I faced with something similar..
Here is my related dmesg output:
[ 723.041613] show_signal_msg: 15 callbacks suppressed
[ 723.041621] compiz[10514]: segfault at 71 ip 7f11c6eb0565 sp
7fff84340770 error 4 in libunityshell.so[7f11c6db4000+225000]
[ 902.798182] compiz[14654]: segfault a
Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: compiz (Ubuntu)
Status: New => Confirmed
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Title:
comp
** Package changed: ubuntu => compiz (Ubuntu)
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https://bugs.launchpad.net/bugs/904086
Title:
compiz kernel null pointer mutex_lock
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