[PATCH v5 03/22] cachefiles: introduce on-demand read mode

[Matthew Wilcox]

On Mon, Mar 21, 2022 at 11:18:05PM +0800, JeffleXu wrote:
> >> Besides, IMHO read-write lock shall be more performance friendly, since
> >> most cases are the read side.
> > 
> > That's almost never true.  rwlocks are usually a bad idea because you
> > still have to bounce the cacheline, so you replace lock contention
> > (which you can see) with cacheline contention (which is harder to
> > measure).  And then you have questions about reader/writer fairness
> > (should new readers queue behind a writer if there's one waiting, or
> > should a steady stream of readers be able to hold a writer off
> > indefinitely?)
> 
> Interesting, I didn't notice it before. Thanks for explaining it.

No problem.  It's hard to notice.

> BTW what I want is just
> 
> ```
> PROCESS 1		PROCESS 2
> =========		=========
> #lock			#lock
> set DEAD state		if (not DEAD)
> flush xarray		   enqueue into xarray
> #unlock			#unlock
> ```
> 
> I think it is a generic paradigm. So it seems that the spinlock inside
> xarray is already adequate for this job?

Absolutely; just use xa_lock() to protect both setting & testing the
flag.