Re: [Threads-devel] read/write locks and pthreads

From: Anthony Williams (anthony_at_[hidden])
Date: 2007-04-12 08:06:13

Russell Kliese <russell_at_[hidden]> writes:

> I've been trying to come up with a timed read/write lock for Linux to 
> replace the implementation that was pulled from Boost. After looking 
> over some of the messages is the mailing list, I decided to investigate 
> wrapping up pthreads.

Did you not get the new version I posted? I've attached it again in case it
went astray.

> __rw_readers has to be zero, therefore it looks like the read lock must 
> be released before obtaining a write lock (when a write lock is 
> obtained, this value is incremented). This seems bad to me as it makes 
> it tricky to handle upgrading and downgrading locks.
>
> Am I missing something here?

Yes. POSIX does not allow upgrading and downgrading --- a read lock is always
a read lock, and a write lock is always a write lock. The latest POSIX spec
says about pthread_rwlock_rdlock:

    "The calling thread may deadlock if at the time the call is made it holds
    a write lock."

and about pthread_rwlock_wrlock:

    "The calling thread may deadlock if at the time the call is made it holds
    the read-write lock (whether a read or write lock)."

Anthony
-- 
Anthony Williams
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