Re: rwsem: down_read_unfair() proposal

[Michel Lespinasse <walken@google.com>]

On Wed, May 05, 2010 at 11:03:40AM +0100, David Howells wrote:
> If the system is as heavily loaded as you say, how do you prevent
> writer starvation?  Or do things just grind along until sufficient
> threads are queued waiting for a write lock?

Reader/Writer fairness is not disabled in the general case - it only is
for a few specific readers such as /proc/<pid>/maps. In particular, the
do_page_fault path, which holds a read lock on mmap_sem for potentially long
(~disk latency) periods of times, still uses a fair down_read() call.
In comparison, the /proc/<pid>/maps path which we made unfair does not
normally hold the mmap_sem for very long (it does not end up hitting disk);
so it's been working out well for us in practice.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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