When seq_nr wraps around, the next reorder job with seq 0 is hashed to
the first CPU in padata_do_serial(). Correspondingly, need reset pd->cpu
to the first one when pd->processed wraps around. Otherwise, if the
number of used CPUs is not a power of 2, padata_find_next() will be
checking a wrong list, hence deadlock.

Fixes: 6fc4dbcf0276 ("padata: Replace delayed timer with immediate workqueue in padata_reorder")
Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
---
 kernel/padata.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/padata.c b/kernel/padata.c
index f85f8bd788d0..833740d75483 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -291,8 +291,12 @@ static void padata_reorder(struct padata_priv *padata)
 		struct padata_serial_queue *squeue;
 		int cb_cpu;
 
-		cpu = cpumask_next_wrap(cpu, pd->cpumask.pcpu);
 		processed++;
+		/* When sequence wraps around, reset to the first CPU. */
+		if (unlikely(processed == 0))
+			cpu = cpumask_first(pd->cpumask.pcpu);
+		else
+			cpu = cpumask_next_wrap(cpu, pd->cpumask.pcpu);
 
 		cb_cpu = padata->cb_cpu;
 		squeue = per_cpu_ptr(pd->squeue, cb_cpu);
-- 
2.50.1

On Sun, Aug 17, 2025 at 12:30:15AM +0800, Xiao Liang wrote:
> When seq_nr wraps around, the next reorder job with seq 0 is hashed to
> the first CPU in padata_do_serial(). Correspondingly, need reset pd->cpu
> to the first one when pd->processed wraps around. Otherwise, if the
> number of used CPUs is not a power of 2, padata_find_next() will be
> checking a wrong list, hence deadlock.
> 
> Fixes: 6fc4dbcf0276 ("padata: Replace delayed timer with immediate workqueue in padata_reorder")
> Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
> ---
>  kernel/padata.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt