Re: frequent lockups in 3.18rc4

[Chris Mason]

I'm not sure if this is related, but running trinity here, I noticed it
was stuck at 100% system time on every CPU.  perf report tells me we are
spending all of our time in spin_lock under the sync system call.

I think it's coming from contention in the bdi_queue_work() call from
inside sync_inodes_sb, which is spin_lock_bh(). 

I wonder if we're just spinning so hard on this one bh lock that we're
starving the watchdog?

Dave, do you have spinlock debugging on?  

-chris
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