proc: faster open/close of files without ->release hook

The whole point of code in fs/proc/inode.c is to make sure ->release
hook is called either at close() or at rmmod time.

All if it is unnecessary if there is no ->release hook.

Save allocation+list manipulations under spinlock in that case.

Link: http://lkml.kernel.org/r/20180214063033.GA15579@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/inode.c

@@ -342,31 +342,36 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	 *
 	 * Save every "struct file" with custom ->release hook.
 	 */
-	pdeo = kmalloc(sizeof(struct pde_opener), GFP_KERNEL);
-	if (!pdeo)
-		return -ENOMEM;
-
-	if (!use_pde(pde)) {
-		kfree(pdeo);
+	if (!use_pde(pde))
 		return -ENOENT;
-	}
-	open = pde->proc_fops->open;
+
 	release = pde->proc_fops->release;
+	if (release) {
+		pdeo = kmalloc(sizeof(struct pde_opener), GFP_KERNEL);
+		if (!pdeo) {
+			rv = -ENOMEM;
+			goto out_unuse;
+		}
+	}
 
+	open = pde->proc_fops->open;
 	if (open)
 		rv = open(inode, file);
 
-	if (rv == 0 && release) {
-		/* To know what to release. */
-		pdeo->file = file;
-		pdeo->closing = false;
-		pdeo->c = NULL;
-		spin_lock(&pde->pde_unload_lock);
-		list_add(&pdeo->lh, &pde->pde_openers);
-		spin_unlock(&pde->pde_unload_lock);
-	} else
-		kfree(pdeo);
+	if (release) {
+		if (rv == 0) {
+			/* To know what to release. */
+			pdeo->file = file;
+			pdeo->closing = false;
+			pdeo->c = NULL;
+			spin_lock(&pde->pde_unload_lock);
+			list_add(&pdeo->lh, &pde->pde_openers);
+			spin_unlock(&pde->pde_unload_lock);
+		} else
+			kfree(pdeo);
+	}
 
+out_unuse:
 	unuse_pde(pde);
 	return rv;
 }