From: Florian Weimer <fweimer@redhat.com>
To: Carlos O'Donell <carlos@redhat.com>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
gcc@gcc.gnu.org, libc-alpha@sourceware.org,
Iain Sandoe <iain@sandoe.co.uk>,
aburgess@redhat.com, lttng-dev@lists.lttng.org
Subject: Re: New TLS usage in libgcc_s.so.1, compatibility impact
Date: Mon, 15 Jan 2024 16:35:34 +0100 [thread overview]
Message-ID: <87jzoa6249.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <81279c5d-0b60-0e37-abe9-0936688b14fa@redhat.com> (Carlos O'Donell's message of "Mon, 15 Jan 2024 09:47:26 -0500")
* 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
next prev parent reply other threads:[~2024-01-15 15:35 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-13 12:49 Florian Weimer
2024-01-15 12:46 ` Szabolcs Nagy
2024-01-15 13:55 ` Adhemerval Zanella Netto
2024-01-15 14:47 ` Carlos O'Donell
2024-01-15 15:35 ` Florian Weimer [this message]
2024-01-15 15:38 ` Iain Sandoe
2024-01-15 16:44 ` Florian Weimer
2024-01-15 16:29 ` Joseph Myers
2024-01-15 19:05 ` [lttng-dev] " Mathieu Desnoyers
2024-01-15 19:42 ` Florian Weimer
2024-01-15 20:08 ` Mathieu Desnoyers
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