Re: segfault on x86_64 using musl libc

  • From: John Spencer <maillist-luajit@xxxxxxxxxxx>
  • To: luajit@xxxxxxxxxxxxx, Mike Pall <mike-1207@xxxxxxxxxx>
  • Date: Tue, 17 Jul 2012 02:46:23 +0200

On 07/16/2012 04:05 PM, Mike Pall wrote:
Apparently musl doesn't set up thread-local storage correctly.

we had a discussion on the musl mailing list about the TLS issue,
and it turned out that I mistakenly built gcc without --disable-tls.

(musl currently doesn't support TLS for various reasons.)

with gcc fixed, luajit seems to work fine (at least for singlethreaded usage).

Rich came up with a clever portable (POSIX) way that could be used instead
of the existing non-threadsafe fallback code for OSX/OpenBSD/non-TLS.

please see forwarded Mail content below.

From:     Rich Felker <d...@xxxxxxxxxx>
Subject:     Re: [musl] thread local storage

On Mon, Jul 16, 2012 at 02:57:22PM -0400, Gregor Richards wrote:
> On 07/16/2012 03:02 PM, John Spencer wrote:
> >2 out of 14 sabotage followers wanted to use a musl-based system
> >as a platform for luajit (and then were never seen again).
> >so i looked into adding it...
> >
> >luajit builds without problems on musl, but then crashes due to a
> >lack of TLS.
> >
> >is it planned to add this feature ? iirc it wasn't mentioned on
> >the latest roadmap...
> >
> >
> >
> With a quick perusal of the LuaJIT source, this is the only instance
> of TLS I see:
> #if LJ_TARGET_OSX || defined(__OpenBSD__)
> /* Sorry, no thread safety for OSX. Complain to Apple, not me. */
> static _Unwind_Exception static_uex;
> #else
> static __thread _Unwind_Exception static_uex;
> #endif
> Convince it to use the same exception as OS X and OpenBSD and you
> should be in business.

This is broken and non-thread-safe. Not a good idea. Instead try:

#define static_uex (*(_Unwind_Exception *)pthread_getspecific(static_uex_key))

where static_uex_key is a pthread_key_t initialized earlier with:
pthread_key_create(&static_uex_key, 0);

And where the thread-specific value of the key is set in thread
startup as:
_Unwind_Exception static_uex_local;
pthread_setspecific(static_uex_key, &static_uex_local);

The simplicity and generality of this solution is why __thread was
just a stupid idea to begin with...


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