[tip:perf/core] perf: Fix event->ctx locking

From: tip-bot for Peter Zijlstra
Date: Wed Feb 04 2015 - 09:40:11 EST

Commit-ID: f63a8daa5812afef4f06c962351687e1ff9ccb2b
Gitweb: http://git.kernel.org/tip/f63a8daa5812afef4f06c962351687e1ff9ccb2b
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Fri, 23 Jan 2015 12:24:14 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 4 Feb 2015 08:07:10 +0100

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@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20150123125834.209535886@xxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/events/core.c | 244 +++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 207 insertions(+), 37 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b358cb3..417a96b 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);

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