rqspinlock: Enclose lock/unlock within lock entry acquisitions

There is a tiny window between the fast-path cmpxchg and the grabbing of
the lock entry where an NMI could land, attempt the same lock that was
just acquired, and end up timing out. This is not ideal. Instead, move
the lock entry acquisition from the fast path to before the cmpxchg, and
remove the grabbing of the lock entry in the slow path, assuming it was
already taken by the fast path.

There is a similar case when unlocking the lock. If the NMI lands
between the WRITE_ONCE and smp_store_release, it is possible that we end
up in a situation where the NMI fails to diagnose the AA condition,
leading to a timeout.

Note the changes to the comments in release_held_lock_entry and
res_spin_unlock. They talk about prevention of the following scenario:
(assuming we don't place unlock release after WRITE_ONCE)

grab entry A
lock A
grab entry B
lock B
unlock B
   smp_store_release(B->locked, 0)
							grab entry B
							lock B
							grab entry A
							lock A
							! <detect ABBA>
   WRITE_ONCE(rqh->locks[rqh->cnt - 1], NULL)

If the store release were placed after the WRITE_ONCE, the other CPU
would not observe B in the table of the CPU unlocking the lock B.

Avoiding this while it was convenient was a prudent choice, but since it
leads to missed diagnosis of AA deadlocks in case of NMIs, it does not
make sense to keep such ordering any further. Moreover, while this
particular schedule is a misdiagnosis, the CPUs are obviously
participating in an ABBA deadlock otherwise, and are only lucky to avoid
an error before due to the aforementioned race.

Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>

Author: kkdwvd

include/asm-generic/rqspinlock.h

@@ -129,8 +129,8 @@ static __always_inline void release_held_lock_entry(void)
 	 * <error> for lock B
 	 * release_held_lock_entry
 	 *
-	 * try_cmpxchg_acquire for lock A
 	 * grab_held_lock_entry
+	 * try_cmpxchg_acquire for lock A
 	 *
 	 * Lack of any ordering means reordering may occur such that dec, inc
 	 * are done before entry is overwritten. This permits a remote lock
@@ -139,13 +139,8 @@ static __always_inline void release_held_lock_entry(void)
 	 * CPU holds a lock it is attempting to acquire, leading to false ABBA
 	 * diagnosis).
 	 *
-	 * In case of unlock, we will always do a release on the lock word after
-	 * releasing the entry, ensuring that other CPUs cannot hold the lock
-	 * (and make conclusions about deadlocks) until the entry has been
-	 * cleared on the local CPU, preventing any anomalies. Reordering is
-	 * still possible there, but a remote CPU cannot observe a lock in our
-	 * table which it is already holding, since visibility entails our
-	 * release store for the said lock has not retired.
+	 * The case of unlock is treated differently due to NMI reentrancy, see
+	 * comments in res_spin_unlock.
 	 *
 	 * In theory we don't have a problem if the dec and WRITE_ONCE above get
 	 * reordered with each other, we either notice an empty NULL entry on
@@ -175,10 +170,16 @@ static __always_inline int res_spin_lock(rqspinlock_t *lock)
 {
 	int val = 0;
 
-	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL))) {
-		grab_held_lock_entry(lock);
+	/*
+	 * Grab the deadlock detection entry before doing the cmpxchg, so that
+	 * reentrancy due to NMIs between the succeeding cmpxchg and creation of
+	 * held lock entry can correctly detect an acquisition attempt in the
+	 * interrupted context.
+	 */
+	grab_held_lock_entry(lock);
+
+	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
 		return 0;
-	}
 	return resilient_queued_spin_lock_slowpath(lock, val);
 }
 
@@ -192,28 +193,32 @@ static __always_inline void res_spin_unlock(rqspinlock_t *lock)
 {
 	struct rqspinlock_held *rqh = this_cpu_ptr(&rqspinlock_held_locks);
 
-	if (unlikely(rqh->cnt > RES_NR_HELD))
-		goto unlock;
-	WRITE_ONCE(rqh->locks[rqh->cnt - 1], NULL);
-unlock:
 	/*
-	 * Release barrier, ensures correct ordering. See release_held_lock_entry
-	 * for details.  Perform release store instead of queued_spin_unlock,
-	 * since we use this function for test-and-set fallback as well. When we
-	 * have CONFIG_QUEUED_SPINLOCKS=n, we clear the full 4-byte lockword.
+	 * Release barrier, ensures correct ordering. Perform release store
+	 * instead of queued_spin_unlock, since we use this function for the TAS
+	 * fallback as well. When we have CONFIG_QUEUED_SPINLOCKS=n, we clear
+	 * the full 4-byte lockword.
+	 */
+	smp_store_release(&lock->locked, 0);
+	if (likely(rqh->cnt <= RES_NR_HELD))
+		WRITE_ONCE(rqh->locks[rqh->cnt - 1], NULL);
+	/*
+	 * Unlike release_held_lock_entry, we do the lock word release before
+	 * rewriting the entry back to NULL. This opens up a window where
+	 * another CPU could acquire this lock, and then observe it in our table
+	 * on the current CPU, leading to possible misdiagnosis of ABBA when we
+	 * get reordered with a grab_held_lock_entry's writes (see the case
+	 * described in release_held_lock_entry comments).
 	 *
-	 * Like release_held_lock_entry, we can do the release before the dec.
-	 * We simply care about not seeing the 'lock' in our table from a remote
-	 * CPU once the lock has been released, which doesn't rely on the dec.
+	 * This could be avoided if we did the smp_store_release right before
+	 * the dec, ensuring that the remote CPU could only acquire this lock
+	 * and never observe this lock in our table.
 	 *
-	 * Unlike smp_wmb(), release is not a two way fence, hence it is
-	 * possible for a inc to move up and reorder with our clearing of the
-	 * entry. This isn't a problem however, as for a misdiagnosis of ABBA,
-	 * the remote CPU needs to hold this lock, which won't be released until
-	 * the store below is done, which would ensure the entry is overwritten
-	 * to NULL, etc.
+	 * However, that opens up a window where reentrant NMIs on this same
+	 * CPU could have their AA heuristics fail to fire if they land between
+	 * the WRITE_ONCE and unlock release store, which would result in a
+	 * timeout.
 	 */
-	smp_store_release(&lock->locked, 0);
 	this_cpu_dec(rqspinlock_held_locks.cnt);
 }
 

kernel/bpf/rqspinlock.c

@@ -275,7 +275,7 @@ int __lockfunc resilient_tas_spin_lock(rqspinlock_t *lock)
 	int val, ret = 0;
 
 	RES_INIT_TIMEOUT(ts);
-	grab_held_lock_entry(lock);
+	/* Deadlock detection entry already held after failing slow path. */
 
 	/*
 	 * Since the waiting loop's time is dependent on the amount of
@@ -397,10 +397,7 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 		goto queue;
 	}
 
-	/*
-	 * Grab an entry in the held locks array, to enable deadlock detection.
-	 */
-	grab_held_lock_entry(lock);
+	/* Deadlock detection entry already held after failing slow path. */
 
 	/*
 	 * We're pending, wait for the owner to go away.
@@ -448,11 +445,7 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val)
 	 */
 queue:
 	lockevent_inc(lock_slowpath);
-	/*
-	 * Grab deadlock detection entry for the queue path.
-	 */
-	grab_held_lock_entry(lock);
-
+	/* Deadlock detection entry already held after failing slow path. */
 	node = this_cpu_ptr(&rqnodes[0].mcs);
 	idx = node->count++;
 	tail = encode_tail(smp_processor_id(), idx);