Re: [PATCH 4/4 RFC] RCU: synchronize_sched() without migration

[Dipankar Sarma <dipankar@in.ibm.com>]

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