Re: [RFC][PATCH v2 -next 2/2] Adding lock operations to kmsg_dump()/pstore_dump()

[Don Zickus <dzickus@redhat.com>]

On Fri, Oct 21, 2011 at 05:21:35PM -0400, Seiji Aguchi wrote:
> pstore_dump()/kmsg_dump() may be called everywhere in kernel.
> So we have to care about following cases.
> 
>   - Panic path
>     In this case, Logging message process is serialized via smp_send_stop().
>     So, we can bust spin_locks.
> 
>     Currently, kmsg_dump() may be called twice (KMSG_DUMP_PANIC and KMSG_DUMP_EMERGY)
>     So, for avoiding deadlock, I suggest to bust locks rather than skipping them.
> 
>   - NMI context 
>      While a cpu is in NMI handler, other cpus may be running.
>      So, trylock should be called so that lockdep cheking works.
>  
>   - Process context
>     In this case, we can simply take locks.

It ain't pretty but it moves things towards a more reliable message dump.
The odds of us needing to bust the spinlocks are really small.  Most of
the time no one reads the pstore filesystem.

I would love to figure out a prettier solution for this locking mess, but
I can't think of anything.  We have customers who want to utilize this
technology, so I am trying to make sure it is stable and robust for now.
A little selfish I suppose.  But we are open to ideas?

Acked-by: Don Zickus <dzickus@redhat.com>