[libvirt] [PATCH 1/3] virthread: Introduce virRWLockInitPreferWriter

[Daniel P. Berrange]

On Mon, Jul 24, 2017 at 01:12:59PM -0400, John Ferlan wrote:
> 
> 
> On 07/19/2017 10:31 AM, Michal Privoznik wrote:
> > We already have virRWLockInit. But this uses pthread defaults
> > which prefer reader to initialize the RW lock. This may lead to
> > writer starvation. Therefore we need to have the counterpart that
> > prefers writers. Now, according to the
> > pthread_rwlockattr_setkind_np() man page setting
> > PTHREAD_RWLOCK_PREFER_WRITER_NP attribute is no-op. Therefore we
> > need to use PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
> > attribute. So much for good enum value names.
> > 
> > Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> > ---
> >  src/libvirt_private.syms |  1 +
> >  src/util/virthread.c     | 35 +++++++++++++++++++++++++++++++++++
> >  src/util/virthread.h     |  1 +
> >  3 files changed, 37 insertions(+)
> > 
> 
> This has broken the CI build, freebsd is not happy:
> 
> ../../src/util/virthread.c:133:42: error: use of undeclared identifier
> 'PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP'
>     pthread_rwlockattr_setkind_np(&attr,
> PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
>                                          ^
> 1 error generated.

It is not just FreeBSD, it also breaks OS-X and Win32.

The suffix '_np' / '_NP' is shorthand for nNon portable" - these
are glibc inventions. IMHO we should not really use this in our
code as if we're going to make assumptions that writers are not
starved as a result, because writers will be starved on any
non-Linux platform.

IOW, I think we need to just revert this.


Regards,
Daniel
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