Re: [PATCH v3 1/3] mm/kmemleak: avoid soft lockup when scanning task stacks

[Davidlohr Bueso]

On Mon, 15 Jun 2026, Breno Leitao wrote:

> kmemleak_scan() walks every thread and scans its kernel stack under a
> single rcu_read_lock() with no reschedule point. On a host with very
> many threads -- amplified by KASAN/lockdep in debug builds -- this loop
> can hog a CPU long enough to trip the soft lockup watchdog:
>
>  watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537]
>   scan_block
>   kmemleak_scan
>   kmemleak_scan_thread
>   kthread
>
> A cond_resched() cannot be added directly: the loop runs inside an RCU
> read-side critical section.
>
> Walk the tasks one PID at a time with find_ge_pid(), taking the RCU read
> lock only to look up and pin each task. The stack is then scanned with no
> lock held, so cond_resched() runs between tasks and the scan stops early
> on scan_should_stop(). This follows the next_tgid()/task_seq_get_next()
> iteration pattern and keeps each RCU critical section short.
>
> Fixes: c4b28963fd79 ("mm/kmemleak: rely on rcu for task stack scanning")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

LGTM

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>