[PATCH] files: Use rcu lock to get the file structures for better performance

[Muchun Song]

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.

Signed-off-by: Muchun Song <songmuc...@bytedance.com>
---
 fs/proc/fd.c         | 31 ++++++++++++++++++++++++-------
 kernel/bpf/syscall.c | 17 +++++++++++------
 kernel/kcmp.c        | 15 ++++++++++-----
 3 files changed, 45 insertions(+), 18 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 81882a13212d3..5d5b0f091d32a 100644
--- a/fs/proc/fd.c
+++ 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);
                        f_flags = file->f_flags;
                        if (close_on_exec(fd, fdt))
                                f_flags |= O_CLOEXEC;
-
-                       get_file(file);
                        ret = 0;
                }
-               spin_unlock(&files->file_lock);
+               rcu_read_unlock();
                put_files_struct(files);
        }
 
@@ -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();
                put_files_struct(files);
        }
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8608d6e1b0e0e..441c91378a1fc 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3451,14 +3451,19 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
        if (!files)
                return -ENOENT;
 
-       err = 0;
-       spin_lock(&files->file_lock);
+       rcu_read_lock();
+again:
        file = fcheck_files(files, fd);
-       if (!file)
+       if (file) {
+               if (!get_file_rcu(file)) {
+                       /* we loop to catch the new file (or NULL pointer) */
+                       goto again;
+               }
+               err = 0;
+       } else {
                err = -EBADF;
-       else
-               get_file(file);
-       spin_unlock(&files->file_lock);
+       }
+       rcu_read_unlock();
        put_files_struct(files);
 
        if (err)
diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index b3ff9288c6cc9..3b4f2a54186f2 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -120,13 +120,18 @@ static int kcmp_epoll_target(struct task_struct *task1,
        if (!files)
                return -EBADF;
 
-       spin_lock(&files->file_lock);
+       rcu_read_lock();
+again:
        filp_epoll = fcheck_files(files, slot.efd);
-       if (filp_epoll)
-               get_file(filp_epoll);
-       else
+       if (filp_epoll) {
+               if (!get_file_rcu(filp_epoll)) {
+                       /* we loop to catch the new file (or NULL pointer) */
+                       goto again;
+               }
+       } else {
                filp_tgt = ERR_PTR(-EBADF);
-       spin_unlock(&files->file_lock);
+       }
+       rcu_read_unlock();
        put_files_struct(files);
 
        if (filp_epoll) {
-- 
2.11.0