Messages in this thread | | | Date | Mon, 1 Feb 2010 08:23:34 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock |
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On Mon, 1 Feb 2010, Mathieu Desnoyers wrote: > > However, this does not deal with mm_cpumask update, and we cannot use > the per-cpu rq lock, as it's a process-wide data structure updated with > clear_bit/set_bit in switch_mm(). So at the very least, we would have to > add memory barriers in switch_mm() on some architectures to deal with > this.
I'd much rather have a "switch_mm()" is a guaranteed memory barrier logic, because quite frankly, I don't see how it ever couldn't be one anyway. It fundamentally needs to do at least a TLB context switch (which may be just switching an ASI around, not flushing the whole TLB, of course), and I bet that for 99% of all architectures, that is already pretty much guaranteed to be equivalent to a memory barrier.
It certainly is for x86. "mov to cr0" is serializing (setting any control register except cr8 is serializing). And I strongly suspect other architectures will be too.
Btw, one reason to strongly prefer "switch_mm()" over any random context switch is that at least it won't affect inter-thread (kernel or user-land) switching, including switching to/from the idle thread.
So I'd be _much_ more open to a "let's guarantee that 'switch_mm()' always implies a memory barrier" model than to playing clever games with spinlocks.
Linus
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