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

[Hillf Danton]

On Sat, 31 Oct 2026 09:45:39 +1100 NeilBrown wrote:
> 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.

It is on the way [11].

[11] Subject: [PATCH 1/2] asm-generic/bitops: Add set_bit_release()
https://lore.kernel.org/lkml/20261021095512.1345191-3-phasta@xxxxxxxxxx/

> 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