From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Florian Weimer <fweimer@redhat.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 15:08:33 -0500 [thread overview]
Message-ID: <7e0e21b4-94f8-4ca5-8caf-31d8864a4ace@efficios.com> (raw)
In-Reply-To: <87ply24c3h.fsf@oldenburg.str.redhat.com>
On 2024-01-15 14:42, Florian Weimer wrote:
> * Mathieu Desnoyers:
>
[...]
>
> General use of lttng should be fine, I think, only the malloc wrapper
> has this problem.
The purpose of the nesting counter TLS variable in the malloc wrapper
is to catch situations like this where a global-dynamic TLS access
(or any unexpected memory access done as a side-effect from calling
libc) from within LTTng-UST instrumentation would internally attempt to
call recursively into the malloc wrapper. In that nested case, we skip
the instrumentation and call the libc function directly.
I agree with your conclusion that only this nesting counter gating variable
actually needs to be initial-exec.
>
>> 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?
Unfortunately not easily, because by design LTTng-UST is meant to be a
singleton per-process. Changing this would have far-reaching impacts on
interactions with the LTTng-UST tracepoint instrumentation, as well as
impacts on synchronization between the LTTng-UST agent thread and
application calling fork/clone. Also AFAIR, the LTTng session daemon
(at least until recently) does not expect multiple concurrent
registrations from a given process.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
prev parent reply other threads:[~2024-01-15 20:08 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
2024-01-15 20:08 ` Mathieu Desnoyers [this message]
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