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SubjectRe: [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@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.

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