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

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Message-ID: <YjiX5oXYkmN6WrA3@casper.infradead.org>
Date:   Mon, 21 Mar 2022 15:21:10 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     JeffleXu <jefflexu@...ux.alibaba.com>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com, xiang@...nel.org,
        chao@...nel.org, linux-erofs@...ts.ozlabs.org,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        linux-fsdevel@...r.kernel.org, joseph.qi@...ux.alibaba.com,
        bo.liu@...ux.alibaba.com, tao.peng@...ux.alibaba.com,
        gerry@...ux.alibaba.com, eguan@...ux.alibaba.com,
        linux-kernel@...r.kernel.org, luodaowen.backend@...edance.com
Subject: Re: [PATCH v5 03/22] cachefiles: introduce on-demand read mode

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.