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

On 2026/07/15 4:30, Maxi Saparov wrote:
> From: Maxi Saparov <[email protected]>
> 
> 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 <[email protected]>

Thank you for the improvement, the patch looks good and tested ok.
I tweaked only the comment style.
https://github.com/makedumpfile/makedumpfile/commit/fe9858fb7a59a786fb1c00ddc6f1e2a31031dde9

Thanks,
Kazu

> ---
>   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));
> +				}
>   			}
>   		}
>