On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney <paul...@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 >> <paul...@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.pr...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
