Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

[Manfred Spraul]

Hi Oleg,

On 12/29/24 12:57 PM, Oleg Nesterov wrote:
> On 12/28, Manfred Spraul wrote:
> > On 12/28/24 4:22 PM, Oleg Nesterov wrote:
> > > Now suppose that another CPU executes wake() between LOAD(CONDITION)
> > > and list_add(entry, head). With your patch wait() will miss the event.
> > > The same for __pollwait(), I think...
> ...
>
> > It could still work for prepare_to_wait and thus fs/pipe, since then the
> > smb_mb() in set_current_state prevents earlier execution.

From now, I'll try to follow standard patterns:

every memory barrier must be paired. And adding barriers to common 
functions for potentially rare situations is now allowed.

(unless it is a bugfix).

And then enumerate all codepaths:

For the wait_event users: We have a smp_mb() in prepare_to_wait(), it 
could pair with the barrier in wq_has_sleepers().

> Not sure, please see the note about __pollwait() above.
>
> I think that your patch (and the original patch from WangYuli) has the same
> proble with pipe_poll()->poll_wait()->__pollwait().

What is the memory barrier for pipe_poll()?

There is poll_wait()->__pollwait()->add_wait_queue()->spin_unlock(). 
thus only store_release.

And then READ_ONCE(), i.e. no memory barrier.

Thus the CPU would be free to load pipe->head and pipe->tail before 
adding the entry to the poll table.

Correct?

--

    Manfred