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SubjectRe: rcu: endless stalls
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On Mon, 2012-06-11 at 20:10 +0200, Mike Galbraith wrote: 
> On Mon, 2012-06-11 at 11:01 -0700, Paul E. McKenney wrote:

> > > 2aa15890 - mm: prevent concurrent unmap_mapping_range() on the same inode
> >
> > I confess, you lost me on this one. You believe that this commit is
> > the cause of the RCU CPU stall warnings?
>
> 4096 tasks on 4096 CPUs exit (well, try to) simultaneously.
>
> Call Trace:
> __mutex_lock_slowpath+0x94/0x150
> mutex_lock+0x1a/0x40
> unlink_file_vma+0x3f/0xf0
> free_pgtables+0x40/0x100
> exit_mmap+0xb0/0x120
> mmput+0x49/0x120
> exit_mm+0x122/0x160
> do_exit+0x179/0x8d0
> do_group_exit+0x3d/0xb0
> sys_exit_group+0x12/0x20
>
> Monster box dies screaming.

That commit landed in stable, box with way too many cores (NR_CPUS=0!!)
chokes instantly with loads of spinners. Ok, so zillion CPUs grabbing a
mutex in lockstep is a bad idea (_having_ zillion?), but is there pilot
error involved in a logjam like this?

-Mike



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