Re: gdb and multi-threaded (NPTL) programs

[John Fodor <john_fodor@mac.com>]

Daniel Jacobowitz wrote:
> On Fri, Mar 24, 2006 at 02:57:19PM -0500, John Fodor wrote:
> 
>>"There is an unfortunate side effect.  If one thread stops for a
>>breakpoint, or for some other reason, and another thread is blocked in a
>>system call, then the system call may return prematurely.  This is a
>>consequence of the interaction between multiple threads and the signals
>>that GDB uses to implement breakpoints and other events that stop
>>execution.
> 
> 
> Really, in my opinion, it's a kernel bug; the syscall should
> automatically restart in this case.  Some syscalls now do that, on
> current kernels.  Others don't.  It's hard to fix this without breaking
> them in other ways.
> 
> In this case the syscall is sys_futex.  When interrupted, futex_wait
> returns -EINTR.  This is documented to happen whether the signal was
> handled or not.  Maybe adding a fifth signal restart option to the
> existing four in the Linux kernel could fix this: ERESTARTNOSIGNAL.
> That wouldn't be hard to implement if you want to try it.  You'd have
> to do some thinking about the semantics of futexes to make sure it was
> safe.

Sounds like a good idea. Let me know how it goes :)

> 
> 
>>Hmmm... so people who use POSIX threads have to put every syscall into a 
>>loop, ignoring EINTR? What if it's a real timeout? Sorry this does not 
>>seem reasonable to me.
> 
> 
> Let's be precise here: "what if it's a real signal".  sem_wait does not
> time out.  sem_timedwait returns ETIMEDOUT, not EINTR, for timeouts.

I wasn't referring to sem_wait specifically. I was thinking of any 
general syscall that will return EINTR after a SIGALARM. But you're 
right, could be some other signal.

> 
> 
>>Will there be a fix in the future to this unfortunate side-effect? How 
>>do NPTL programmers single-step their programs today? Using syscalls in 
>>loops? Using a different debugger?
> 
> 
> In practice this does not bother most programmers.  If your application
> uses signals, it often needs to do this anyway!
> 

If you use signals you can set SA_RESTART for catchable signals. What 
we're talking about here is a syscall wrapper just so we can single-step 
with gdb. Lot's of S/W examples don't have these wrappers. Anyway I do 
appreciate you help. Thanks.

Can someone send me a nice wrapper macro?