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

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Message-ID: <20200522114348.GL28818@bombadil.infradead.org>
Date:   Fri, 22 May 2020 04:43:48 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     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 03:52:39PM +0800, Muchun Song wrote:
> On Fri, May 22, 2020 at 12:47 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > @@ -160,14 +168,23 @@ static int proc_fd_link(struct dentry *dentry, struct path *path)
> > >               unsigned int fd = proc_fd(d_inode(dentry));
> > >               struct file *fd_file;
> > >
> > > -             spin_lock(&files->file_lock);
> > > +             rcu_read_lock();
> > > +again:
> > >               fd_file = fcheck_files(files, fd);
> > >               if (fd_file) {
> > > +                     if (!get_file_rcu(fd_file)) {
> > > +                             /*
> > > +                              * we loop to catch the new file
> > > +                              * (or NULL pointer).
> > > +                              */
> > > +                             goto again;
> > > +                     }
> > >                       *path = fd_file->f_path;
> > >                       path_get(&fd_file->f_path);
> > > +                     fput(fd_file);
> > >                       ret = 0;
> > >               }
> > > -             spin_unlock(&files->file_lock);
> > > +             rcu_read_unlock();
> >
> > Why is it an improvement to increment/decrement the refcount on the
> > struct file here, rather than take/release the spinlock?
> >
> 
> lock-free vs spinlock.

bananas vs oranges.

How do you think refcounts work?  How do you think spinlocks work?

> Do you think spinlock would be better than the lock-free method?
> Actually I prefer the rcu lock.

Why?  You don't seem to understand the tradeoffs.