RE: [PATCH] disable lock annotation with clang 3.4.2

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Monday, 13 February 2023 15.55
> 
> On Mon, Feb 13, 2023 at 03:44:55PM +0100, David Marchand wrote:
> > Venerable RHEL7 clang 3.4.2 has (at least) two issues with lock
> > annotations.
> >
> > A first one with regards to the attribute position:
> > ../lib/vhost/vhost.h:518:2: error: GCC does not allow
> > 	assert_exclusive_lock attribute in this position on a
> > 	function definition [-Werror,-Wgcc-compat]
> >         __rte_assert_exclusive_lock(&vq->access_lock)
> >         ^
> > ../lib/eal/include/rte_lock_annotations.h:29:38: note: expanded
> > 	from macro '__rte_assert_exclusive_lock'
> >         __attribute__((assert_exclusive_lock(__VA_ARGS__)))
> >                                             ^
> >
> > This can be worked around by splitting and having the allocation on
> the
> > function declaration.
> >
> > But on the other hand, clang 3.4.2 does not seem to propagate those
> > annotations in presence of a __builtin_expect (i.e. unlikely()), like
> > for example when calling if (unlikely(rte_spinlock_trylock() == 0)).
> >
> > Those annotations were only working with clang in any case, so
> restrict
> > to clang versions newer than 3.5.0.
> >
> > Fixes: 657a98f38940 ("eal: annotate spinlock, rwlock and seqlock")
> >
> > Signed-off-by: David Marchand <david.marchand@redhat.com>
> > ---
> >  drivers/meson.build | 2 +-
> >  lib/meson.build     | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/meson.build b/drivers/meson.build
> > index bddc4a6cc4..0618c31a69 100644
> > --- a/drivers/meson.build
> > +++ b/drivers/meson.build
> > @@ -168,7 +168,7 @@ foreach subpath:subdirs
> >          enabled_drivers += name
> >          lib_name = '_'.join(['rte', class, name])
> >          cflags += '-DRTE_LOG_DEFAULT_LOGTYPE=' +
> '.'.join([log_prefix, name])
> > -        if annotate_locks and cc.has_argument('-Wthread-safety')
> > +        if annotate_locks and cc.get_id() == 'clang' and
> cc.version().version_compare('>=3.5.0')
> >              cflags += '-DRTE_ANNOTATE_LOCKS'
> >              cflags += '-Wthread-safety'
> >          endif
> 
> Are we likely to see any issues with this with any other compilers?
> Should
> we look to do a built-test in meson to determine feature support rather
> than checking clang versions explicitly?
> 
> On the plus side, checking clang version like this makes it clear when
> we
> can drop the conditional.

I prefer this over auto-detection in meson, for the reason mentioned by Bruce.

Furthermore, different compilers may use different syntax in the code, so it is impossible to detect generically; the auto-detection would be per compiler anyway.

Acked-by: Morten Brørup <mb@smartsharesystems.com>