NPTL pthread_cancel and libgcc_s.so.2 vs. libgcc_s.so.1 (Why the DSO version bump?)

Dave,

When I attempt to test syscall cancellation with NPTL threads the interface complains that:

libgcc_s.so.1 must be installed for pthread_cancel to work

I don't have this installed, instead I have libgcc_s.so.2 installed by gcc 4.x during the tls toolchain compile/install.

The code in question is nptl/sysdeps/pthread/unwind-resume.c, which does this:

---
static void
init (void)
{
void *resume, *personality;
void *handle;

  handle = __libc_dlopen ("libgcc_s.so.1");

  if (handle == NULL 
      || (resume = __libc_dlsym (handle, "_Unwind_Resume")) == NULL 
      || (personality = __libc_dlsym (handle, "__gcc_personality_v0")) == NULL)
    __libc_fatal ("libgcc_s.so.1 must be installed for pthread_cancel to work\n");

libgcc_s_resume = resume; libgcc_s_personality = personality;
}
---
or
---
void
pthread_cancel_init (void)
{
void *resume, *personality, *forcedunwind, *getcfa; void *handle;

if (__builtin_expect (libgcc_s_getcfa != NULL, 1))

    return; 

  handle = __libc_dlopen ("libgcc_s.so.1");

if (handle == NULL

      || (resume = __libc_dlsym (handle, "_Unwind_Resume")) == NULL 
      || (personality = __libc_dlsym (handle, "__gcc_personality_v0")) == NULL 
      || (forcedunwind = __libc_dlsym (handle, "_Unwind_ForcedUnwind"))
         == NULL 
      || (getcfa = __libc_dlsym (handle, "_Unwind_GetCFA")) == NULL 
#ifdef ARCH_CANCEL_INIT
      || ARCH_CANCEL_INIT (handle)
#endif
      )
    __libc_fatal ("libgcc_s.so.1 must be installed for pthread_cancel to work\n");

libgcc_s_resume = resume;
libgcc_s_personality = personality;
libgcc_s_forcedunwind = forcedunwind;
libgcc_s_getcfa = getcfa;
}
---

It wants to "forward" the local calls for _Unwind_Resume, __gcc_personality_v0, _Unwind_ForcedUnwind and _Unwind_GetCFA to those in ligcc_s.so.1.

The call sequence is like this:

(Enter cancellable syscall)
...
__libc_enable_asynccancel
__do_cancel

    __pthread_unwind
      (Calls _Unwind_GetCFA ...)

...
(Do the syscall)
...
__libc_disable_asynccancel
...
(Done)

This is called to enable unwinding out of a cancelled syscall. What was the reason for DSO version bump in libgcc_s.so.1?

c.

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It was done to distinguish between libgcc_s's built with sjlj and dwarf2 eh support, respectively. They use incompatible unwind routines and it's not possible to support both exception handling techniques with one library.

Dave

J. David Anglin                                  [email protected]
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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So libgcc_s.so.1 is for dwarf2?
While libgcc_s.so.2 is for sjlj?

Thus NPTL thread cancellation seems to requires dwarf2 eh support. How do I enable sjlj versus dwarf2 (--enable-dwarf2-exceptions?)

Our build script is here if you want to have a look:
http://cvs.parisc-linux.org/build-tools/build-tls-tools?rev=1.5&view=markup

c.

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