Messages in this thread | | | Date | Mon, 8 Nov 2010 15:57:25 +0100 | From | Oleg Nesterov <> | Subject | Q: sys_perf_event_open() && PF_EXITING |
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I am puzzled by PF_EXITING check in find_lively_task_by_vpid().
How can it help? The task can call do_exit() right after the check.
And why do we need it? The comment only says "Can't attach events to a dying task". Maybe it tries protect sys_perf_event_open() against perf_event_exit_task_context(), but it can't.
c93f7669 "perf_counter: Fix race in attaching counters to tasks and exiting" says:
There is also a race between perf_counter_exit_task and find_get_context; this solves the race by moving the get_ctx that was in perf_counter_alloc into the locked region in find_get_context, so that once find_get_context has got the context for a task, it won't get freed even if the task calls perf_counter_exit_task.
OK, the code was changed since that commit, but afaics "it won't be freed" is still true.
However,
It doesn't matter if new top-level (non-inherited) counters get attached to the context after perf_counter_exit_task has detached the context from the task. They will just stay there and never get scheduled in until the counters' fds get closed, and then perf_release will remove them from the context and eventually free the context.
This looks wrong. perf_release() does free_event()->put_ctx(), this pairs get_ctx() after alloc_perf_context().
But __perf_event_init_context() sets ctx->refcount = 1, and I guess this reference should be dropped by ctx->task ? If yes, then it is not OK to attach the event after sys_perf_event_open().
No?
Hmm. jump_label_inc/dec looks obviously racy too. Say, free_event() races with perf_event_alloc(). There is a window between atomic_xxx() and jump_label_update(), afaics it is possible to call jump_label_disable() when perf_task_events/perf_swevent_enabled != 0.
Oleg.
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