Messages in this thread | | | Date | Fri, 22 Jan 2010 03:43:34 +0100 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled checks in tracing hot path |
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On Thu, Jan 21, 2010 at 09:05:17PM -0500, Steven Rostedt wrote: > On Fri, 2010-01-22 at 02:16 +0100, Frederic Weisbecker wrote: > > Every time we enter the function profiler tracing callbacks, we first > > check if the function profiling is enabled. > > > > This check is useless because we register the function graph > > callbacks only after the hashlist has been initialized. > > Unfortunately, since the previous patch is incorrect, it makes this one > buggy too. > > If you remove the check to ftrace_profile_enabled, the call to the > profiled code could have been preempted and pending to be called. > > Stop machine may remove all calls to the tracing, but it only affects > new hits. Pending hits may still exist. > > If you remove this check, and the user re-enables the profiling, then > all PER_CPU hashs will be reset. If in the process of this happening, > the task with the pending trace wakes up, it may access the PER_CPU list > and corrupt it.
Indeed.
> Now for the reason I Cc'd Paul and Mathieu... > > If we had a synchronize_sched() like function that would wait and return > when all preempted tasks have been scheduled again and went to either > userspace or called schedule directly, then we could actually do this. > > After unregistering the function graph trace, you call this > "synchronize_tasks()" and it will guarantee that all currently preempted > tasks have either went to userspace or have called schedule() directly. > Then it would be safe to remove this check.
Good point!
I fear that would require heavy hooks in the scheduler though...
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