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

[Davidlohr Bueso]

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