linux-kernel - Re: [External] Re: [PATCH] files: Use rcu lock to get the file structures for better performance

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Date:   Thu, 21 May 2020 18:16:23 +0200
From:   Greg KH <greg@...ah.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Matthew Wilcox <willy@...radead.org>, adobriyan@...il.com,
        ast@...nel.org, daniel@...earbox.net, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        john.fastabend@...il.com, kpsingh@...omium.org,
        ebiederm@...ssion.com, bernd.edlinger@...mail.de,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [External] Re: [PATCH] files: Use rcu lock to get the file
 structures for better performance

On Fri, May 22, 2020 at 12:06:46AM +0800, Muchun Song wrote:
> On Thu, May 21, 2020 at 11:21 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Thu, May 21, 2020 at 08:38:35PM +0800, Muchun Song wrote:
> > > There is another safe way to get the file structure without
> > > holding the files->file_lock. That is rcu lock, and this way
> > > has better performance. So use the rcu lock instead of the
> > > files->file_lock.
> >
> > What makes you think this is safe?  Are you actually seeing contention
> > on this spinlock?
> >
> 
> I have read the doc which is in the Documentation/filesystems/files.txt.
> If my understanding is correct, I think it is safe to use rcu lock.

Did you test this and prove that it is safe and "faster"?  If so, you
always have to show that in your changelog.  Please fix it up and
resend.

thanks,

greg k-h