* David Howells <[email protected]> wrote:
> They still aren't allowed to. Consider:
>
> CPU 1 CPU 2
> ======================= =======================
> -->down_read(&A);
> <--down_read(&A);
> -->down_write(&A);
> --- SLEEPING ---
> -->down_read(&A);
> --- DEADLOCKED ---
i think you misunderstood what nested locking means in the lockdep case.
(and that is my fault, for not adding enough documentation to
down_write_nested() and down_read_nested().)
nested locking does not mean the same instance is allowed to nest! It
only allows different-instance nesting, 'nesting within the same lock
class'. (Lockdep has a very broad notion of 'lock class', to achieve the
collection of very generic locking rules and to do as generic validation
as possible. See Documentation/lockdep-design.txt for more details.)
Rw-locks on the other hand have special permission to nest for the same
instance too. See commit 6c9076ec9cd448f43bbda871352a7067f456ee26:
lockdep so far only allowed read-recursion for the same lock instance.
This is enough in the overwhelming majority of cases, but a hostap case
triggered and reported by Miles Lane relies on same-class
different-instance recursion. So we relax the restriction on read-lock
recursion.
(This change does not allow rwsem read-recursion, which is still
forbidden.)
also please see the testcases in lib/locking-selftest.c, we specifically
test the rwsem scenario you outlined above, to make sure the validator
immediately flags it:
------------------------
| Locking API testsuite:
----------------------------------------------------------------------------
| spin |wlock |rlock |mutex | wsem | rsem |
--------------------------------------------------------------------------
[...]
recursive read-lock: | ok | | ok |
recursive read-lock #2: | ok | | ok |
[...]
Ingo
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