Messages in this thread | | | Date | Sun, 29 Nov 2009 10:51:22 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [rfc] "fair" rw spinlocks |
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On Sat, Nov 28, 2009 at 09:30:18AM -0800, Linus Torvalds wrote: > > > 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).
In other words, any fair rwlock must unconditionally grant read access to any CPU that already read-holds the lock, regardless of the state of any writer wannabees. Or is there another requirement I am missing?
> 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?
Indeed, RCU goes a step further, permitting new readers unconditionally, regardless of writer state. ;-)
> 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.
But the required unfairness is limited to unconditionally granting recursive read requests, right? If I understand correctly, if a given CPU does not already read-hold the lock, then we can safely make that CPU wait for a writer that might otherwise be starved. Again, is there another requirement that I am missing?
Thanx, Paul
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