Re: [PATCH] sunrpc: fix lost wakeups when stopping service threads

[NeilBrown]

On Sat, 31 Oct 2026, Kal Cutter Conley wrote:
> Stopping NFS can leave rpc.nfsd 0 in an uninterruptible wait with
> this stack:
> 
>   svc_stop_kthreads [sunrpc]
>   svc_set_num_threads [sunrpc]
>   nfsd_svc [nfsd]
>   write_threads [nfsd]
>   nfsctl_transaction_write [nfsd]
> 
> SP_NEED_VICTIM and SP_VICTIM_REMAINS remain set while all remaining
> nfsd threads sleep in svc_recv() and none has RQ_VICTIM set.
> 
> The stop request is published with unordered set_bit() operations.
> svc_pool_wake_idle_thread() checks the idle list and task state before
> calling wake_up_process(). If it sees an empty list or TASK_RUNNING,
> it skips the wakeup and its memory barrier. The requester can therefore
> miss a sleeping worker while that worker misses the stop request.
> 
> Order work publication before both lockless checks, even when no
> wakeup occurs. Pair with set_current_state() and llist_add() in
> svc_thread_wait_for_work(), so the worker observes the pending work or
> the requester observes the idle worker and wakes it.
> 
> Fixes: 60749cbe3d8a ("sunrpc: change sp_nrthreads from atomic_t to unsigned int.")
> Signed-off-by: Kal Cutter Conley <kalcutterc@xxxxxxxxxx>
> ---
>  net/sunrpc/svc.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index af8d87c1de83..f3cc63acc5e1 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -29,6 +29,8 @@
>  #include <linux/sunrpc/clnt.h>
>  #include <linux/sunrpc/bc_xprt.h>
>  
> +#include <asm/barrier.h>
> +
>  #include <trace/events/sunrpc.h>
>  
>  #include "fail.h"
> @@ -693,6 +695,13 @@ void svc_pool_wake_idle_thread(struct svc_pool *pool)
>  	struct svc_rqst	*rqstp;
>  	struct llist_node *ln;
>  
> +	/*
> +	 * Order work publication before the idle-list and task-state checks,
> +	 * even when wake_up_process() is skipped. Pairs with the barriers in
> +	 * set_current_state() and llist_add() in svc_thread_wait_for_work().
> +	 */
> +	smp_mb();
> +

I don't like this.  Stand-alone barriers are, I think, harder to read
than barriers combined with operations, like smp_load_acquire() or
similar.  And this one seems quite detached from anything.

If I understand correctly, the possible race is between
svc_thread_wait_for_work which includes:

		llist_add(&rqstp->rq_idle, &pool->sp_idle_threads);
		if (likely(svc_thread_should_sleep(rqstp)))

and svc_stop_kthread which includes:

		set_bit(SP_NEED_VICTIM, &pool->sp_flags);
		svc_pool_wake_idle_thread(pool);

The first sets ->sp_idle_threads.first and then checks SP_NEED_VICTIM
and the second sets SP_NEED_VICTIM and then checks
->sp_idle_threads.first.

So we need barriers to ensure the values are seen is the expected order.
Have I missed anything?

llist_add() always calls try_cmpxchg() which is fully ordered, so no
extra barriers are needed there.

There is no barrier between the set_bit)( and the READ_ONCE() in
svc_pool_wake_idle_thread(), so you added one.  I agree that is correct
and solves the problem.
But it is only this call to svc_pool_wake_idle_thread() that needs a
barrier, not all calls.  So I would prefer the barrier went in
svc_stop_kthreads().   Do you have a reason for not including it there?

We do need a full memory barrier - a "release" on the set_bit and an
"acquire" on the test of ->sp_idle_threads.first

I would be happy with smp_load_acquire(->sp_idle_theads_first) in
svc_pool_wake_idle_thread().  I'm not sure it is needed though.

I would like set_bit_release(SP_NEED_VICTIM) in svc_stop_kthreads()
but we don't have that.
We could do
   atomic_long_fetch_or_release(BIT(SP_NEED_VICTIM), (atomic_long_t*)&pool->sp_flags)
but that is ugly.

So I think the best solution is

		set_bit(SP_NEED_VICTIM, &pool->sp_flags);
		/* Order SP_NEED_VICTIM before testing sp_idle_threads,
                 * matching svc_thread_wait_for_work() which orders
                 * setting sp_idle_threads before testing the bit
		 */
+               smp_mb__after_atomic();
		svc_pool_wake_idle_thread(pool);

We probably need the same in svc_wake_up() after the set_bit().

Does this make the barriers easier to understand?

Thoughts?

Thanks,
NeilBrown


>  	rcu_read_lock();
>  	ln = READ_ONCE(pool->sp_idle_threads.first);
>  	if (ln) {
> 
> base-commit: b803428098f6fcd584db9f0499217b80e465bb34
> -- 
> 2.53.0
> 
>