Re: [PATCH] loop: make autoclear operation asynchronous

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2021/12/02 16:22, Christoph Hellwig wrote:
> On Wed, Dec 01, 2021 at 11:41:23PM +0900, Tetsuo Handa wrote:
>> OK. Here is a patch.
>> Is this better than temporarily dropping disk->open_mutex ?
> 
> This looks much better, and also cleans up the horrible locking warts
> in __loop_clr_fd.
> 

What "the horrible locking warts" refer to? Below approach temporarily
drops disk->open_mutex. I think there is no locking difference between
synchronous and asynchronous...

Anyway, I resent
https://lkml.kernel.org/r/d1f760f9-cdb2-f40d-33d8-bfa517c731be@i-love.sakura.ne.jp
in order to apply before "loop: replace loop_validate_mutex with loop_validate_spinlock".

---
 drivers/block/loop.c | 33 +++++++--------------------------
 1 file changed, 7 insertions(+), 26 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index ba76319b5544..31d3fbe67fea 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1082,7 +1082,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 	return error;
 }
 
-static void __loop_clr_fd(struct loop_device *lo, bool release)
+static void __loop_clr_fd(struct loop_device *lo)
 {
 	struct file *filp;
 	gfp_t gfp = lo->old_gfp_mask;
@@ -1153,31 +1153,15 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
 	if (lo->lo_flags & LO_FLAGS_PARTSCAN) {
 		int err;
 
-		/*
-		 * open_mutex has been held already in release path, so don't
-		 * acquire it if this function is called in such case.
-		 *
-		 * If the reread partition isn't from release path, lo_refcnt
-		 * must be at least one and it can only become zero when the
-		 * current holder is released.
-		 */
-		if (!release)
-			mutex_lock(&lo->lo_disk->open_mutex);
+		mutex_lock(&lo->lo_disk->open_mutex);
 		err = bdev_disk_changed(lo->lo_disk, false);
-		if (!release)
-			mutex_unlock(&lo->lo_disk->open_mutex);
+		mutex_unlock(&lo->lo_disk->open_mutex);
 		if (err)
 			pr_warn("%s: partition scan of loop%d failed (rc=%d)\n",
 				__func__, lo->lo_number, err);
 		/* Device is gone, no point in returning error */
 	}
 
-	/*
-	 * lo->lo_state is set to Lo_unbound here after above partscan has
-	 * finished. There cannot be anybody else entering __loop_clr_fd() as
-	 * Lo_rundown state protects us from all the other places trying to
-	 * change the 'lo' device.
-	 */
 	lo->lo_flags = 0;
 	if (!part_shift)
 		lo->lo_disk->flags |= GENHD_FL_NO_PART;
@@ -1185,11 +1169,6 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
 	lo->lo_state = Lo_unbound;
 	mutex_unlock(&lo->lo_mutex);
 
-	/*
-	 * Need not hold lo_mutex to fput backing file. Calling fput holding
-	 * lo_mutex triggers a circular lock dependency possibility warning as
-	 * fput can take open_mutex which is usually taken before lo_mutex.
-	 */
 	fput(filp);
 }
 
@@ -1222,7 +1201,7 @@ static int loop_clr_fd(struct loop_device *lo)
 	lo->lo_state = Lo_rundown;
 	mutex_unlock(&lo->lo_mutex);
 
-	__loop_clr_fd(lo, false);
+	__loop_clr_fd(lo);
 	return 0;
 }
 
@@ -1747,7 +1726,9 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 		 * In autoclear mode, stop the loop thread
 		 * and remove configuration after last close.
 		 */
-		__loop_clr_fd(lo, true);
+		mutex_unlock(&lo->lo_disk->open_mutex);
+		__loop_clr_fd(lo);
+		mutex_lock(&lo->lo_disk->open_mutex);
 		return;
 	} else if (lo->lo_state == Lo_bound) {
 		/*
-- 
2.18.4