Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 3 Jul 2017 10:14:20 -0700

On Mon, Jul 03, 2017 at 10:39:49AM -0400, Alan Stern wrote:
> On Sat, 1 Jul 2017, Manfred Spraul wrote:
> 
> > As we want to remove spin_unlock_wait() and replace it with explicit
> > spin_lock()/spin_unlock() calls, we can use this to simplify the
> > locking.
> > 
> > In addition:
> > - Reading nf_conntrack_locks_all needs ACQUIRE memory ordering.
> > - The new code avoids the backwards loop.
> > 
> > Only slightly tested, I did not manage to trigger calls to
> > nf_conntrack_all_lock().
> > 
> > Fixes: b16c29191dc8
> > Signed-off-by: Manfred Spraul <[email-masked]>
> > Cc: <[email-masked]>
> > Cc: Sasha Levin <[email-masked]>
> > Cc: Pablo Neira Ayuso <[email-masked]>
> > Cc: [email-masked]
> > ---
> >  net/netfilter/nf_conntrack_core.c | 44 +++++++++++++++++++++------------------
> >  1 file changed, 24 insertions(+), 20 deletions(-)
> > 
> > diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> > index e847dba..1193565 100644
> > --- a/net/netfilter/nf_conntrack_core.c
> > +++ b/net/netfilter/nf_conntrack_core.c
> > @@ -96,19 +96,24 @@ static struct conntrack_gc_work conntrack_gc_work;
> >  
> >  void nf_conntrack_lock(spinlock_t *lock) __acquires(lock)
> >  {
> > +	/* 1) Acquire the lock */
> >  	spin_lock(lock);
> > -	while (unlikely(nf_conntrack_locks_all)) {
> > -		spin_unlock(lock);
> >  
> > -		/*
> > -		 * Order the 'nf_conntrack_locks_all' load vs. the
> > -		 * spin_unlock_wait() loads below, to ensure
> > -		 * that 'nf_conntrack_locks_all_lock' is indeed held:
> > -		 */
> > -		smp_rmb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
> > -		spin_unlock_wait(&nf_conntrack_locks_all_lock);
> > -		spin_lock(lock);
> > -	}
> > +	/* 2) read nf_conntrack_locks_all, with ACQUIRE semantics */
> > +	if (likely(smp_load_acquire(&nf_conntrack_locks_all) == false))
> > +		return;
> 
> As far as I can tell, this read does not need to have ACQUIRE
> semantics.
> 
> You need to guarantee that two things can never happen:
> 
>     (1) We read nf_conntrack_locks_all == false, and this routine's
> 	critical section for nf_conntrack_locks[i] runs after the
> 	(empty) critical section for that lock in 
> 	nf_conntrack_all_lock().
> 
>     (2) We read nf_conntrack_locks_all == true, and this routine's 
> 	critical section for nf_conntrack_locks_all_lock runs before 
> 	the critical section in nf_conntrack_all_lock().
> 
> In fact, neither one can happen even if smp_load_acquire() is replaced
> with READ_ONCE().  The reason is simple enough, using this property of
> spinlocks:
> 
> 	If critical section CS1 runs before critical section CS2 (for 
> 	the same lock) then: (a) every write coming before CS1's
> 	spin_unlock() will be visible to any read coming after CS2's
> 	spin_lock(), and (b) no write coming after CS2's spin_lock()
> 	will be visible to any read coming before CS1's spin_unlock().
> 
> Thus for (1), assuming the critical sections run in the order mentioned
> above, since nf_conntrack_all_lock() writes to nf_conntrack_locks_all
> before releasing nf_conntrack_locks[i], and since nf_conntrack_lock()
> acquires nf_conntrack_locks[i] before reading nf_conntrack_locks_all,
> by (a) the read will always see the write.
> 
> Similarly for (2), since nf_conntrack_all_lock() acquires 
> nf_conntrack_locks_all_lock before writing to nf_conntrack_locks_all, 
> and since nf_conntrack_lock() reads nf_conntrack_locks_all before 
> releasing nf_conntrack_locks_all_lock, by (b) the read cannot see the 
> write.

And the Linux kernel memory model (https://lwn.net/Articles/718628/
and https://lwn.net/Articles/720550/) agrees with Alan.  Here is
a litmus test, which emulates spin_lock() with xchg_acquire() and
spin_unlock() with smp_store_release():

------------------------------------------------------------------------

C C-ManfredSpraul-L1G1xchgnr.litmus

(* Expected result: Never.  *)

{
}

P0(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held)
{
	/* Acquire local lock. */
	r10 = xchg_acquire(lcl, 1);
	r1 = READ_ONCE(*nfcla);
	if (r1) {
		smp_store_release(lcl, 0);
		r11 = xchg_acquire(gbl, 1);
		r12 = xchg_acquire(lcl, 1);
		smp_store_release(gbl, 0);
	}
	r2 = READ_ONCE(*gbl_held);
	WRITE_ONCE(*lcl_held, 1);
	WRITE_ONCE(*lcl_held, 0);
	smp_store_release(lcl, 0);
}

P1(int *nfcla, spinlock_t *gbl, int *gbl_held, spinlock_t *lcl, int *lcl_held)
{
	/* Acquire global lock. */
	r10 = xchg_acquire(gbl, 1);
	WRITE_ONCE(*nfcla, 1);
	r11 = xchg_acquire(lcl, 1);
	smp_store_release(lcl, 0);
	r2 = READ_ONCE(*lcl_held);
	WRITE_ONCE(*gbl_held, 1);
	WRITE_ONCE(*gbl_held, 0);
	smp_store_release(gbl, 0);
}

exists
((0:r2=1 \/ 1:r2=1) /\ 0:r10=0 /\ 0:r11=0 /\ 0:r12=0 /\ 1:r10=0 /\ 1:r11=0)

------------------------------------------------------------------------

The memory model says that the forbidden state does not happen:

------------------------------------------------------------------------

States 25
0:r10=0; 0:r11=0; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=0; 0:r11=0; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=1; 1:r2=0;
0:r10=0; 0:r11=0; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=1; 1:r2=1;
0:r10=0; 0:r11=0; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=1; 1:r2=0;
0:r10=0; 0:r11=0; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=1; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=1; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=1; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=1; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=1; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=1; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=1; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=0; 0:r11=1; 0:r12=1; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=0; 0:r11=1; 0:r12=1; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=1; 0:r11=0; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=1; 0:r11=0; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=1; 0:r11=0; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=1; 0:r11=0; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=1; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=1; 0:r11=1; 0:r12=0; 0:r2=0; 1:r10=0; 1:r11=0; 1:r2=1;
0:r10=1; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=0;
0:r10=1; 0:r11=1; 0:r12=0; 0:r2=1; 1:r10=0; 1:r11=0; 1:r2=1;
No
Witnesses
Positive: 0 Negative: 260
Condition exists ((0:r2=1 \/ 1:r2=1) /\ 0:r10=0 /\ 0:r11=0 /\ 0:r12=0 /\ 1:r10=0 /\ 1:r11=0)
Observation C-ManfredSpraul-L1G1xchgnr Never 0 260

------------------------------------------------------------------------

(Note the line "Positive: 0 Negative: 260", in other words, there
were no scenarios that matched the "exists" clause and 260 that did
not match.)

Of course, testing is also required.  ;-)

Manfred, any objections to my changing your patch as Alan suggests?

							Thanx, Paul