Messages in this thread | | | Date | Fri, 8 Nov 2002 09:25:24 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [Linux-ia64] reader-writer livelock problem |
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On 8 Nov 2002, Jeremy Fitzhardinge wrote: > > The normal way of solving this fairness problem is to make pending write > locks block read lock attempts, so that the reader count is guaranteed > to drop to zero as read locks are released. I haven't looked at the > Linux implementation of rwlocks, so I don't know how hard this is to > do. Or perhaps there's some other reason for not implementing it this > way?
There's another reason for not doing it that way: allowing readers to keep interrupts on even in the presense of interrupt uses of readers.
If you do the "pending writes stop readers" approach, you get
cpu1 cpu2
read_lock() - get
write_lock_irq() - pending
irq happens - read_lock() - deadlock
and that means that you need to make readers protect against interrupts even if the interrupts only read themselves.
NOTE! I'm not saying the existing practice is necessarily a good tradeoff, and maybe we should just make sure to find all such cases and turn the read_lock() calls into read_lock_irqsave() and then make the rw-locks block readers on pending writers. But it's certainly more work and cause for subtler problems than just naively changing the rw implementation.
Linus
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