From mboxrd@z Thu Jan 1 00:00:00 1970 From: Heiko Carstens Subject: Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context Date: Tue, 6 Jun 2017 19:20:15 +0200 Message-ID: <20170606172015.GA3023@osiris> References: <20170605220919.GA27820@linux.vnet.ibm.com> <1496700591-30177-1-git-send-email-paulmck@linux.vnet.ibm.com> <20170606105343.ibhzrk6jwhmoja5t@hirez.programming.kicks-ass.net> <20170606152705.GD6681@osiris> <20170606161551.dy5lr6mo6vqujk5d@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christian Borntraeger , Paolo Bonzini , "Paul E. McKenney" , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, kvm@vger.kernel.org, Linus Torvalds , Martin Schwidefsky , linux-s390 To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <20170606161551.dy5lr6mo6vqujk5d@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, Jun 06, 2017 at 06:15:51PM +0200, Peter Zijlstra wrote: > On Tue, Jun 06, 2017 at 05:27:06PM +0200, Heiko Carstens wrote: > > On Tue, Jun 06, 2017 at 04:45:57PM +0200, Christian Borntraeger wrote: > > > > As a side note, I am asking myself, though, why we do need the > > > preempt_disable/enable for the cases where we use the opcodes > > > like lao (atomic load and or to a memory location) and friends. > > > > Because you want the atomic instruction to be executed on the local cpu for > > which you have to per cpu pointer. If you get preempted to a different cpu > > between the ptr__ assignment and lan instruction it might be executed not > > on the local cpu. It's not really a correctness issue. > > As per the previous email, I think it is a correctness issue wrt CPU > hotplug. Yes, I realized that just a minute after I sent the above. > > However in reality it doesn't matter at all, since all distributions we > > care about have preemption disabled. > > Well, either you support PREEMPT=y or you don't :-) If you do, it needs > to be correct, irrespective of what distro's do with it. That is true. Our s390 specific percpu ops are supposed to be correct for PREEMPT=y, and that's apparently the only reason why I added the preempt disable/enable pairs back then. I just had to remember why I did that ;) > > So this_cpu_inc() should just generate three instructions: two to calculate > > the percpu pointer and an additional asi for the atomic increment, with > > operand specific serialization. This is supposed to be a lot faster than > > disabling/enabling interrupts around a non-atomic operation. > > So typically we joke about s390 that it has an instruction for this > 'very-complicated-thing', but here you guys do not, what gives? ;-) Tough luck. :)