The core generated is:

wb_process_queue (frame=0x0, file=0x0, flush_all=0 '\0')
    at write-behind.c:1728
#1  0xb80d2cfa in wb_setattr_cbk (frame=Cannot access memory at address 0x8
) at write-behind.c:1074
#2  0xb7687246 in afr_setattr_unwind (frame=Cannot access memory at address 0x8
) at afr-inode-write.c:751
#3  0xb7688c98 in afr_setattr_wind_cbk (frame=Cannot access memory at address 0x8
) at afr-inode-write.c:812
#4  0xb76bd0c1 in client_setattr_cbk (frame=Cannot access memory at address 0x8
) at client-protocol.c:4703
#5  0xb76a9b25 in protocol_client_interpret (this=Cannot access memory at address 0x8
)   
    at client-protocol.c:5915
#6  0xb76a9ccf in protocol_client_pollin (this=Cannot access memory at address 0x8
) at client-protocol.c:6209
#7  0xb76b9c42 in notify (this=Cannot access memory at address 0x8
) at client-protocol.c:6328
#8  0xb809bd9f in xlator_notify (xl=Cannot access memory at address 0x8
#9  0xb6e48aed in socket_event_poll_in (this=0x0) at socket.c:713
#10 0xb6e48bab in socket_event_handler (fd=Cannot access memory at address 0x8
) at socket.c:813
#11 0x00000000 in ?? ()
(gdb) bt
#0  wb_process_queue (frame=0x0, file=0x0, flush_all=0 '\0')
    at write-behind.c:1728
#1  0xb80d2cfa in wb_setattr_cbk (frame=Cannot access memory at address 0x8
) at write-behind.c:1074
#2  0xb7687246 in afr_setattr_unwind (frame=Cannot access memory at address 0x8
) at afr-inode-write.c:751
#3  0xb7688c98 in afr_setattr_wind_cbk (frame=Cannot access memory at address 0x8
) at afr-inode-write.c:812
#4  0xb76bd0c1 in client_setattr_cbk (frame=Cannot access memory at address 0x8
) at client-protocol.c:4703
#5  0xb76a9b25 in protocol_client_interpret (this=Cannot access memory at address 0x8
)
    at client-protocol.c:5915
#6  0xb76a9ccf in protocol_client_pollin (this=Cannot access memory at address 0x8
) at client-protocol.c:6209
#7  0xb76b9c42 in notify (this=Cannot access memory at address 0x8
) at client-protocol.c:6328
#8  0xb809bd9f in xlator_notify (xl=Cannot access memory at address 0x8
) at xlator.c:929
#9  0xb6e48aed in socket_event_poll_in (this=0x0) at socket.c:713
#10 0xb6e48bab in socket_event_handler (fd=Cannot access memory at address 0x8
) at socket.c:813
#11 0x00000000 in ?? ()