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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-12-08 20:26:55 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2015-01-06 11:05:28 -0800
commite3663b1024d1f94688e5233440ad67a9bc10b94e (patch)
tree429ec7e23feb1193df7b45d8633a822adb77905b /init
parent6ccd2ecd422644277b7d8b37222e3af3f43ea9ae (diff)
downloadlinux-stericsson-e3663b1024d1f94688e5233440ad67a9bc10b94e.tar.gz
rcu: Handle gpnum/completed wrap while dyntick idle
Subtle race conditions can result if a CPU stays in dyntick-idle mode long enough for the ->gpnum and ->completed fields to wrap. For example, consider the following sequence of events: o CPU 1 encounters a quiescent state while waiting for grace period 5 to complete, but then enters dyntick-idle mode. o While CPU 1 is in dyntick-idle mode, the grace-period counters wrap around so that the grace period number is now 4. o Just as CPU 1 exits dyntick-idle mode, grace period 4 completes and grace period 5 begins. o The quiescent state that CPU 1 passed through during the old grace period 5 looks like it applies to the new grace period 5. Therefore, the new grace period 5 completes without CPU 1 having passed through a quiescent state. This could clearly be a fatal surprise to any long-running RCU read-side critical section that happened to be running on CPU 1 at the time. At one time, this was not a problem, given that it takes significant time for the grace-period counters to overflow even on 32-bit systems. However, with the advent of NO_HZ_FULL and SMP embedded systems, arbitrarily long idle periods are now becoming quite feasible. It is therefore time to close this race. This commit therefore avoids this race condition by having the quiescent-state forcing code detect when a CPU is falling too far behind, and setting a new rcu_data field ->gpwrap when this happens. Whenever this new ->gpwrap field is set, the CPU's ->gpnum and ->completed fields are known to be untrustworthy, and can be ignored, along with any associated quiescent states. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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