
 int perf_event_task_disable(void)
 {
+        struct perf_event_context *ctx;
         struct perf_event *event;
 
         mutex_lock(&current->perf_event_mutex);
-        list_for_each_entry(event, &current->perf_event_list, owner_entry)
-                perf_event_for_each_child(event, perf_event_disable);
+        list_for_each_entry(event, &current->perf_event_list, owner_entry) {
+                ctx = perf_event_ctx_lock(event);
+                perf_event_for_each_child(event, _perf_event_disable);
+                perf_event_ctx_unlock(event, ctx);
+        }
         mutex_unlock(&current->perf_event_mutex);
 
         return 0;
@@ -7269,6 +7413,15 @@ out:
         return ret;
 }
 
+static void mutex_lock_double(struct mutex *a, struct mutex *b)
+{
+        if (b < a)
+                swap(a, b);
+
+        mutex_lock(a);
+        mutex_lock_nested(b, SINGLE_DEPTH_NESTING);
+}
+
 /**
  * sys_perf_event_open - open a performance event, associate it to a task/cpu
  *
@@ -7284,7 +7437,7 @@ SYSCALL_DEFINE5(perf_event_open,
         struct perf_event *group_leader = NULL, *output_event = NULL;
         struct perf_event *event, *sibling;
         struct perf_event_attr attr;
-        struct perf_event_context *ctx;
+        struct perf_event_context *ctx, *uninitialized_var(gctx);
         struct file *event_file = NULL;
         struct fd group = {NULL, 0};
         struct task_struct *task = NULL;
@@ -7482,9 +7635,14 @@ SYSCALL_DEFINE5(perf_event_open,
         }
 
         if (move_group) {
-                struct perf_event_context *gctx = group_leader->ctx;
+                gctx = group_leader->ctx;
+
+                /*
+                 * See perf_event_ctx_lock() for comments on the details
+                 * of swizzling perf_event::ctx.
+                 */
+                mutex_lock_double(&gctx->mutex, &ctx->mutex);
 
-                mutex_lock(&gctx->mutex);
                 perf_remove_from_context(group_leader, false);
 
                 /*
@@ -7499,15 +7657,19 @@ SYSCALL_DEFINE5(perf_event_open,
                         perf_event__state_init(sibling);
                         put_ctx(gctx);
                 }
-                mutex_unlock(&gctx->mutex);
-                put_ctx(gctx);
+        } else {
+                mutex_lock(&ctx->mutex);
         }
 
         WARN_ON_ONCE(ctx->parent_ctx);
-        mutex_lock(&ctx->mutex);
 
         if (move_group) {
+                /*
+                 * Wait for everybody to stop referencing the events through
+                 * the old lists, before installing it on new lists.
+                 */
                 synchronize_rcu();
+
                 perf_install_in_context(ctx, group_leader, group_leader->cpu);
                 get_ctx(ctx);
                 list_for_each_entry(sibling, &group_leader->sibling_list,
@@ -7519,6 +7681,11 @@ SYSCALL_DEFINE5(perf_event_open,
 
         perf_install_in_context(ctx, event, event->cpu);
         perf_unpin_context(ctx);
+
+        if (move_group) {
+                mutex_unlock(&gctx->mutex);
+                put_ctx(gctx);
+        }
         mutex_unlock(&ctx->mutex);
 
         put_online_cpus();
@@ -7626,7 +7793,11 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
         src_ctx = &per_cpu_ptr(pmu->pmu_cpu_context, src_cpu)->ctx;
         dst_ctx = &per_cpu_ptr(pmu->pmu_cpu_context, dst_cpu)->ctx;
 
-        mutex_lock(&src_ctx->mutex);
+        /*
+         * See perf_event_ctx_lock() for comments on the details
+         * of swizzling perf_event::ctx.
+         */
+        mutex_lock_double(&src_ctx->mutex, &dst_ctx->mutex);
         list_for_each_entry_safe(event, tmp, &src_ctx->event_list,
                                  event_entry) {
                 perf_remove_from_context(event, false);
@@ -7634,11 +7805,9 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
                 put_ctx(src_ctx);
                 list_add(&event->migrate_entry, &events);
         }
-        mutex_unlock(&src_ctx->mutex);
 
         synchronize_rcu();
 
-        mutex_lock(&dst_ctx->mutex);
         list_for_each_entry_safe(event, tmp, &events, migrate_entry) {
                 list_del(&event->migrate_entry);
                 if (event->state >= PERF_EVENT_STATE_OFF)
@@ -7648,6 +7817,7 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
                 get_ctx(dst_ctx);
         }
         mutex_unlock(&dst_ctx->mutex);
+        mutex_unlock(&src_ctx->mutex);
 }
 EXPORT_SYMBOL_GPL(perf_pmu_migrate_context);