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Commit e2c85cb1 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

documentation: Present updated RCU guarantee 



Recent memory-model work deduces the relationships of RCU read-side
critical sections and grace periods based on the relationships of
accesses within a critical section and accesses preceding and following
the grace period.  This commit therefore adds this viewpoint.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 1001354c

Documentation/RCU/Design/Requirements/Requirements.html

+24 −1
Original line number Diff line number Diff line
@@ -547,7 +547,7 @@ The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to 
	It could reuse a value formerly fetched from this same pointer.

	It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time

	manner, resulting in <i>load tearing</i>, in turn resulting a bytewise

	mash-up of two distince pointer values.

	mash-up of two distinct pointer values.

	It might even use value-speculation optimizations, where it makes

	a wrong guess, but by the time it gets around to checking the

	value, an update has changed the pointer to match the wrong guess.
@@ -659,6 +659,29 @@ systems with more than one CPU: 
	In other words, a given instance of <tt>synchronize_rcu()</tt>

	can avoid waiting on a given RCU read-side critical section only

	if it can prove that <tt>synchronize_rcu()</tt> started first.



	<p>

	A related question is &ldquo;When <tt>rcu_read_lock()</tt>

	doesn't generate any code, why does it matter how it relates

	to a grace period?&rdquo;

	The answer is that it is not the relationship of

	<tt>rcu_read_lock()</tt> itself that is important, but rather

	the relationship of the code within the enclosed RCU read-side

	critical section to the code preceding and following the

	grace period.

	If we take this viewpoint, then a given RCU read-side critical

	section begins before a given grace period when some access

	preceding the grace period observes the effect of some access

	within the critical section, in which case none of the accesses

	within the critical section may observe the effects of any

	access following the grace period.



	<p>

	As of late 2016, mathematical models of RCU take this

	viewpoint, for example, see slides&nbsp;62 and&nbsp;63

	of the

	<a href="http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.2016.10.04c.LCE.pdf">2016 LinuxCon EU</a>

	presentation.

</font></td></tr>

<tr><td>&nbsp;</td></tr>

</table>