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

Florian Weimer fweimer at redhat.com
Mon Jan 15 14:42:58 EST 2024

* Mathieu Desnoyers:

> On 2024-01-13 07:49, Florian Weimer via lttng-dev wrote:
>> This commit
>> commit 8abddb187b33480d8827f44ec655f45734a1749d
>> Author: Andrew Burgess <andrew.burgess at embecosm.com>
>> Date:   Sat Aug 5 14:31:06 2023 +0200
>>      libgcc: support heap-based trampolines
>>           Add support for heap-based trampolines on x86_64-linux,
>> aarch64-linux,
>>      and x86_64-darwin. Implement the __builtin_nested_func_ptr_created and
>>      __builtin_nested_func_ptr_deleted functions for these targets.
>>           Co-Authored-By: Maxim Blinov <maxim.blinov at embecosm.com>
>>      Co-Authored-By: Iain Sandoe <iain at sandoe.co.uk>
>>      Co-Authored-By: Francois-Xavier Coudert <fxcoudert at gcc.gnu.org>
>> added TLS usage to libgcc_s.so.1.  The way that libgcc_s is
>> currently
>> built, it ends up using a dynamic TLS variant on the Linux targets.
>> This means that there is no up-front TLS allocation with glibc (but
>> there would be one with musl).
> Trying to wrap my head around this:
> If I get this right, the previous behavior was that glibc did allocate
> global-dynamic variables from libraries which are preloaded and loaded
> on c startup as if they were initial-exec, but now that libgcc_s.so.1
> has a dynamic TLS variable, all those libraries loaded on c startup that
> have global-dynamic TLS do not get the initial allocation special
> treatment anymore. Is that more or less correct ?

Ahh.  I had forgotten about this aspect.  The allocation from the static
TLS area still happens as before.

> I've prepared a change for lttng-ust to move the lttng-ust libc wrapper
> "malloc nesting" guard variable from global-dynamic to initial-exec:
> https://review.lttng.org/c/lttng-ust/+/11677 Fix: libc wrapper: use initial-exec for malloc_nesting TLS

I don't know if this is completely sufficient if there are other TLS
variables in the lttng stack.

> This should help for the infinite recursion issue, but if my understanding
> is correct about the impact of effectively changing the behavior used
> for global-dynamic variables in preloaded and on-startup-loaded libraries
> introduced by this libgcc change, I suspect we have other new issues here,
> such as problems with async-signal safety of other global-dynamic variables
> within LTTng-UST.

This didn't change, and the allocation is not done lazily (contrary to
what I might have written before).  But even on the __tls_get_addr fast
path, we check the TLS generation counter, and if that has changed, we
do extra bookkeeping work.  TLS usage in libgcc_s.so.1 means that in the
now-failing test, the generation counter changed.  Before bug 19924

  TLS performance degradation after dlopen 

was fixed, we did not do this bookkeeping work, which is why the problem
didn't occur.

General use of lttng should be fine, I think, only the malloc wrapper
has this problem.

> But moving all TLS variables used by lttng-ust from global-dynamic to
> initial-exec is tricky, because a prior attempt to do so introduced
> regressions in use-cases where lttng-ust was dlopen'd by Java or
> Python, AFAIU situations where the runtimes were already using most of
> the extra memory pool for dlopen'd libraries initial-exec variables,
> causing dlopen of lttng-ust to fail.

Oh, right, that makes it quite difficult.  Could you link a private copy
of the libraries into the wrapper that uses initial-exec TLS?


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