Re: [PATCH] stackdepot: don't WARN when the depot runs out of pools

[Marco Elver]

On Tue, 3 Nov 2026 at 12:01, Breno Leitao <leitao@xxxxxxxxxx> wrote:
>
> depot_init_pool() emits WARN_ONCE() when the depot runs out of pools.
> It runs with the raw pool_lock held and IRQs disabled, and
> stack_depot_save_flags() wraps that section in printk_deferred_enter()
> so that it does not print to the consoles with the lock held.
>
> That deferral only applies to legacy consoles. ON nbcon WARN() puts the
> CPU in emergency mode, and in emergency mode printk() flushes all
> pending records to the nbcon consoles from the calling context. With
> a slow serial console (pl011 is an nbcon console) and a backlog of
> messages, that flush can outlast the hard lockup threshold, all of it
> with pool_lock held and IRQs disabled, while every other CPU that needs
> to save a new stack trace spins on pool_lock.
>
> This was hit on an arm64 host running a KASAN and kmemleak enabled
> linux-next kernel under stress-ng:
>
>    Stack depot reached limit capacity
>    WARNING: lib/stackdepot.c:302 at stack_depot_save_flags+0x6b0/0x6b8, CPU#56: stress-ng-dev/3466160
>    smp: csd: Detected non-responsive CSD lock (#1) on CPU#66, waiting 5000000030 ns for CPU#19 do_nothing+0x0/0x18(0x0).
>    smp: csd: Detected non-responsive CSD lock (#2) on CPU#4, waiting 5000000030 ns for CPU#61 ipi_mb+0x0/0x18(0x0).
>    watchdog: CPU53: Watchdog detected hard LOCKUP on cpu 54
>    Kernel panic - not syncing: Hard LOCKUP
>
> The rest of the WARN splat never made it to the log: CPU 56 was still
> flushing the console backlog from the "WARNING:" line when the hard
> lockup detector fired.
>
> Running out of depot space is an expected condition under heavy debug
> load, not a kernel bug, so report it with pr_warn_once() instead.
>
> At normal priority the nbcon consoles are printed by the printer
> threads, and the legacy consoles honour printk_deferred.
>
> This also keeps a full depot from crashing machines running with
> panic_on_warn.
>
> Fixes: cd11016e5f52 ("mm, kasan: stackdepot implementation. Enable stackdepot for SLAB")
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  lib/stackdepot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 90c52f2e0d3f42..dea6338f73c1cd 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -299,7 +299,7 @@ static bool depot_init_pool(void **prealloc)
>                 /* Bail out if we reached the pool limit. */
>                 WARN_ON_ONCE(pools_num > stack_max_pools); /* should never happen */
>                 WARN_ON_ONCE(!new_pool); /* to avoid unnecessary pre-allocation */
> -               WARN_ONCE(1, "Stack depot reached limit capacity");
> +               pr_warn_once("Stack depot reached limit capacity\n");
>                 return false;
>         }
>
>
> ---
> base-commit: 2637bdec0535a2044cd830eba9a79363651d9043
> change-id: 20261103-stackdepot-full-warn-b29a9d89f1a5
>
> Best regards,
> --
> Breno Leitao <leitao@xxxxxxxxxx>
>