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doc: rcu: Better clarify the rcu_segcblist ->len field 

An important note under the rcu_segcblist description could use a more
detailed description. Especially explanation of the scenario where the
->head field may be temporarily NULL making it not wise to rely on it
to determine if callbacks are associated with the rcu_segcblist.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Joel Fernandes (Google) 2018-09-25 11:26:00 -07:00 committed by Paul E. McKenney
parent b54d9db260
commit 82eccec851
1 changed files with 15 additions and 8 deletions
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23
Documentation/RCU/Design/Data-Structures/Data-Structures.html
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@ -928,17 +928,24 @@ this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>-&gt;head</tt>
pointer.
The reason for this is that all the ready-to-invoke callbacks
(that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
all at once at callback-invocation time.
all at once at callback-invocation time (<tt>rcu_do_batch</tt>), due
to which <tt>-&gt;head</tt> may be set to NULL if there are no not-done
callbacks remaining in the <tt>rcu_segcblist</tt>.
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 <tt>RCU_DONE_TAIL</tt> segment.
Either way, the <tt>-&gt;len</tt> and <tt>-&gt;len_lazy</tt> counts
are adjusted after the corresponding callbacks have been invoked, and so
again it is the <tt>-&gt;len</tt> count that accurately reflects whether
or not there are callbacks associated with this <tt>rcu_segcblist</tt>
structure.
callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment and
<tt>-&gt;head</tt> once again points to the start of the segment.
In short, the head field can briefly be <tt>NULL</tt> even though the
CPU has callbacks present the entire time.
Therefore, it is not appropriate to test the <tt>-&gt;head</tt> pointer
for <tt>NULL</tt>.
<p>In contrast, the <tt>-&gt;len</tt> and <tt>-&gt;len_lazy</tt> counts
are adjusted only after the corresponding callbacks have been invoked.
This means that the <tt>-&gt;len</tt> count is zero only if
the <tt>rcu_segcblist</tt> structure really is devoid of callbacks.
Of course, off-CPU sampling of the <tt>-&gt;len</tt> count requires
the use of appropriate synchronization, for example, memory barriers.
careful use of appropriate synchronization, for example, memory barriers.
This synchronization can be a bit subtle, particularly in the case
of <tt>rcu_barrier()</tt>.