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Date: Mon, 3 Jul 2017 16:04:14 -0400 (EDT)
From: Alan Stern <stern@...land.harvard.edu>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc: Manfred Spraul <manfred@...orfullife.com>,
<linux-kernel@...r.kernel.org>, <netfilter-devel@...r.kernel.org>,
<netdev@...r.kernel.org>, <oleg@...hat.com>,
<akpm@...ux-foundation.org>, <mingo@...hat.com>,
<dave@...olabs.net>, <tj@...nel.org>, <arnd@...db.de>,
<linux-arch@...r.kernel.org>, <will.deacon@....com>,
<peterz@...radead.org>, <parri.andrea@...il.com>,
<torvalds@...ux-foundation.org>,
Pablo Neira Ayuso <pablo@...filter.org>,
Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
Florian Westphal <fw@...len.de>,
"David S. Miller" <davem@...emloft.net>, <coreteam@...filter.org>
Subject: Re: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with
lock/unlock pair
On Mon, 3 Jul 2017, Paul E. McKenney wrote:
> 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 <manfred@...orfullife.com>
> > > Cc: <stable@...r.kernel.org>
> > > Cc: Sasha Levin <sasha.levin@...cle.com>
> > > Cc: Pablo Neira Ayuso <pablo@...filter.org>
> > > Cc: netfilter-devel@...r.kernel.org
> > > ---
> > > 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);
This litmus test is incomplete, because it omits the assignment setting
nf_conntrack_locks_all back to false when the global lock is released.
You should insert
smp_store_release(*nfcla, 0);
right here.
> 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)
With that addition, the litmus test fails unless the read of nfcla in
P0 is an smp_load_acquire. So Manfred's patch should not be changed.
Alan Stern
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