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

From: Muchun Song
Date: Thu May 21 2020 - 08:39:00 EST


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 <songmuchun@xxxxxxxxxxxxx>
---
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