Messages in this thread | | | From | (Eric W. Biederman) | Date | Sat, 05 Dec 2009 19:12:28 -0800 | Subject | Re: [rfc] "fair" rw spinlocks |
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Linus Torvalds <torvalds@linux-foundation.org> writes:
> 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.
Atomically sending signal to every member of a process group, is the big fly in the ointment I am aware of. Last time I looked I could not see how to convert it rcu.
Fundamentally: "kill -KILL -pgrp" should be usable to kill all of the processes in a process group, and "kill -KILL -1" should be usable to kill everything except the sender and init. Something I have seen in shutdown scripts on more than one occasion.
This is a subtle in the sense that it won't show up in simple tests if you get it wrong.
This is a pain because we occasionally signal a process group from interrupt context.
The trouble as I recall is how to ensure new processes see the signal.
Eric
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