gdb backtrace and pthread_cond_wait()
-
12-09-2019 - |
Question
This is on a Redhat EL5 machine w/ a 2.6.18-164.2.1.el5 x86_64 kernel using gcc 4.1.2 and gdb 7.0.
When I run my application with gdb and break in while it's running, several of my threads show the following call stack when I do a backtrace:
#0 0x000000000051d7da in pthread_cond_wait ()
#1 0x0000000100000000 in ?? ()
#2 0x0000000000c1c3b0 in ?? ()
#3 0x0000000000c1c448 in ?? ()
#4 0x00000000000007dd in ?? ()
#5 0x000000000051d630 in ?? ()
#6 0x00007fffffffdc90 in ?? ()
#7 0x000000003b1ae84b in ?? ()
#8 0x00007fffffffdd50 in ?? ()
#9 0x0000000000000000 in ?? ()
Is this a symptom of a common problem?
Is there a known issue with viewing the call stack while waiting on a condition?
Solution
The problem is that pthread_cond_wait
is written in hand-coded assembly, and apparently doesn't have proper unwind descriptor (required on x86_64 to unwind the stack) in your build of glibc
. This problem may have recently been fixed here.
You can try to build and install the latest glibc (note: if you screw up installation, your machine will likely become unbootable; approach with extreme caution!), or just live with "bogus" stack traces from pthread_cond_wait
.
OTHER TIPS
Generally, synchronization is required when multiple threads share a single resource.
In such a case, when you interrupt the program, you'll see only 1 thread is running (i.e., accessing the resource) and other threads are waiting within pthread_cond_wait()
.
So I don't think pthread_cond_wait()
itself is problematic.
If your program hangs with deadlock or performance doesn't scale, it might be caused by pthread_cond_wait()
.
That looks like a corrupt stack trace to me
for example:
#9 0x0000000000000000 in ?? ()
There shouldn't be code at NULL