Description of problem:
Glusterd do not response any request to its 24007 port

From glusterd process gdb, it can be seen the thread8 stucked at LOCK($iobref->lock), and thread9 stuck at waiting for signal priv->notify.cond which only thread8 will notify.  
So glusterd cannot response any request like mount command, cli request, glusterfsd startup.

Thread 9 (Thread 0x7f0855a25700 (LWP 1991)):
#0  0x00007f085c0485bc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007f085702edab in socket_event_poll_err (this=0x7f084c045bf0, gen=7, idx=7) at socket.c:1201
#2  0x00007f085703399c in socket_event_handler (fd=13, idx=7, gen=7, data=0x7f084c045bf0, poll_in=1, poll_out=0, poll_err=0)
    at socket.c:2480
#3  0x00007f085d2f65e9 in event_dispatch_epoll_handler (event_pool=0xf1db00, event=0x7f0855a24e84) at event-epoll.c:587
#4  0x00007f085d2f68c0 in event_dispatch_epoll_worker (data=0xf2c0c0) at event-epoll.c:663
#5  0x00007f085c0425da in start_thread () from /lib64/libpthread.so.0
#6  0x00007f085b918eaf in clone () from /lib64/libc.so.6

Thread 8 (Thread 0x7f0856226700 (LWP 1990)):
#0  0x00007f085c04b85c in __lll_lock_wait () from /lib64/libpthread.so.0
#1  0x00007f085c04e657 in __lll_lock_elision () from /lib64/libpthread.so.0
#2  0x00007f085d2c0ae6 in iobref_unref (iobref=0x7f08480033e0) at iobuf.c:944
#3  0x00007f085d046f29 in rpc_transport_pollin_destroy (pollin=0x7f0848047f10) at rpc-transport.c:123
#4  0x00007f0857033319 in socket_event_poll_in (this=0x7f084c045bf0, notify_handled=_gf_true) at socket.c:2322
#5  0x00007f0857033932 in socket_event_handler (fd=13, idx=7, gen=7, data=0x7f084c045bf0, poll_in=1, poll_out=0, poll_err=0)

The way to reproduce:  
Restart gluster server and clients in the same time, the reproduce ratio once in several hundred restarts 

analysis
The function socket_event_poll_in could can be called for same socket fd in the same time, that could be the reason why thread8 stuck at the place "LOCK($iobref->lock)" and iobref free in another thread just now.