From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Wed, 24 Aug 2011 23:52:09 +0000 (-0700)
Subject: rcu: Move propagation of ->completed from rcu_start_gp() to rcu_report_qs_rsp()
X-Git-Tag: v3.2-rc1~165^2^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=afe24b122eb6edb5f1cb942570ac8d766105c7fc;p=pandora-kernel.git

rcu: Move propagation of ->completed from rcu_start_gp() to rcu_report_qs_rsp()

It is possible for the CPU that noted the end of the prior grace period
to not need a new one, and therefore to decide to propagate ->completed
throughout the rcu_node tree without starting another grace period.
However, in so doing, it releases the root rcu_node structure's lock,
which can allow some other CPU to start another grace period.  The first
CPU will be propagating ->completed in parallel with the second CPU
initializing the rcu_node tree for the new grace period.  In theory
this is harmless, but in practice we need to keep things simple.

This commit therefore moves the propagation of ->completed to
rcu_report_qs_rsp(), and refrains from marking the old grace period
as having been completed until it has finished doing this.  This
prevents anyone from starting a new grace period concurrently with
marking the old grace period as having been completed.

Of course, the optimization where a CPU needing a new grace period
doesn't bother marking the old one completed is still in effect:
In that case, the marking happens implicitly as part of initializing
the new grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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