Re: INFO: task hung in __get_super

[Tetsuo Handa]

This bug report is getting no feedback, but I guess that this bug is in
block or mm or locking layer rather than fs layer.

NMI backtrace for this bug tends to report that sb_bread() from fill_super()
 from mount_bdev() is stalling is the cause of keep holding s_umount_key for
more than 120 seconds. What is strange is that NMI backtrace for this bug tends
to point at rcu_read_lock()/pagecache_get_page()/radix_tree_deref_slot()/
rcu_read_unlock() which is expected not to stall.

Since CONFIG_RCU_CPU_STALL_TIMEOUT is set to 120 (and actually +5 due to
CONFIG_PROVE_RCU=y) which is longer than CONFIG_DEFAULT_HUNG_TASK_TIMEOUT,
maybe setting CONFIG_RCU_CPU_STALL_TIMEOUT to smaller values (e.g. 25) can
give us some hints...