Compatibility with GCC < 4.7 and ATOMICS about selinux HOT 6 CLOSED

Well, albeit slower, the use of atomics there could be removed and the existing mutex (spec->regex_compiled) be used within the mutex. Then their are no atomics. Not sure how badly that would affct performance, but I would wager it won't be noticeable.

from selinux.

Just using the mutex was deemed unacceptable for performance reasons by the Android team when the patch was first proposed that addressed the thread safety issues in selabel_lookup.
Not opposed to the use of syncs on older gcc.

from selinux.

Normal (non-atomic) accesses to 'spec->regex_compiled' would have equivalent effects to today's code.

Atomic accesses would only be needed if you were trying to implement thread-safe algorithm without any locks. But you're not; you're just doing a first pass "should we even attempt to lock?" filter. In the current code, the pre-lock accesses to regex_compiled will race whether atomic or not; it's the mutex that ensures safety.

from selinux.

Yes, I questioned the need for atomics in the original patch. At the time, the author of the patch said "I was thinking of that, but I figure I don't want to fix a multi threading issue by introducing a potential / even more subtle multi threading issue by relying on the compiler / architecture to do what I want. The atomics make it explicit that if compile_regex() returns success, that the regex is ready to be used, which isn't necessarily a guarantee that can be made across architectures without them."
If that's a bogus argument, then I'm open to a patch to get rid of it, but probably ought to include the original author on that as well.

from selinux.

I'm very very skeptical (see in particular POSIX 4.12 "Memory Synchronization"), but as an incremental improvement I'll just address the build issue.

from selinux.

I guess I'll submit to [email protected] too, but that list doesn't seem to have public archives so I wouldn't be able to include a link here.

0001-Fix-build-break-around-_atomic-with-GCC-4.7.patch.txt

from selinux.