Description of problem:

After the latest upgrade lightdm-gtk-greeter started to hit heavily on CPU.  On otherwise idle system 'top' shows it consistently around 36 - 40 percent of CPU.  stracing this process reveals that it sits in a tight loop doing over and over again this:

stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2388, ...}) = 0
recvmsg(7, 0x7fff9c568a00, 0)           = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=12, events=POLLIN}, {fd=4, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(7, 0x7fff9c567d00, 0)           = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=7, events=POLLIN|POLLOUT}], 1, 4294967295) = 1 ([{fd=7, revents=POLLOUT}])
writev(7, [{"\22\0\16\0 \0\200\0T\1\0\0\6\0\0\0 \30\4\0\10\0\0\0\7\0\0\0\0\0\0\0"..., 500}, {NULL, 0}, {"", 0}], 3) = 500
poll([{fd=7, events=POLLIN}], 1, 4294967295) = 1 ([{fd=7, revents=POLLIN}])
recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"\34\0\207c \0\200\0T\1\0\0\327\r\3\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096}], msg_controllen=0, msg_flags=0}, 0) = 96
recvmsg(7, 0x7fff9c5683b0, 0)           = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=12, events=POLLIN}, {fd=4, events=POLLIN}], 4, 0) = 0 (Timeout)
recvmsg(7, 0x7fff9c5688d0, 0)           = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(7, 0x7fff9c568a00, 0)           = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=12, events=POLLIN}, {fd=4, events=POLLIN}], 4, 63) = 0 (Timeout)
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2388, ...}) = 0

It turns out that 'show-clock=true' option in lightdm-gtk-greeter.conf triggers the above.  Turning it off restores sanity.  OTOH downgrading to
lightdm-gtk-1.6.1-3.fc21.x86_64 also kills fireworks even with 'show-clock=true' turned on.

Version-Release number of selected component (if applicable):
lightdm-gtk-1.8.1-1.fc21

How reproducible:
always

Additional info:
lightdm-gtk-1.6.1-3.fc20, as used by Fedora 20, also does not have any issues with showing a clock