Re: [PATCH RFC] nfsd: serialize layout stateid morphing operations

[Christoph Hellwig <hch@lst.de>]

On Thu, Sep 17, 2015 at 07:58:24AM -0400, Jeff Layton wrote:
> In order to allow the client to make a sane determination of what
> happened with racing LAYOUTGET/LAYOUTRETURN/CB_LAYOUTRECALL calls, we
> must ensure that the seqids return accurately represent the order of
> operations. The simplest way to do that is to ensure that operations on
> a single stateid are serialized.
> 
> This patch adds a mutex to the layout stateid, and locks it when
> checking the layout stateid's seqid. The mutex is held over the entire
> operation and released after the seqid is bumped.
> 
> Note that in the case of CB_LAYOUTRECALL we must move the increment of
> the seqid and setting into a new cb "prepare" operation. The lease
> infrastructure will call the lm_break callback with a spinlock held, so
> and we can't take the mutex in that codepath.

I can't say I like the long running mutex all that much.  What kinds
of reproducers do you have where the current behavior causes problems?