Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive

[Michel Lespinasse]

On Fri, Mar 29, 2013 at 2:57 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, 2013-03-28 at 19:50 -0700, Michel Lespinasse wrote:
>> So, there are a few things I don't like about spin_unlock_wait():
>>
>> 1- From a lock ordering point of view, it is strictly equivalent to
>> taking the lock and then releasing it - and yet, lockdep won't catch
>> any deadlocks that involve spin_unlock_wait. (Not your fault here,
>> this should be fixed as a separate change in lockdep. I manually
>> looked at the lock ordering here and found it safe).
>
> Ooh, I never noticed that, but indeed this shouldn't be hard to cure.
>
>> 2- With the current ticket lock implementation, a stream of lockers
>> can starve spin_unlock_wait() forever. Once again, not your fault and
>> I suspect this could be fixed - I expect spin_unlock_wait() callers
>> actually only want to know that the lock has been passed on, not that
>> it actually got to an unlocked state.
>
> I suppose the question is do we want to fix it or have both semantics
> and use lock+unlock where appropriate.

We'd have to look at the users to be sure, but I strongly expect they
don't need to get in line waiting - it's sufficient to just wait for
the head of the queue to move (or for the queue to be empty).

There are actually very few users - Just drivers/ata/libata-eh.c for
the spin_unlock_wait() function, and a couple more (kernel/task_work.c
and kernel/exit.c) for the raw_spin_unlock_wait variant. Guess I'm not
the only one to dislike that function :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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