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documentation: Sharpen up the no-readers quick quiz

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Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Paul E. McKenney 2016-03-15 13:40:28 -07:00
parent 6146f8df48
commit 5413e24c94
1 changed files with 8 additions and 5 deletions
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13
Documentation/RCU/Design/Requirements/Requirements.html
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@ -1002,18 +1002,21 @@ obligation to wait for these new readers.
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Suppose that synchronize_rcu() did wait until all readers had completed.
Would the updater be able to rely on this?
Suppose that synchronize_rcu() did wait until <i>all</i>
readers had completed instead of waiting only on
pre-existing readers.
For how long would the updater be able to rely on there
being no readers?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
No.
For no time at all.
Even if <tt>synchronize_rcu()</tt> were to wait until
all readers had completed, a new reader might start immediately after
<tt>synchronize_rcu()</tt> completed.
Therefore, the code following
<tt>synchronize_rcu()</tt> cannot rely on there being no readers
in any case.
<tt>synchronize_rcu()</tt> can <i>never</i> rely on there being
no readers.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>