Re: [Libunwind-devel] Re: [patch 2/2] Allow caller to block signals.

[Mark Rabkin]

Paul, Arun -- thank you both for the clarifications, they were very helpful.


On 9/25/09 1:11 PM, "Paul Pluzhnikov" <address@hidden> wrote:

> On Fri, Sep 25, 2009 at 12:00 PM, Mark Rabkin <address@hidden> wrote:
> 
>> I read this:
>> http://lists.nongnu.org/archive/html/libunwind-devel/2009-09/msg00000.html
>> 
>> But I was a little unclear on the main motivation.
>> 
>> Was the motivation that the existing signal blocking mechanism didn't
>> prevent re-entry?  If it didn't prevent it, did it cause crashes or aborted
>> stack traces or what was the symptom?
> 
> Existing mechanism did prevent re-entry, but at a cost of lots of
> sigprocmask()s. We have one particular (emulated) environment, where these
> are quite costly.
> 
>> If you're now planning to block additional signals in your own client code
>> ("at the top level"),
> 
> We don't actually need to block them, we just need to make sure that the
> same thread doesn't reenter libunwind from a signal handler....
> 
>> are you removing the calls inside libunwind to ensure
>> it doesn't unblock them (as it does SETMASK),
> 
> The remaining sigprocmask calls are following this pattern:
> 
>   sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
>   ret = dl_iterate_phdr (check_callback, as);
>   sigprocmask (SIG_SETMASK, &saved_mask, NULL);
> 
> so (AFAICT) they wouldn't unblock anything that was blocked before.
> 
> But I see now that I should have eliminated these remaining sigprocmasks
> as well. Patch attached.
> 
>> or are you just removing them
>> for performance since they're unnecessary as your initial message implies?
> 
> Yes.
> 
>> Finally, I just wanted to get a clarification on the effects of linking
>> libunwind -- it was my impression that linking it may also affect the stack
>> unwinding done when, for instance, a C++ exception is thrown.
> 
> Not by default. In default configuration on x86_64, I get:
> 
> nm -D src/.libs/libunwind.so | grep ' T ' | grep -v ' _U'
> 0000000000001380 T backtrace
> 
> So only calls to (glibc) backtrace would be affected.
> 
> Cheers,