From: Florian Weimer <fweimer@redhat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: gcc@gcc.gnu.org, libc-alpha@sourceware.org,
Iain Sandoe <iain@sandoe.co.uk>,
aburgess@redhat.com, lttng-dev@lists.lttng.org,
Szabolcs Nagy <szabolcs.nagy@arm.com>
Subject: Re: [lttng-dev] New TLS usage in libgcc_s.so.1, compatibility impact
Date: Mon, 15 Jan 2024 20:42:58 +0100 [thread overview]
Message-ID: <87ply24c3h.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <1c32a469-9bef-4b04-9696-0f875bb3727f@efficios.com> (Mathieu Desnoyers's message of "Mon, 15 Jan 2024 14:05:32 -0500")
* Mathieu Desnoyers:
> On 2024-01-13 07:49, Florian Weimer via lttng-dev wrote:
>> This commit
>> commit 8abddb187b33480d8827f44ec655f45734a1749d
>> Author: Andrew Burgess <andrew.burgess@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@embecosm.com>
>> Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>
>> Co-Authored-By: Francois-Xavier Coudert <fxcoudert@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
<https://sourceware.org/bugzilla/show_bug.cgi?id=19924>
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?
Thanks,
Florian
next prev parent reply other threads:[~2024-01-15 19:43 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
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 [this message]
2024-01-15 20:08 ` Mathieu Desnoyers
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