fs: use __fput_sync in close(2)

close(2) is a special case which guarantees a shallow kernel stack,
making delegation to task_work machinery unnecessary. Said delegation is
problematic as it involves atomic ops and interrupt masking trips, none
of which are cheap on x86-64. Forcing close(2) to do it looks like an
oversight in the original work.

Moreover presence of CONFIG_RSEQ adds an additional overhead as fput()
-> task_work_add(..., TWA_RESUME) -> set_notify_resume() makes the
thread returning to userspace land in resume_user_mode_work(), where
rseq_handle_notify_resume takes a SMAP round-trip if rseq is enabled for
the thread (and it is by default with contemporary glibc).

Sample result when benchmarking open1_processes -t 1 from will-it-scale
(that's an open + close loop) + tmpfs on /tmp, running on the Sapphire
Rapid CPU (ops/s):
stock+RSEQ:     1329857
stock-RSEQ:     1421667 (+7%)
patched:        1523521 (+14.5% / +7%) (with / without rseq)

Patched result is the same regardless of rseq as the codepath is avoided.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/file_table.c

@@ -461,11 +461,8 @@ void fput(struct file *file)
  */
 void __fput_sync(struct file *file)
 {
-	if (atomic_long_dec_and_test(&file->f_count)) {
-		struct task_struct *task = current;
-		BUG_ON(!(task->flags & PF_KTHREAD));
+	if (atomic_long_dec_and_test(&file->f_count))
 		__fput(file);
-	}
 }
 
 EXPORT_SYMBOL(fput);

fs/open.c

@@ -1503,7 +1503,7 @@ SYSCALL_DEFINE2(creat, const char __user *, pathname, umode_t, mode)
  * "id" is the POSIX thread ID. We use the
  * files pointer for this..
  */
-int filp_close(struct file *filp, fl_owner_t id)
+static int filp_flush(struct file *filp, fl_owner_t id)
 {
 	int retval = 0;
 
@@ -1520,10 +1520,18 @@ int filp_close(struct file *filp, fl_owner_t id)
 		dnotify_flush(filp, id);
 		locks_remove_posix(filp, id);
 	}
-	fput(filp);
 	return retval;
 }
 
+int filp_close(struct file *filp, fl_owner_t id)
+{
+	int retval;
+
+	retval = filp_flush(filp, id);
+	fput(filp);
+
+	return retval;
+}
 EXPORT_SYMBOL(filp_close);
 
 /*
@@ -1533,7 +1541,20 @@ EXPORT_SYMBOL(filp_close);
  */
 SYSCALL_DEFINE1(close, unsigned int, fd)
 {
-	int retval = close_fd(fd);
+	int retval;
+	struct file *file;
+
+	file = close_fd_get_file(fd);
+	if (!file)
+		return -EBADF;
+
+	retval = filp_flush(file, current->files);
+
+	/*
+	 * We're returning to user space. Don't bother
+	 * with any delayed fput() cases.
+	 */
+	__fput_sync(file);
 
 	/* can't restart close syscall because file table entry was cleared */
 	if (unlikely(retval == -ERESTARTSYS ||