[PATCH] [makedumpfile] Avoid glibc lazy-unwind machinery in parallel teardown

[Maxi Saparov]

From: Maxi Saparov <masaparov at coreweave.com>

Worker threads end with pthread_exit(), and the consumer thread then
pthread_cancel()s every worker even after a fully successful pass.
Both make glibc lazily dlopen() libgcc_s.so.1 for stack unwinding
(misc/unwind-link.c), and glibc aborts the process when that load
fails:

  libgcc_s.so.1 must be installed for pthread_exit to work

In a minimal kdump initramfs nothing has loaded libgcc_s.so.1 by the
time the copy finishes, so the process-wide first load happens exactly
when every worker exits and is cancelled concurrently at peak
memory pressure in the capture kernel. Observed intermittently with
1.7.7, glibc 2.39 and --num-threads 2 on arm64 crash captures, aborting
right after "Copying data" reached 100% with libgcc_s.so.1 present and
loadable, leaving an otherwise complete dump marked incomplete.

This bug only affects dynamically linked builds, which is currently what
most mainstream distributions ship (Debian / Ubuntu build with
LINKTYPE=dynamic). Statically linked builds pull the unwinder
(libgcc_eh.a) at build time so there is no concern with dlopen failing
on teardown.

Avoid entering the unwind machinery at all on the success path:

- return from the worker function instead of calling pthread_exit();
  the return value still reaches pthread_join(), but a plain return
  never enters glibc's unwinder,
- pthread_cancel() the workers only when bailing out on an error,
  where they may still be blocked producing pages.  On success every
  worker is already exiting on its own, so joining is sufficient.

Signed-off-by: Maxi Saparov <masaparov at coreweave.com>
---
 makedumpfile.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/makedumpfile.c b/makedumpfile.c
index 46d9ac7..5c8b60e 100644
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -8790,7 +8790,9 @@ fail:
 	if (bitmap_memory_parallel.buf != NULL)
 		free(bitmap_memory_parallel.buf);
 
-	pthread_exit(retval);
+	/* Plain return: pthread_exit() would enter glibc's unwind path,
+	 * which requires loading libgcc_s.so.1. */
+	return retval;
 }
 
 int
@@ -8994,12 +8996,16 @@ finish:
 
 out:
 	if (threads != NULL) {
-		for (i = 0; i < info->num_threads; i++) {
-			if (threads[i] != NULL) {
-				res = pthread_cancel(*threads[i]);
-				if (res != 0 && res != ESRCH)
-					ERRMSG("Can't cancel thread %d. %s\n",
-							i, strerror(res));
+		/* Cancel only on error; on success the workers are already
+		 * exiting and joining is sufficient. */
+		if (!ret) {
+			for (i = 0; i < info->num_threads; i++) {
+				if (threads[i] != NULL) {
+					res = pthread_cancel(*threads[i]);
+					if (res != 0 && res != ESRCH)
+						ERRMSG("Can't cancel thread %d. %s\n",
+								i, strerror(res));
+				}
 			}
 		}
 
-- 
2.54.0