[PATCH v7 2/2] lib: Use backoff polling to wait for loop device nodes

On systems where loop device node creation is asynchronous (such as
Android containers or systems with slow udev startup), calling stat()
or open() immediately after LOOP_CTL_GET_FREE can transiently fail
because the device file (e.g. /dev/loopX) has not been fully populated
in time.

Introduce an exponential-backoff retry loop in both
tst_find_free_loopdev() and tst_attach_device() by reusing the LTP
native TST_RETRY_FN_EXP_BACKOFF() macro (starting at 1us delay,
doubling each try, with a base timeout limit of 1 second that scales
automatically with LTP_TIMEOUT_MUL) to wait for the device node to be
successfully populated.

This improves the robustness of loop device allocations on asynchronous
virtualized environments while minimizing unnecessary delays on responsive
systems.

Signed-off-by: Wake Liu <[email protected]>
---
 lib/tst_device.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/lib/tst_device.c b/lib/tst_device.c
index 744173ffe..00304711b 100644
--- a/lib/tst_device.c
+++ b/lib/tst_device.c
@@ -82,7 +82,7 @@ static int set_dev_path(char *dev, char *path, size_t path_len)
 
 int tst_find_free_loopdev(char *path, size_t path_len)
 {
-	int ctl_fd, dev_fd, rc, i;
+	int ctl_fd, dev_fd, rc, i, path_set;
 	struct loop_info loopinfo;
 	char buf[PATH_MAX];
 
@@ -93,8 +93,13 @@ int tst_find_free_loopdev(char *path, size_t path_len)
 		rc = ioctl(ctl_fd, LOOP_CTL_GET_FREE);
 		close(ctl_fd);
 		if (rc >= 0) {
-			if (path && set_dev_loop_path(rc, path, path_len))
-				tst_brkm(TBROK, NULL, "Could not stat loop device %i", rc);
+			if (path) {
+				path_set = TST_RETRY_FN_EXP_BACKOFF(
+					set_dev_loop_path(rc, path, path_len),
+					TST_RETVAL_EQ0, 1);
+				if (path_set)
+					tst_brkm(TBROK, NULL, "Could not stat loop device %i", rc);
+			}
 			tst_resm(TINFO, "Found free device %d '%s'",
 				rc, path ?: "");
 			return rc;
@@ -161,7 +166,7 @@ int tst_attach_device(const char *dev, const char *file)
 			 LO_NAME_SIZE);
 	}
 
-	dev_fd = open(dev, O_RDWR);
+	dev_fd = TST_RETRY_FN_EXP_BACKOFF(open(dev, O_RDWR), TST_RETVAL_GE0, 1);
 	if (dev_fd < 0) {
 		tst_resm(TWARN | TERRNO, "open('%s', O_RDWR) failed", dev);
 		return 1;
-- 
2.55.0.654.g21b8a5bc05-goog


-- 
Mailing list info: https://lists.linux.it/listinfo/ltp