[PATCH 3/4] fsx: fix infinite/too long loops when generating ranges for copy_file_range

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

While running generic/521 I've had fsx taking a lot of CPU time and not
making any progress for several hours. Attaching gdb to the fsx process
revealed that fsx was in the loop that generates the ranges for a
copy_file_range operation, in particular the loop seemed to never end
because the range defined by 'offset2' kept overlapping with the range
defined by 'offset'.
So far this happened one time only in one of my test VMs with generic/521.

Fix this by breaking out of the loop after trying 30 times, like we
currently do for dedupe operations, which results in logging the operation
as skipped.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 ltp/fsx.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/ltp/fsx.c b/ltp/fsx.c
index 5949ebf0..89a5f60e 100644
--- a/ltp/fsx.c
+++ b/ltp/fsx.c
@@ -2042,17 +2042,25 @@ test(void)
 			break;
 		}
 	case OP_COPY_RANGE:
-		TRIM_OFF_LEN(offset, size, file_size);
-		offset -= offset % readbdy;
-		if (o_direct)
-			size -= size % readbdy;
-		do {
-			offset2 = random();
-			TRIM_OFF(offset2, maxfilelen);
-			offset2 -= offset2 % writebdy;
-		} while (range_overlaps(offset, offset2, size) ||
-			 offset2 + size > maxfilelen);
-		break;
+		{
+			int tries = 0;
+
+			TRIM_OFF_LEN(offset, size, file_size);
+			offset -= offset % readbdy;
+			if (o_direct)
+				size -= size % readbdy;
+			do {
+				if (tries++ >= 30) {
+					size = 0;
+					break;
+				}
+				offset2 = random();
+				TRIM_OFF(offset2, maxfilelen);
+				offset2 -= offset2 % writebdy;
+			} while (range_overlaps(offset, offset2, size) ||
+				 offset2 + size > maxfilelen);
+			break;
+		}
 	}
 
 have_op:
-- 
2.11.0