Messages in this thread | | | Date | Mon, 30 Nov 2009 18:13:33 +0100 | From | Nick Piggin <> | Subject | Re: [rfc] "fair" rw spinlocks |
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On Mon, Nov 30, 2009 at 09:05:57AM -0800, Linus Torvalds wrote: > > > On Mon, 30 Nov 2009, Paul E. McKenney wrote: > > > > My suggestion would be to put the nesting counter in the task structure > > to avoid this problem. > > It still doesn't end up being all that cheap. Now you'd need to disable > preemption in order to fix the race between the local counter and the real > lock. > > That should be cheaper than cli/sti, but the downside is that now you need > that task struct pointer (both for the preemption disable and the > counter), so now you're adding some register pressure too. Of course, > maybe you don't even want to inline it anyway, in which case that doesn't > matter. > > One advantage with your suggestion of using preemption is that (unlike irq > disables) you can keep the preemt counter over the whole lock, so you > don't need to re-do the preempt disable/enable in both read-lock and > read-unlock. > > So you might end up with something like (UNTESTED!): > > static void tasklist_write_lock(void) > { > spin_lock_irq(&tasklist_lock); > } > > static void tasklist_write_unlock(void) > { > spin_unlock_irq(&tasklist_lock); > }
Two questions. Firstly, does tasklist_lock benefit much from read side paralellism? Looking at some of the critical sections some seem to hold it for quite a while (over task and thread iterations). So it might not be the right thing to convert it to a spinlock?
Secondly:
> static void tasklist_read_lock(void) > { > preempt_disable(); > if (!current->tasklist_count++)
What happens if an interrupt and nested tasklist_read_lock() happens here?
> spin_lock(&tasklist_lock); > } > > static void tasklist_read_unlock(void) > { > if (!--current->tasklist_count) > spin_unlock(&tasklist_lock); > preempt_enable(); > } > > And the upside, of course, is that a spin_unlock() is cheaper than a > read_unlock() (no serializing atomic op), so while there is overhead, > there are also some advantages.. Maybe that atomic op advantage is enough > to offset the extra instructions. > > And maybe we could use 'raw_spin_[un]lock()' in the above read-[un]lock > sequences, since tasklist_lock is pretty special, and since we do the > preempt disable by hand (no need to do it again in the spinlock code). > That looks like it might cut down the overhead of all of the above to > almost nothing for what is probably the common case today (ie preemption > enabled). > > Linus
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