tjh.dev
arm64: spinlock: serialise spin_unlock_wait against concurrent lockers
Boqun Feng reported a rather nasty ordering issue with spin_unlock_wait
on architectures implementing spin_lock with LL/SC sequences and acquire
semantics:
| CPU 1 CPU 2 CPU 3
| ================== ==================== ==============
| spin_unlock(&lock);
| spin_lock(&lock):
| r1 = *lock; // r1 == 0;
| o = READ_ONCE(object); // reordered here
| object = NULL;
| smp_mb();
| spin_unlock_wait(&lock);
| *lock = 1;
| smp_mb();
| o->dead = true;
| if (o) // true
| BUG_ON(o->dead); // true!!
The crux of the problem is that spin_unlock_wait(&lock) can return on
CPU 1 whilst CPU 2 is in the process of taking the lock. This can be
resolved by upgrading spin_unlock_wait to a LOCK operation, forcing it
to serialise against a concurrent locker and giving it acquire semantics
in the process (although it is not at all clear whether this is needed -
different callers seem to assume different things about the barrier
semantics and architectures are similarly disjoint in their
implementations of the macro).
This patch implements spin_unlock_wait using an LL/SC sequence with
acquire semantics on arm64. For v8.1 systems with the LSE atomics, the
exclusive writeback is omitted, since the spin_lock operation is
indivisible and no intermediate state can be observed.
Signed-off-by: Will Deacon <will.deacon@arm.com>