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

[NeilBrown]

On Sat, 31 Oct 2026, Hillf Danton wrote:
> 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_threads_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/

Thanks for that pointer.  It helped me realise that I was wrong about
the sort of barriers needed.  A set_bit_release() would not help.
And an acquire on the subsequent read of sp_idle_threads also wouldn't
help.

A release is between two writes:
T1:
  foo = 1
  smp_store_release(&bar, 2)
puts a write barrier between the two.

An acquire is between two reads.
T2:
  y = smp_load_acquire(&bar);
  x = foo

These ensure that if T2 sees '2' in bar, it will definitely see '1' in foo.

But we don't have two reads or two writes, we have a write-then-read,
and that requires a full memory barrier.

So we would want set_bit_mb() (corresponding to smp_store_mb()).
As we don't have that,
   set_bit()
   smp_mb__after_atomic()

is the right way to fix this - acquire/release barriers would not be
effective. 

As I noted, this is needed in both svc_wake_up() and
svc_stop_kthreads().

Thanks,
NeilBrown

 
> > 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
>