Re: [Linux-ia64] Linux kernel deadlock caused by spinlock bug

[Matthew Wilcox <willy@debian.org>]

On Mon, Jul 29, 2002 at 03:37:17PM -0500, Van Maren, Kevin wrote:
> I changed the code to allow the writer bit to remain set even if
> there is a reader.  By only allowing a single processor to set
> the writer bit, I don't have to worry about pending writers starving
> out readers.  The potential writer that was able to set the writer bit
> gains ownership of the lock when the current readers finish.  Since
> the retry for read_lock does not keep trying to increment the reader
> count, there are no other required changes.

however, this is broken.  linux relies on being able to do

read_lock(x);
func()
  -> func()
       -> func()
            -> read_lock(x);

if a writer comes between those two read locks, you're toast.

i suspect the right answer for the contention you're seeing is an improved
get_timeofday which is lockless.

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