Messages in this thread | | | From | Pranith Kumar <> | Date | Wed, 23 Jul 2014 10:01:14 -0400 | Subject | Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU |
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On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Wed, Jul 23, 2014 at 09:23:47AM -0400, Pranith Kumar wrote: >> On Wed, Jul 23, 2014 at 8:09 AM, Paul E. McKenney >> <paulmck@linux.vnet.ibm.com> wrote: >> > On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote: >> >> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from >> >> invoke_rcu_core() which is called from __call_rcu_core(). >> >> >> >> Each of these three functions checks if the cpu is online. We can remove the >> >> redundant ones. This commit removes one of these redundant check. >> >> >> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com> >> > >> > Sorry, but no. There can be a long delay between raise_softirq() and >> > this function starting, particularly if ksoftirqd gets involved. The >> > CPU could easily go offline in the meantime. >> > >> >> That makes sense. I guess one of the other two checks in >> __call_rcu_core() or invoke_rcu_core() can go then? > > What would have to be the case for this suggested change to be safe? > Does that condition in fact hold? >
The only scenario which is unsafe is when this thread is preempted and scheduled on a dying CPU. In that case we should turn off interrupts.
Actually, checking the code, I see that interrupts are turned off already before we call _call_rcu_core(). So I am not sure such a case will happen. On the other hand invoke_rcu_core() will be a one line function which might as well be in-lined to avoid the double cpu online check.
What am I missing?
-- Pranith
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