In the previous episode
Molnar had posted his own version of the mutex patch, adding a new
synchronization primitive to the kernel. Ingo has continued to refine this
patch set, with frequent releases; the current version is
. This patch set has faced
ongoing resistance from Andrew Morton, who didn't see the reasons for
adding a new mutual exclusion mechanism to the kernel. Andrew, instead,
wished that the developers would concentrate on fixing the problems with
the current semaphore code.
Perhaps the most significant development since then has been a private conversation between Andrew and
Ingo. There is, it seems, a plan in place which would replace the
current semaphore implementation entirely. Almost all current semaphore
users are implementing simple mutual exclusion areas, so they would be
converted over to the new mutex type directly. An estimated 90% of current
semaphore users fall into this category. Of the remaining users, about 90% employ
semaphores to indicate event completion. The task of converting those
users to the completion type
has been ongoing for some time; replacing semaphores would require
finishing this job. Finally, an estimated 1% of the semaphores in the
kernel are used for their counting feature; they can be converted over to a
(not yet posted) architecture-independent counter type.
Once all that work is done, semaphores could be removed from the kernel
altogether. Says Andrew: "It's a lot of churn, but we'll end up with
a better end result and a somewhat-net-simpler kernel, so I'm
happy." Linus, meanwhile, has offered some suggestions for
improvements (already incorporated by Ingo) and stated: "At that point I'd like to
switch to mutexes just because the code is cleaner!"
Since then, most of the discussion has been concerned with the details of
the mutex implementation rather than whether it is fundamentally a good
idea or not. The main objections would appear to have been overcome.
So, unless something new comes up, it looks like this change is going
to happen; the only question is "when." The next couple of weeks will
determine whether the mutex code will be part of 2.6.16 or not. Then all
that's left is the long task of converting all semaphore users over and,
finally, removing the old semaphore code.
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