On 07/05/2013 08:00 AM, Laine Stump wrote:
On 07/04/2013 01:43 AM, Caizhifeng wrote:
Hi ALL,
In order to catch the calltrace of deadlock in libvirtd, I modified the function virMutexLock as follows:
But, unfortunatly, sometimes, deadlock happened in child process after virFork,
The problem is that backtrace() is not "async signal safe". The section "Async-signal-safe functions" of "man 7 signal" explains what this is in the context of a signal handler, but the conditions in a child process just after fork() are really the same - a lock was acquired in one thread of the parent, then *while that lock is being held* a different thread of the parent calls fork(), which duplicates all of the process' memory (including the lock) then creates a new process. In the new process, the lock comes into existence marked as being held, but there is no thread to unlock it, so when the child attempts to acquire the lock, it waits forever.
FYI, I'm in the middle of fixing such a bug in our usage of virSetUIDGID, which is another instance of calling a non-async-safe function after fork. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org