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SubjectRe: [PATCH RFC 01/26] netfilter: Replace spin_unlock_wait() with lock/unlock pair
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@colorfullife.com>
> Cc: <stable@vger.kernel.org>
> Cc: Sasha Levin <sasha.levin@oracle.com>
> Cc: Pablo Neira Ayuso <pablo@netfilter.org>
> Cc: netfilter-devel@vger.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.

Alan Stern

> +
> + /* fast path failed, unlock */
> + spin_unlock(lock);
> +
> + /* Slow path 1) get global lock */
> + spin_lock(&nf_conntrack_locks_all_lock);
> +
> + /* Slow path 2) get the lock we want */
> + spin_lock(lock);
> +
> + /* Slow path 3) release the global lock */
> + spin_unlock(&nf_conntrack_locks_all_lock);
> }
> EXPORT_SYMBOL_GPL(nf_conntrack_lock);
>
> @@ -149,18 +154,17 @@ static void nf_conntrack_all_lock(void)
> int i;
>
> spin_lock(&nf_conntrack_locks_all_lock);
> - nf_conntrack_locks_all = true;
>
> - /*
> - * Order the above store of 'nf_conntrack_locks_all' against
> - * the spin_unlock_wait() loads below, such that if
> - * nf_conntrack_lock() observes 'nf_conntrack_locks_all'
> - * we must observe nf_conntrack_locks[] held:
> - */
> - smp_mb(); /* spin_lock(&nf_conntrack_locks_all_lock) */
> + nf_conntrack_locks_all = true;
>
> for (i = 0; i < CONNTRACK_LOCKS; i++) {
> - spin_unlock_wait(&nf_conntrack_locks[i]);
> + spin_lock(&nf_conntrack_locks[i]);
> +
> + /* This spin_unlock provides the "release" to ensure that
> + * nf_conntrack_locks_all==true is visible to everyone that
> + * acquired spin_lock(&nf_conntrack_locks[]).
> + */
> + spin_unlock(&nf_conntrack_locks[i]);
> }
> }



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