Re: MT-safe annotations for gcvt and related functions

[Alexandre Oliva <aoliva at redhat dot com>]

On Dec 16, 2014, Florian Weimer <fweimer@redhat.com> wrote:

> On 12/16/2014 08:25 PM, Alexandre Oliva wrote:
>> AFAICT, whereas __printf_fp does access _NL_CURRENT multiple times, but
>> they all access the same locale object, because they all use the result
>> of dereferencing the pointer to the current locale a single time, as a
>> result of compiler optimization, and as such it doesn't mandate the
>> âlocaleâ annotation, although it certainly wouldn't hurt to add it.

> Interesting, I wasn't aware that the rules for these annotations work
> out that way.  I thought all access to the global locale object were
> racy.

Here's a patch that appends some more comments to the definition of
locale discussing this point.

Ok to install?


for ChangeLog

	* manual/intro.texi (locale): Expand, in comments, rationale
	on not marking single dereferences of the locale pointer.
---
 manual/intro.texi |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/manual/intro.texi b/manual/intro.texi
index d4045f2..6cd5776 100644
--- a/manual/intro.texi
+++ b/manual/intro.texi
@@ -730,6 +730,22 @@ constant in these contexts, which makes the former safe.
 @c locale multiple times may invoke all sorts of undefined behavior
 @c because of the unexpected locale changes.
 
+@c The âmultiple timesâ above is relevant.  Functions that dereference
+@c the locale pointer only once, even if to access the locale object
+@c multiple times (_NL_CURRENT's design enables this sort of compiler
+@c optimization, and current compilers can be counted on to perform
+@c them), will behave consistently with that one locale object they
+@c reference throughout.  So, we will generally not annotate such
+@c functions with the @code{locale} keyword, even though calling such a
+@c function multiple times in sequence will not ensure the same locale
+@c object is used.
+
+@c We have to take care, however, of our functions that *internally*
+@c call such functions multiple times: these have to be marked with
+@c locale, because each internal call may end up using a different
+@c locale and thus the internal calling function will behave
+@c inconsistently overall.
+
 
 @item @code{env}
 @cindex env


-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer