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

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Message-Id: <20180923235512.GB4222@linux.ibm.com>
Date:   Sun, 23 Sep 2018 16:55:12 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about ->head field of rcu_segcblist

On Sun, Sep 23, 2018 at 07:31:37PM -0400, Joel Fernandes wrote:
> On Sun, Sep 23, 2018 at 7:30 PM Joel Fernandes <joel@...lfernandes.org> 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.
> 
> Just to clarify, I meant set to NULL assuming all cbs were done
> waiting and ready to be invoked.

Ah, good, then that is correct.  But even then, being NULL doesn't mean
no callbacks because they might be temporarily held by rcu_do_batch().

							Thanx, Paul