Re: Multithreaded locks.c

[Andrew Morton <andrewm@uow.edu.au>]

Alan Cox wrote:
> 
> > have got it right.  Does anyone know what this part of the
> > flock(2) manpage means?
> >
> >        A  single file may not simultaneously have both shared and
> >        exclusive locks.
> 
> AFAIK its saying LOCK_EX is exclusive and blocks shared locks and vice
> versa.  Its a standard reader-writer lock setup. LOCK_EX is writer, LOCK_SH
> is reader.

Thanks, Alan.  You're right, and this is indeed how the current
implementation behaves.  The DG/UX manpage makes a bit more sense
here.

One interesting factoid: if a process currently has an exclusive
lock, and it changes that to an exclusive lock (ie: no change),
this has the effect of dropping the lock and reclaiming it.

The DG/UX dg_flock() departs from BSD in that "Upgrading a lock 
will be an atomic operation.  That is, a lock is not released
to upgrade that lock".  Linux doesn't do that - the lock is
effectively dropped and reclaimed when we go from LOCK_SH to LOCK_EX.

Even the DG/UX manpage doesn't say what happens when you sidegrade
the lock.  LOCK_EX->LOCK_EX :)
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