[Bug nptl/7057] pthread rwlock does not implement 'writer preferred' option

["carlos at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=7057

Carlos O'Donell <carlos at redhat dot com> changed:

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                 CC|                            |carlos at redhat dot com

--- Comment #3 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Steven Munroe from comment #2)
> Uli I think this question deserves a more complete explanation. 
> 
> This seems like a reasonable request. If you disagree then it would not hurt 
> to explain why.
> 
> There seems to be a valid concern related to reliable implementation of 
> recursive read locks with writer priority. As in this discusion:
> 
> http://sources.redhat.com/ml/libc-alpha/2000-01/msg00055.html
> 
> If this your concern then saying so would help resolve/close this issue.

I'm making a statement here that this is exactly the case. 

Adding support for PTHREAD_RWLOCK_PREFER_WRITER_NP is not possible without
serious negative consequences.

It is not a bug, it is simply a conflicting requirement between
PTHREAD_RWLOCK_PREFER_WRITER_NP (a non-portable extension) and POSIX that
cannot be satisfied without significant difficulty, and because of that we
don't support PTHREAD_RWLOCK_PREFER_WRITER_NP (but can't get rid of it for ABI
reasons), instead we have an additional constant
PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP which ensures the readers are
non-recursive and thus writer preference can be implemented without the obvious
deadlock.

I am rewriting the Linux man-pages document to state this, which was the
decision made on the mailing list but was never followed up.

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