Messages in this thread |  | | | Date | Fri, 13 May 2011 14:48:03 +0200 | | From | Frederic Weisbecker <> | | Subject | Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely callable from scheduler |
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On Fri, May 13, 2011 at 06:48:26AM -0400, Steven Rostedt wrote: > On Thu, 2011-05-12 at 22:32 +0200, Frederic Weisbecker wrote: > > Avoid potential scheduler recursion and deadlock from the > > stacktrace code by avoiding rescheduling when we re-enable > > preemption. > > I'm curious to where you saw this deadlock? As I have the function stack > tracer using preempt_disable_notrace and enable_notrace without any > issues, and it traces all functions in the kernel[*]. I have no issue > with using raw_local_irq_save/restore() if it is to protect the per_cpu > variable from interrupt corruption, but I don't see the problem with > recursion. > > There's only one function I had to worry about with preempt disable, not > the entire scheduler. That was the function preempt_schedule(). This > function is called by preempt_enable() and that will cause an infinite > loop if you have something in preempt_schedule() call preempt_enable(). > > Remember that ftrace_preempt_disable/enable() crap that I did to try to > avoid the scheduler deadlock? I found it was complex and unnecessary > because the scheduler itself was not an issue, it was only > preempt_schedule(). I replaced all that crappy code with a single line > that added notrace to preempt_schedule() and everything just worked. > > Thus, if you disable interrupts to protect the cpu data, that's fine, > and say so in the change log. I really like to know if you really saw > this deadlock. Yes enabling preemption in the scheduler may recurse, but > it will only do so once. > > I still argue that interrupt enabling is slow. I've seen a large slow > down of the code by switching stack tracer from preempt disable to irq > disable. I used perf to see why, and it told me that disabling > interrupts as fine, but enabling interrupts can cost you quite a bit. > > -- Steve > > [*] of course function tracing does not trace other notrace functions. >
I haven't observed any deadlock. trace events disable preemption and other tracers do too (my changelog was buggy).
I just worried about potential other users, like a WARN_ON in the scheduler or so.
My worry is the following scenario:
schedule() { acquire(rq) set_tsk_need_resched WARN_ON() { stack_trace() { preempt_enable() { preempt_schedule() { acquire(rq) } } } } } I don't know if it happens that one set TIF_NEED_RESCHED remotely, or if TIF_NEED_RESCHED can be set when we hold the rq, and then we can be followed by a WARN_ON, ... So I preferred to be careful.
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