Messages in this thread | | | Date | Mon, 7 Dec 2009 10:18:16 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [rfc] "fair" rw spinlocks |
| |
On Sat, Dec 05, 2009 at 07:12:28PM -0800, Eric W. Biederman wrote: > 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.
Is it required that all of the processes see the signal before the corresponding interrupt handler returns? (My guess is "no", which enables a trick or two, but thought I should ask.)
> The trouble as I recall is how to ensure new processes see the signal.
And can we afford to serialize signals to groups of processes? Not necessarily one at a time, but a limited set at a given time? Alternatively, a long list of pending group signals for each new task to walk?
Thanx, Paul
| |