some questions about fast-path-lock

I got some idea from the README under storage/lmgr and read some code of
LockAcquireExtended , but I still have some questions now.

LWLockAcquire(&MyProc->backendLock, LW_EXCLUSIVE);
if (FastPathStrongRelationLocks->count[fasthashcode] != 0)
acquired = false;
else
acquired = FastPathGrantRelationLock(locktag->locktag_field2,
lockmode);

1. In the README, it says: "A key point of this algorithm is that it
must be possible to verify the
absence of possibly conflicting locks without fighting over a shared LWLock
or
spinlock. Otherwise, this effort would simply move the contention
bottleneck
from one place to another."

but in the code, there is LWLockAcquire in the above code. Actually I
can't think out how can we proceed without a lock.

2. Why does the MyProc->backendLock work? it is MyProc not a global
lock.

3. for the line, acquired =
FastPathGrantRelationLock(locktag->locktag_field2,
lockmode); I think it should be able to replaced with "acquired =
true" (but obviously I'm wrong) . I read "FastPathGrantRelationLock" but
can't understand it.

Any hint will be helpful. thanks!