[LTP] [PATCH v2] lib: tst_device: Use exponential backoff for loop device detach

[Sebastian Chlad]

detach_loop_fd() retries LOOP_CLR_FD 40 times in 50ms steps, a fixed
2s timeout that ignores LTP_TIMEOUT_MUL. When another process still
holds the loop device at cleanup for longer than that, e.g. on a
loaded machine, the test ends with an unwanted
"ioctl(/dev/loopN, LOOP_CLR_FD, 0) no ENXIO for too long" TWARN.

Use TST_RETRY_FN_EXP_BACKOFF() instead, as attach already does since
d90d20046 ("lib: Use backoff polling to wait for loop device nodes").
The detach finishes quickly when the device is released right away,
polling slows down when it is not, and the total timeout (about 2s by
default) is scaled by tst_multiply_timeout().

Suggested-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Sebastian Chlad <sebastianchlad@gmail.com>
---
Changes in v2:
- Use TST_RETRY_FN_EXP_BACKOFF() instead of scaling the retry count
  with tst_multiply_timeout() (Cyril).
- Link to v1:
  https://lore.kernel.org/ltp/20261022115410.23845-1-sebastian.chlad@suse.com/

 lib/tst_device.c | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/lib/tst_device.c b/lib/tst_device.c
index 00304711b..bf77c9ded 100644
--- a/lib/tst_device.c
+++ b/lib/tst_device.c
@@ -249,9 +249,14 @@ uint64_t tst_get_device_size(const char *dev_path)
 	return size/1024/1024;
 }
 
+static int loop_clr_fd_busy(int dev_fd)
+{
+	return !ioctl(dev_fd, LOOP_CLR_FD, 0) || errno == EBUSY;
+}
+
 static int detach_loop_fd(const char *dev, int *dev_fd)
 {
-	int ret, i, retval = 1;
+	int ret, retval = 1;
 
 	/* keep trying to clear LOOPDEV until we get ENXIO, a quick succession
 	 * of attach/detach might not give udev enough time to complete
@@ -259,22 +264,19 @@ static int detach_loop_fd(const char *dev, int *dev_fd)
 	 * Since 18048c1af783 ("loop: Fix a race between loop detach and loop open")
 	 * device is detached only after last close.
 	 */
-	for (i = 0; i < 40; i++) {
-		ret = ioctl(*dev_fd, LOOP_CLR_FD, 0);
+	ret = TST_RETRY_FN_EXP_BACKOFF(loop_clr_fd_busy(*dev_fd),
+				       TST_RETVAL_EQ0, 2);
 
-		if (ret && (errno == ENXIO)) {
-			retval = 0;
-			goto exit;
-		}
-
-		if (ret && (errno != EBUSY)) {
-			tst_resm(TWARN,
-				 "ioctl(%s, LOOP_CLR_FD, 0) unexpectedly failed with: %s",
-				 dev, tst_strerrno(errno));
-			goto exit;
-		}
+	if (!ret && (errno == ENXIO)) {
+		retval = 0;
+		goto exit;
+	}
 
-		usleep(50000);
+	if (!ret) {
+		tst_resm(TWARN,
+			 "ioctl(%s, LOOP_CLR_FD, 0) unexpectedly failed with: %s",
+			 dev, tst_strerrno(errno));
+		goto exit;
 	}
 
 	tst_resm(TWARN,
-- 
2.51.0