Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers

[Davidlohr Bueso <dave@stgolabs.net>]

On Fri, 11 Oct 2019, Manfred Spraul wrote:
>But you are right, there are two different scenarios:
>
>1) thread already in another wake_q, wakeup happens immediately after 
>the cmpxchg_relaxed().
>
>This scenario is safe, due to the smp_mb__before_atomic() in wake_q_add()
>
>2) thread woken up but e.g. a timeout, see ->state=STATE_READY, 
>returns to user space, calls sys_exit.
>
>This must not happen before get_task_struct acquired a reference.
>
>And this appears to be unsafe: get_task_struct() is refcount_inc(), 
>which is refcount_inc_checked(), which is according to lib/refcount.c 
>fully unordered.
>
>Thus: ->state=STATE_READY can execute before the refcount increase.
>
>Thus: ->state=STATE_READY needs a smp_store_release(), correct?

What if we did the reference count explicitly, and then just use
wake_q_add_safe()? That would avoid the extra barrier, __pipelined_op()
would become:

      list_del();
      get_task_struct();
      wake_q_add_safe();
      WRITE_ONCE(->state, STATE_READY);

Thanks,
Davidlohr