The ptrace_start function in kernel/ptrace.c in the Linux kernel 2.6.18 does not properly handle simultaneous execution of the do_coredump function, which allows local users to cause a denial of service (deadlock) via vectors involving the ptrace system call and a coredumping thread.
https://bugzilla.redhat.com/show_bug.cgi?id=504263
https://bugzilla.redhat.com/attachment.cgi?id=346742
https://bugzilla.redhat.com/attachment.cgi?id=346615
http://www.vmware.com/security/advisories/VMSA-2009-0016.html