Messages in this thread | | | Date | Mon, 30 Nov 2009 14:49:10 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [rfc] "fair" rw spinlocks |
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On Mon, 30 Nov 2009, Thomas Gleixner wrote: > > I'm aware of that. The number of places where we read_lock > tasklist_lock is 79 in 36 files right now. That's not a horrible task > to go through them one by one and do a case by case conversion with a > proper changelog. That would only leave the write_lock sites.
The write_lock sites should be fine, since just changing them to a spinlock should be 100% semantically equivalent - except for the lack of interrupt disable. And the lack of interrupt disable will result in a nice big deadlock if some interrupt really does take the spinlock, which is much easier to debug than a subtle race that would get the wrong read value.
> We can then either do the rw_lock to spin_lock conversion or keep the > rw_lock which has no readers anymore and behaves like a spinlock for a > transition time so reverts of one of the read_lock -> rcu patches > could be done to debug stuff.
So as per the above, I wouldn't worry about the write lockers. Might as well change it to a spinlock, since that's what it will act as. It's not as if there is any chance that the spinlock code is subtly buggy.
So the only reason to keep it as a rwlock would be if you decide to do the read-locked cases one by one, and don't end up with all of them converted. Which is a reasonable strategy too, of course. We don't _have_ to convert them all - if the main problem is some starvation issue, it's sufficient to convert just the main read-lock cases so that writers never get starved.
But converting it all would be nice, because that whole
write_lock_irq(&tasklist_lock);
to
spin_lock(&tasklist_lock);
conversion would likely be a measurable performance win. Both because spinlocks are fundamentally faster (no atomic on unlock), and because you get rid of the irq disable/enable. But in order to get there, you'd have to convert _all_ the read-lockers, so you'd miss the opportunity to only convert the easy cases.
Linus
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