[LTP] [PATCH v6] lib: Use backoff polling to wait for loop device nodes

[Wake Liu]

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 <wakel@google.com>
---
v5 -> v6:
 - Correct timing descriptions in commit message to match the actual behavior of TST_RETRY_FN_EXP_BACKOFF() macro (1us initial delay, 1s base timeout).

v4 -> v5:
 - Declare tst_multiply_timeout() in tst_common.h to fix old API build failures in CI.

v3 -> v4:
 - Rebase onto the latest LTP HEAD.
 - Remove self-referencing Link in commit message.

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.

 include/tst_common.h |  2 ++
 lib/tst_device.c     | 13 +++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/tst_common.h b/include/tst_common.h
index e1f7c7907..d09edbbbe 100644
--- a/include/tst_common.h
+++ b/include/tst_common.h
@@ -26,6 +26,8 @@
 #define LTP_ALIGN(x, a)    __LTP_ALIGN_MASK(x, (typeof(x))(a) - 1)
 #define __LTP_ALIGN_MASK(x, mask)  (((x) + (mask)) & ~(mask))
 
+unsigned int tst_multiply_timeout(unsigned int timeout);
+
 /**
  * TST_RETRY_FUNC() - Repeatedly retry a function with an increasing delay.
  * @FUNC - The function which will be retried
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