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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Sun, 23 Sep 2018 19:30:30 -0400
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Subject: Question about ->head field of rcu_segcblist
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     LKML <linux-kernel@vger.kernel.org>
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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.

Important note: It is the ->len field that determines whether or not
there are callbacks associated with this rcu_segcblist structure, not
the ->head pointer. The reason for this is that all the
ready-to-invoke callbacks (that is, those in the RCU_DONE_TAIL
segment) are extracted all at once at callback-invocation time. If
callback invocation must be postponed, for example, because a
high-priority process just woke up on this CPU, then the remaining
callbacks are placed back on the RCU_DONE_TAIL segment. Either way,
the ->len and ->len_lazy counts are adjusted after the corresponding
callbacks have been invoked, and so again it is the ->lencount that
accurately reflects whether or not there are callbacks associated with
this rcu_segcblist structure. Of course, off-CPU sampling of the ->len
count requires the use of appropriate synchronization, for example,
memory barriers. This synchronization can be a bit subtle,
particularly in the case of rcu_barrier().

Thanks!

 - Joel