[PATCH] fs: provide locked helper variant of close_fd_get_file()

[Jens Axboe <axboe@kernel.dk>]

Assumes current->files->file_lock is already held on invocation. Helps
the caller check the file before removing the fd, if it needs to.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

Al, I need this to get rid of the two-stage operation that io_uring
currently does for close operations. It's proving to be quite a headache
in terms of cancelation, since we must complete part 2 if we did part 1.
If we provide this locked variant helper, then we can ensure that we the
close as one operation, nicely fixing that problem instead of needing to
hack around it.

 fs/file.c     | 36 +++++++++++++++++++++++++-----------
 fs/internal.h |  1 +
 2 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index dab120b71e44..f3a4bac2cbe9 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -22,6 +22,8 @@
 #include <linux/close_range.h>
 #include <net/sock.h>
 
+#include "internal.h"
+
 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
 /* our min() is unusable in constant expressions ;-/ */
@@ -732,36 +734,48 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
 }
 
 /*
- * variant of close_fd that gets a ref on the file for later fput.
- * The caller must ensure that filp_close() called on the file, and then
- * an fput().
+ * See close_fd_get_file() below, this variant assumes current->files->file_lock
+ * is held.
  */
-int close_fd_get_file(unsigned int fd, struct file **res)
+int __close_fd_get_file(unsigned int fd, struct file **res)
 {
 	struct files_struct *files = current->files;
 	struct file *file;
 	struct fdtable *fdt;
 
-	spin_lock(&files->file_lock);
 	fdt = files_fdtable(files);
 	if (fd >= fdt->max_fds)
-		goto out_unlock;
+		goto out_err;
 	file = fdt->fd[fd];
 	if (!file)
-		goto out_unlock;
+		goto out_err;
 	rcu_assign_pointer(fdt->fd[fd], NULL);
 	__put_unused_fd(files, fd);
-	spin_unlock(&files->file_lock);
 	get_file(file);
 	*res = file;
 	return 0;
-
-out_unlock:
-	spin_unlock(&files->file_lock);
+out_err:
 	*res = NULL;
 	return -ENOENT;
 }
 
+/*
+ * variant of close_fd that gets a ref on the file for later fput.
+ * The caller must ensure that filp_close() called on the file, and then
+ * an fput().
+ */
+int close_fd_get_file(unsigned int fd, struct file **res)
+{
+	struct files_struct *files = current->files;
+	int ret;
+
+	spin_lock(&files->file_lock);
+	ret = __close_fd_get_file(fd, res);
+	spin_unlock(&files->file_lock);
+
+	return ret;
+}
+
 void do_close_on_exec(struct files_struct *files)
 {
 	unsigned i;
diff --git a/fs/internal.h b/fs/internal.h
index 77c50befbfbe..c6c85f6ad598 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -132,6 +132,7 @@ extern struct file *do_file_open_root(struct dentry *, struct vfsmount *,
 		const char *, const struct open_flags *);
 extern struct open_how build_open_how(int flags, umode_t mode);
 extern int build_open_flags(const struct open_how *how, struct open_flags *op);
+extern int __close_fd_get_file(unsigned int fd, struct file **res);
 
 long do_sys_ftruncate(unsigned int fd, loff_t length, int small);
 int chmod_common(const struct path *path, umode_t mode);
-- 
2.30.0

-- 
Jens Axboe