Re: [BUG] long freezes on thinkpad t60

[Chuck Ebbert]

On 06/21/2007 12:08 PM, Ingo Molnar wrote:
> yeah - i'm not at all arguing in favor of the BTRL patch i did: i always 
> liked the 'nicer' inner loop of spinlocks, which could btw also easily 
> use MONITOR/MWAIT.

The "nice" inner loop is necessary or else it would generate huge amounts
of bus traffic while spinning.

> So it seems the problem was that if a core kept _truly_ modifying a 
> cacheline via atomics in a high enough frequency, it could artificially 
> starve the other core. (which would keep waiting for the cacheline to be 
> released one day, and which kept the first core from ever making any 
> progress) To me that looks like a real problem on the hardware side - 
> shouldnt cacheline ownership be arbitrated a bit better than that?
> 

A while ago I showed that spinlocks were a lot more fair when doing
unlock with the xchg instruction on x86. Probably the arbitration is all
screwed up because we use a mov instruction, which while atomic is not
locked.

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