On Tue, Aug 07, 2007 at 11:52:26AM -0700, Paul E. McKenney wrote: > The combination of CPU hotplug and PREEMPT_RCU has resulted in deadlocks > due to the migration-based implementation of synchronize_sched() in -rt. > This experimental patch maps synchronize_sched() back onto Classic RCU, > eliminating the migration, thus hopefully also eliminating the deadlocks. > It is not clear that this is a good long-term approach, but it will at > least permit people doing CPU hotplug in -rt kernels additional wiggle > room in their design and implementation. > > The basic approach is to cause the -rt kernel to incorporate rcuclassic.c > as well as rcupreempt.c, but to #ifdef out the conflicting portions of > rcuclassic.c so that only the code needed to implement synchronize_sched() > remains in a PREEMPT_RT build. Invocations of grace-period detection > from the scheduling-clock interrupt go to rcuclassic.c, which then invokes > the corresponding functions in rcupreempt.c (with _rt suffix added to > keep the linker happy). Also applies the RCU_SOFTIRQ to classic RCU. > The bulk of this patch just moves code around. > > If this patch does turn out to be the right approach, the #ifdefs in > kernel/rcuclassic.c might be dealt with. ;-) I think the right thing to do would be to fix cpu hotplug to use a simple reference count, as the latest patches Gautham is working on. That simplifies the implementation from the earlier version with a per-cpu reference counter with RCU. No freezer, no lock. Thanks Dipankar - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
