Re: rcu: endless stalls

[Mike Galbraith]

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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