From: Matthew Wilcox <willy@infradead.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: adobriyan@gmail.com, ast@kernel.org, daniel@iogearbox.net,
kafai@fb.com, songliubraving@fb.com, yhs@fb.com, andriin@fb.com,
john.fastabend@gmail.com, kpsingh@chromium.org,
ebiederm@xmission.com, bernd.edlinger@hotmail.de,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [External] Re: [PATCH] files: Use rcu lock to get the file structures for better performance
Date: Fri, 22 May 2020 04:43:48 -0700 [thread overview]
Message-ID: <20200522114348.GL28818@bombadil.infradead.org> (raw)
In-Reply-To: <CAMZfGtWn4xa-5-0rN2KJzUYioiOOUYX9BFcUDNZS85H11sYDEA@mail.gmail.com>
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@infradead.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.
prev parent reply other threads:[~2020-05-22 11:44 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-21 12:38 [PATCH] files: Use rcu lock to get the file structures for better performance Muchun Song
2020-05-21 15:21 ` Matthew Wilcox
2020-05-21 16:06 ` [External] " Muchun Song
2020-05-21 16:16 ` Greg KH
2020-05-21 16:47 ` Matthew Wilcox
2020-05-22 7:52 ` [External] " Muchun Song
2020-05-22 11:43 ` Matthew Wilcox [this message]
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