Messages in this thread Patch in this message |  | | | Date | Wed, 19 Jan 2011 19:22:07 +0100 | | From | Oleg Nesterov <> | | Subject | [PATCH 1/2] perf: fix find_get_context() vs perf_event_exit_task() race |
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find_get_context() must not install the new perf_event_context if the task has already passed perf_event_exit_task().
If nothing else, this means the memory leak. Initially ctx->refcount == 2, it is supposed that perf_event_exit_task_context() should participate and do the necessary put_ctx().
find_lively_task_by_vpid() checks PF_EXITING but this buys nothing, by the time we call find_get_context() this task can be already dead. To the point, cmpxchg() can succeed when the task has already done the last schedule().
Change find_get_context() to populate task->perf_event_ctxp[] under task->perf_event_mutex, this way we can trust PF_EXITING because perf_event_exit_task() takes the same mutex.
Also, change perf_event_exit_task_context() to use rcu_dereference(). Probably this is not strictly needed, but with or without this change find_get_context() can race with setup_new_exec()->perf_event_exit_task(), rcu_dereference() looks better.
Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- kernel/perf_event.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) --- git/kernel/perf_event.c~3_find_get_context_vs_exit 2011-01-18 16:57:08.000000000 +0100 +++ git/kernel/perf_event.c 2011-01-19 17:41:16.000000000 +0100 @@ -2201,13 +2201,6 @@ find_lively_task_by_vpid(pid_t vpid) if (!task) return ERR_PTR(-ESRCH); - /* - * Can't attach events to a dying task. - */ - err = -ESRCH; - if (task->flags & PF_EXITING) - goto errout; - /* Reuse ptrace permission checks for now. */ err = -EACCES; if (!ptrace_may_access(task, PTRACE_MODE_READ)) @@ -2268,14 +2261,27 @@ retry: get_ctx(ctx); - if (cmpxchg(&task->perf_event_ctxp[ctxn], NULL, ctx)) { - /* - * We raced with some other task; use - * the context they set. - */ + err = 0; + mutex_lock(&task->perf_event_mutex); + /* + * If it has already passed perf_event_exit_task(). + * we must see PF_EXITING, it takes this mutex too. + */ + if (task->flags & PF_EXITING) + err = -ESRCH; + else if (task->perf_event_ctxp[ctxn]) + err = -EAGAIN; + else + rcu_assign_pointer(task->perf_event_ctxp[ctxn], ctx); + mutex_unlock(&task->perf_event_mutex); + + if (unlikely(err)) { put_task_struct(task); kfree(ctx); - goto retry; + + if (err == -EAGAIN) + goto retry; + goto errout; } } @@ -6127,7 +6133,7 @@ static void perf_event_exit_task_context * scheduled, so we are now safe from rescheduling changing * our context. */ - child_ctx = child->perf_event_ctxp[ctxn]; + child_ctx = rcu_dereference(child->perf_event_ctxp[ctxn]); task_ctx_sched_out(child_ctx, EVENT_ALL); /*
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