From: Paul E. McKenney <paul.mckenney@linaro.org>
Date: Thu, 15 Mar 2012 19:16:26 +0000 (-0700)
Subject: rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU
X-Git-Tag: v3.5-rc1~164^2^2~4
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f511fc624642f0bb8cf65aaa28979737514d4746;p=pandora-kernel.git

rcu: Ensure that RCU_FAST_NO_HZ timers expire on correct CPU

Timers are subject to migration, which can lead to the following
system-hang scenario when CONFIG_RCU_FAST_NO_HZ=y:

1.	CPU 0 executes synchronize_rcu(), which posts an RCU callback.

2.	CPU 0 then goes idle.  It cannot immediately invoke the callback,
	but there is nothing RCU needs from ti, so it enters dyntick-idle
	mode after posting a timer.

3.	The timer gets migrated to CPU 1.

4.	CPU 0 never wakes up, so the synchronize_rcu() never returns, so
	the system hangs.

This commit fixes this problem by using mod_timer_pinned(), as suggested
by Peter Zijlstra, to ensure that the timer is actually posted on the
running CPU.

Reported-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

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