[lttng-dev] New TLS usage in libgcc_s.so.1, compatibility impact

[Florian Weimer]

* Carlos O'Donell:

> I agree. TLS should be seen more like .bss/.data rather than something
> that is allocated with malloc().

There wasn't consensus regarding this in 2014.  See below.

> If we leak memory via TLS that is a glibc bug that we can deal with,

This is something that libgcc_s.so.1 does in GCC 14 if the heap
trampolines are used.

> but making it easier to find glibc bugs is also a benefit to the
> community, but not as valuable a benefit as making TLS correctly
> async-signal safe.
>
> Likewise we need to discuss when the memory is allocated, regardless
> of which allocator is used, including allocation up-front at dlopen()
> time.
>> [1] https://sourceware.org/pipermail/libc-alpha/2014-January/047931.html

The change conflated multiple issues: sanitizer support,
async-signal-safe TLS access, and eager allocation of all TLS-related
memory, so that subsequent accesses cannot fail.  My impression was the
main point of contention was eager allocation because it was perceived
as a breaking semantic change.  Nowadays, as long as we are willing to
maintain both allocator variants, we could offer a choice between them
controlled by a tunable.  For sanitizer compatibility, we could perform
eager allocation using malloc.  It's probably a good idea to do it this
way anyway because a separate mmap-based allocator would increase TLB
pressure.

Thanks,
Florian