Messages in this thread | | | Date | Wed, 23 May 2007 17:25:00 +0100 | From | Russell King <> | Subject | Re: [PATCH -rt] ARM TLB flush fix: don't forget to re-enable preemption |
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On Wed, May 23, 2007 at 09:13:57AM -0700, Kevin Hilman wrote: > On Wed, 2007-05-23 at 10:22 +0100, Russell King wrote: > > In which case shouldn't it be at the end of the function so it includes > > the write buffer handling as well? > > > > However, I think I agree with Daniel on this one. I don't see the point > > of the preempt_disable() here. > > Note that my patch simply adds an enable to match the disable added by > the -rt patch. I'm not sure where the disable originally came from, but > there are disable/enable pairs scattered throughout tlbflush.h in the > -rt patch. > > If this one isn't necessary, then the others probably are not either. > In most cases there are 2 mcr instructions inside the critical section. > One for the dsb() and the other for the actual function. > > Russell, is there a reason any of these sections should be atomic?
I don't see any reason for them to be - when switching to another process we'll generally do a full TLB flush anyway, so what's the point in making these flushes atomic?
Consider:
flush_tlb_page() first mcr - invalidates tlb single entry --- context switch, invalidates entire tlb, inc dsb --- something else runs --- context switch, invalidates entire tlb, inc dsb, again --- dsb
That context switch is harmless - we end up with the entire TLB being invalidated and a DSB following. Now consider:
flush_tlb_page() --- context switch, invalidates entire tlb, inc dsb --- something else runs --- context switch, invalidates entire tlb, inc dsb, again --- preempt_disable() first mcr - invalidates tlb single entry dsb preempt_enable()
Any difference? No. Without the preempt disable/enable fiddling? No.
flush_tlb_page() preempt_disable() first mcr - invalidates tlb single entry dsb preempt_enable() --- context switch, invalidates entire tlb, inc dsb --- something else runs --- context switch, invalidates entire tlb, inc dsb, again ---
Any difference? No. Without the preempt disable/enable fiddling? No.
In every case of a preemption occuring in the middle of a tlb operation, the ultimate result is identical irrespective of preempt control sprinkling.
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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