Messages in this thread | | | Date | Thu, 4 Mar 2010 12:56:59 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9) |
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* Linus Torvalds (torvalds@linux-foundation.org) wrote: > > - SA_RUNNING: a way to signal only running threads - as a way for user-space > > based concurrency control mechanisms to deschedule running threads (or, like > > in your case, to implement barrier / garbage collection schemes). > > Hmm. This sounds less fundamentally broken, but at the same time also > _way_ more invasive in the signal handling layer. It's already one of our > more "exciting" layers out there. >
Hrm, thinking about it a bit further, the only way I see we could provide a usable SA_RUNNING flag would be to add hooks to the scheduler. These hooks would somehow have to call user-space code (!) when scheduling in/out a thread. Yes, this sounds utterly broken (since these hooks would have to be preemptable).
The idea is this: if we look, for instance, at the kernel preemptable RCU implementations, they consist of two parts: one is iteration on all CPUs to consider all active CPUs, and the other is a modification of the scheduler to note all preempted tasks that were in a preemptable RCU C.S..
Just for the memory barrier we consider for sys_membarrier(), I had to ensure that the scheduler issues memory barriers to order accesses to user-space memory and mm_cpumask modifications. In reality, what we are doing is to ensure that the operation required on the running thread is done by the scheduler too when scheduling in/out the task.
As soon as we have signal handlers which perform more than a simple memory barrier (e.g. something that has side-effects outside of the processor), I doubt it would ever make sense to only run the handler on running threads unless we have hooks in the scheduler too.
Thanks,
Mathieu
-- Mathieu Desnoyers Operating System Efficiency Consultant EfficiOS Inc. http://www.efficios.com
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