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Re: [patch] Fix threads vs. fork following
Daniel Jacobowitz wrote:
On Mon, Mar 22, 2004 at 12:12:18PM -0500, Daniel Jacobowitz wrote:
On Thu, Mar 11, 2004 at 01:08:40AM +0000, Michael Snyder wrote:
Hey Daniel,
Got a question concerning the code in
linux-nat.c::linux_handle_extended_wait.
You've got a PTRACE_EVENT_FORK event, and now you're going to call
waitpid. You pull a pid out of a list of stopped pids, and wait for
it using waitpid. In your comment, you explain that you don't have to
worry about the pid being a clone, because you didn't ask for pids in
the event mask.
But how is this affected by threads, especially NPTL threads?
I've got a fairly simple test-case (modified from pthreads.c,
I'll attach it), in which a child thread calls fork -- but gdb
apparently tries to wait on the main thread (or perhaps the most
recent event thread). Since that's not the thread that called
fork, waitpid returns -1 with "no child". Gdb reports:
waiting for new child: No child processes.
FWIW, I've tried this on both a single-processor and an SMP machine.
Here's what happened: I was using ptid_get_pid, which gave me the
_process_ ID rather than the _lwp_ ID. I've committed this fix for
HEAD. Should I fix this on the 6.1 branch also?
I don't see why not.
FYI, I've had feedback from one user who says this patch helped get
him past the point at which he was stuck before -- but he's seeing
a new problem which may or may not be related. He has one thread
which calls system("/bin/ls"), and system ("cd <...>"). He can now
get past at least the first few such calls under GDB, but eventually
(and non-deterministically), he sees something like:
Detaching after fork from child process 22087.
---Type <return> to continue, or q <return> to quit---
And then for some unknown percent of the times when the
above occurs, it goes more like this:
Detaching after fork from child process 21502.
Suspended (tty output)
[msnyder@reddwarf gdb]$ fg
/export/msnyder/03r1-2/bin/gdb ...
---Type <return> to continue, or q <return> to quit---
Detaching after fork from child process 21510.
Any thoughts?