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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 12:47 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, May 21, 2020 at 08:38:35PM +0800, Muchun Song wrote:
> > +++ b/fs/proc/fd.c
> > @@ -34,19 +34,27 @@ static int seq_show(struct seq_file *m, void *v)
> > if (files) {
> > unsigned int fd = proc_fd(m->private);
> >
> > - spin_lock(&files->file_lock);
> > + rcu_read_lock();
> > +again:
> > file = fcheck_files(files, fd);
> > if (file) {
> > - struct fdtable *fdt = files_fdtable(files);
> > + struct fdtable *fdt;
> > +
> > + if (!get_file_rcu(file)) {
> > + /*
> > + * we loop to catch the new file (or NULL
> > + * pointer).
> > + */
> > + goto again;
> > + }
> >
> > + fdt = files_fdtable(files);
>
> This is unusual, and may not be safe.
>
> fcheck_files() loads files->fdt. Then it loads file from fdt->fd[].
> Now you're loading files->fdt again here, and it could have been changed
> by another thread expanding the fd table.
>
> You have to write a changelog which convinces me you've thought about
> this race and that it's safe. Because I don't think you even realise
> it's a possibility at this point.

Thanks for your review, it is a problem. I can fix it.

>
> > @@ -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.

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

--
Yours,
Muchun

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