This script:
 while [ 1==1 ] ; do ifconfig eth0 up; usleep 1950000 ;ifconfig eth0 down; dmesg -c ;done
causes in just a second or two:
INFO: task ifconfig:572 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
ifconfig      D 0ff65760     0   572    369 0x00000000
Call Trace:
[c6157be0] [c6008460] 0xc6008460 (unreliable)
[c6157ca0] [c0008608] __switch_to+0x4c/0x6c
[c6157cb0] [c028fecc] schedule+0x184/0x310
[c6157ce0] [c0290e54] __mutex_lock_slowpath+0xa4/0x150
[c6157d20] [c0290c48] mutex_lock+0x44/0x48
[c6157d30] [c01aba74] phy_stop+0x20/0x70
[c6157d40] [c01aef40] ucc_geth_stop+0x30/0x98
[c6157d60] [c01b18fc] ucc_geth_close+0x9c/0xdc
[c6157d80] [c01db0cc] __dev_close+0xa0/0xd0
[c6157d90] [c01deddc] __dev_change_flags+0x8c/0x148
[c6157db0] [c01def54] dev_change_flags+0x1c/0x64
[c6157dd0] [c0237ac8] devinet_ioctl+0x678/0x784
[c6157e50] [c0239a58] inet_ioctl+0xb0/0xbc
[c6157e60] [c01cafa8] sock_ioctl+0x174/0x2a0
[c6157e80] [c009a16c] vfs_ioctl+0xcc/0xe0
[c6157ea0] [c009a998] do_vfs_ioctl+0xc4/0x79c
[c6157f10] [c009b0b0] sys_ioctl+0x40/0x74
[c6157f40] [c00117c4] ret_from_syscall+0x0/0x38

The reason appears to be ucc_geth_stop meets adjust_link as the
PHY reports PHY changes. I belive adjust_link hangs somewhere,
holding the PHY lock, because ucc_geth_stop disabled the
controller HW.
Fix is to stop the PHY before disabling the controller.
Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Reviewed-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>