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

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Message-ID: <CAEXW_YTv2wFoaVW-1yv_S5pRo85Tiad-B4u4EaYBdCx5_KWRkQ@mail.gmail.com>
Date:   Sun, 23 Sep 2018 19:31:37 -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 7:30 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.