Try upgrades and downgrades for POSIX rwlocks

[Attilio Rao]

2011/1/24 John Baldwin <jhb at freebsd.org>:
> Does anyone know if there is a de facto or proposed standard for supporting
> upgrades and downgrades in POSIX rwlocks?  IBM seems to support something
> rather gross where a wrlock() will succeed if the only shared lock is held by
> the current thread.  But then the thread holds both a read and write lock, and
> it has to call unlock twice, the first to drop the write lock, the second to
> drop the read lock.  If we were to add support for upgrades and downgrades I
> would prefer something more along the lines of our in-kernel APIs where there
> are try_upgrade() and downgrade() operations that convert a given lock between
> states.

I'd support us adopting the same semantic rwlock in kernel space have.
An alternative semantic would be what lockmgrs do:
- try to upgrade
- if failed (more than one shared owner then):
  * drop our shared lock
  * try a normal exclusive acquisition

That seems like more linear, even if I'm very much more in favor of
not having something like that (infact, removing LK_UPGRADE is one of
my personal target, as such operation can be implemented in consumers
themselves as well, if they want).

Attilio


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