[PATCH v3] 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() (starting at 1ms delay,
doubling each try, capped at 100ms) 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.

Link: https://lore.kernel.org/ltp/20260803152754.3991113-1-wakel-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org/
Signed-off-by: Wake Liu <[email protected]>
---
v2 -> v3:
 - Reuse LTP native TST_RETRY_FN_EXP_BACKOFF() macro in tst_common.h to simplify code.

v1 -> v2:
 - Strictly clamp backoff delay to 100ms (prevent overflow exceeding cap).
 - Skip usleep on final loop iteration to avoid unnecessary idle delay.
 - Reduce max attempts to 30 (stat) and 15 (open) to bound worst-case timeout.

 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 d3c53a1a1..a3c0992e0 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;
@@ -156,7 +161,7 @@ int tst_attach_device(const char *dev, const char *file)
 	int dev_fd, file_fd;
 	struct loop_info loopinfo;
 
-	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;
-- 
2.55.0.571.g244d577d93-goog


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