From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> commit f63a8daa5812afef4f06c962351687e1ff9ccb2b upstream. 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> Signed-off-by: Amit Pundir <amit.pundir@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 26c40faa8ea4..3964293d1540 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. @@ -1654,7 +1725,7 @@ 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; @@ -1695,6 +1766,19 @@ 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, @@ -2158,7 +2242,7 @@ 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; @@ -2214,9 +2298,21 @@ 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 @@ -2225,10 +2321,25 @@ 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, @@ -3421,7 +3532,16 @@ 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 @@ -3547,12 +3667,13 @@ 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; @@ -3567,7 +3688,7 @@ 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; @@ -3581,14 +3702,11 @@ 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; } @@ -3660,8 +3778,14 @@ 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) @@ -3687,7 +3811,7 @@ 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); @@ -3706,6 +3830,7 @@ 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) @@ -3719,14 +3844,13 @@ 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); } struct period_event { @@ -3831,25 +3955,24 @@ 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); @@ -3896,6 +4019,19 @@ 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) @@ -3918,11 +4054,15 @@ 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(¤t->perf_event_mutex); - list_for_each_entry(event, ¤t->perf_event_list, owner_entry) - perf_event_for_each_child(event, perf_event_enable); + list_for_each_entry(event, ¤t->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(¤t->perf_event_mutex); return 0; @@ -3930,11 +4070,15 @@ int perf_event_task_enable(void) int perf_event_task_disable(void) { + struct perf_event_context *ctx; struct perf_event *event; mutex_lock(¤t->perf_event_mutex); - list_for_each_entry(event, ¤t->perf_event_list, owner_entry) - perf_event_for_each_child(event, perf_event_disable); + list_for_each_entry(event, ¤t->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(¤t->perf_event_mutex); return 0; @@ -7271,6 +7415,15 @@ 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 * @@ -7286,7 +7439,7 @@ 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; @@ -7484,9 +7637,14 @@ 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); /* @@ -7501,15 +7659,19 @@ 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, @@ -7521,6 +7683,11 @@ 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(); @@ -7628,7 +7795,11 @@ 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); @@ -7636,11 +7807,9 @@ 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) @@ -7650,6 +7819,7 @@ 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); -- 2.7.4