Debug a futex lock

Question

I have a process waiting on a futex:

# strace -p 5538
Process 5538 attached - interrupt to quit
futex(0x7f86c9ed6a0c, FUTEX_WAIT, 20, NULL

How can I best debug such a situation? Can I identify who holds the futex? Are there any tools similar to ipcs and ipcrm but for futexes?

Answer 1

Try using gdb -p *PID* and then run where or bt to see a backtrace.

It won't be spectacularly useful with binaries and libraries that have had their debugging symbols stripped, but you may be able to deduce a fair bit from the context. It might be able to indicate to you which part of a complex process is hanging, and then you could examine the right part of the sources to search for the lock.