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Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
- From: Paul Pluzhnikov <ppluzhnikov at google dot com>
- To: Ian Lance Taylor <iant at google dot com>
- Cc: Roland McGrath <roland at hack dot frob dot com>, Richard Henderson <rth at twiddle dot net>, libc-alpha at sourceware dot org, Andrew Hunter <ahh at google dot com>
- Date: Wed, 18 Sep 2013 16:26:23 -0700
- Subject: Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
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Resurrecting an old thread ... which began here:
We really need a TLS that is both async-signal safe, and can be used from
dlopen()ed shared library.
On Wed, Jun 13, 2012 at 6:08 PM, Ian Lance Taylor <firstname.lastname@example.org> wrote:
> We are currently in an unpleasant situation where it is very easy and
> natural to use TLS variables--you just refer to them by name--and using
> them in a signal handler almost always works just fine. Except that in
> some highly specific but not completely implausible circumstances it
> crashes incomprehensibly. This is not a good thing, it's a lurking time
> Perhaps another approach would be to change __tls_get_addr to not use
> malloc, but to use mmap as needed. This of course assumes that mmap is
> in fact async signal safe although it is not on the approved list.
Andrew Hunter has proposed a Google-local glibc patch, that
- introduces async-signal-safe mmap-based allocator into elf/dl-misc.c, and
- updates the rest of the loader to use this allocator when getting space
for non-static TLS.
The allocator is currently quite simple, but wastes space. It could be
made more space-efficient independently of other changes.
Is this something that has a chance of being acceptable into trunk?