Messages in this thread | | | Date | Sat, 28 Nov 2009 09:30:18 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [rfc] "fair" rw spinlocks |
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On Mon, 23 Nov 2009, Nick Piggin wrote: > > Last time this issue came up that I could see, I don't think > there were objections to making rwlocks fair, the main > difficulty seemed to be that we allow reentrant read locks > (so a write lock waiting must not block arbitrary read lockers).
We have at least one major rwlock user - tasklist_lock or whatever. And that one definitely depends on being able to do 'rwlock()' in an interrupt, without other rwlock'ers having to disable irq's (even if there might be a new writer coming in on another cpu).
That usage case _might_ be turned into RCU or something similar, in which case I don't think any major rwlock users remain. However, if that's the case, then why should anybody care about fairness any more either?
So as far as I can tell, we have only one real user of rwlocks where livelocks might be relevant, but that one real user absolutely _requires_ the unfair behavior.
Linus
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