linux-kernel - Re: Question about ->head field of rcu

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Message-ID: <CAEXW_YRTeJHYtoxiTbX+cq3FCRTpigvzRpvApFtUL8HBEozpXA@mail.gmail.com>
Date:   Sun, 23 Sep 2018 22:25:44 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about ->head field of rcu_segcblist

On Sun, Sep 23, 2018 at 7:54 PM Paul E. McKenney <paulmck@...ux.ibm.com> wrote:
>
> On Sun, Sep 23, 2018 at 07:30:30PM -0400, Joel Fernandes wrote:
> > Hi Paul,
> >
> > I was parsing the Data-Structures document and had a question about
> > the following "Important note" text.
> >
> > Could it be clarified in the below text better why "remaining
> > callbacks are placed back on the RCU_DONE_TAIL segment", is a reason
> > for not depending on ->head for determining if no callbacks are
> > associated with the rcu_segcblist? If callbacks are added back to the
> > DONE_TAIL segment, then I would think rcu_head should be != NULL.
> > Infact the "rsclp->head = *rsclp->tails[RCU_DONE_TAIL];" in
> > rcu_segcblist_extract_done_cbs should set the ->head to  NULL if I
> > understand correctly.
>
> The rcu_segcblist_extract_done_cbs() function will set rsclp->head
> to NULL only if there were no non-done callbacks on the rsclp list.
> Otherwise, if there are non-done callbacks, then rsclp->head will
> be set to the first non-done callback.
>
> Either way, the problem is that the done callbacks can be removed
> and re-added, but the count is not adjusted until the re-add.  So
> you have to look at the count to see if there are callbacks.
>
> Testing rsclp->head fails because it can be temporarily NULL, even
> though there are callbacks hanging off of a pointer in rcu_do_batch()'s
> stack frame.
>
> Or am I misunderstanding your question?

Thanks yes that clears it up, I see what you mean that that ->head
field is temporarily volatile and really the ->len tells the real
story :-)

- Joel