Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

[Linus Torvalds]

On Wed, 7 Jan 2009, Linus Torvalds wrote:
> > 
> > "Is get_task_struct() really that bad?"
> 
> Yes. It's an atomic access (two, in fact, since you need to release it 
> too), which is a huge deal if we're talking about a timing-critical 
> section of code.

There's another issue: you also need to lock the thing that gives you the 
task pointer in the first place. So it's not sufficient to do 
get_task_struct(), you also need to do it within a context where you know 
that the pointer is not going away _while_ you do it.

And with the mutexes clearing the ->owner field without even holding the 
spinlock, that is not a guarantee we can easily get any way. Maybe we'd 
need to hold the tasklist_lock or something.

		Linus
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