documentation: Sharpen up the no-readers quick quiz

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<tr><th align="left">Quick Quiz:</th></tr>

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	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?

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<tr><th align="left">Answer:</th></tr>

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	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.

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