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Date: Mon, 24 Dec 2012 21:20:00 +0530 From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> To: Oleg Nesterov <oleg@...hat.com> CC: tglx@...utronix.de, peterz@...radead.org, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au, mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org, vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu, amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl, wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context On 12/23/2012 10:12 PM, Oleg Nesterov wrote: > On 12/23, Srivatsa S. Bhat wrote: >> >> On 12/20/2012 07:12 PM, Oleg Nesterov wrote: >>> >>> We need mb() + rmb(). Plust cli/sti unless this arch has optimized >>> this_cpu_add() like x86 (as you pointed out). >>> >> >> Hey, IIUC, we actually don't need mb() in the reader!! Just an rmb() will do. > > Well. I don't think so. But when it comes to the barriers I am never sure > until Paul confirms my understanding ;) > >> #define reader_nested_percpu() \ >> (__this_cpu_read(reader_percpu_refcnt) & READER_REFCNT_MASK) >> >> #define writer_active() \ >> (__this_cpu_read(writer_signal)) >> >> >> #define READER_PRESENT (1UL << 16) >> #define READER_REFCNT_MASK (READER_PRESENT - 1) >> >> void get_online_cpus_atomic(void) >> { >> preempt_disable(); >> >> /* >> * First and foremost, make your presence known to the writer. >> */ >> this_cpu_add(reader_percpu_refcnt, READER_PRESENT); >> >> /* >> * If we are already using per-cpu refcounts, it is not safe to switch >> * the synchronization scheme. So continue using the refcounts. >> */ >> if (reader_nested_percpu()) { >> this_cpu_inc(reader_percpu_refcnt); >> } else { >> smp_rmb(); >> if (unlikely(writer_active())) { >> ... //take hotplug_rwlock >> } >> } >> >> ... >> >> /* Prevent reordering of any subsequent reads of cpu_online_mask. */ >> smp_rmb(); >> } >> >> The smp_rmb() before writer_active() ensures that LOAD(writer_signal) follows >> LOAD(reader_percpu_refcnt) (at the 'if' condition). And in turn, that load is >> automatically going to follow the STORE(reader_percpu_refcnt) > > But why this STORE should be visible on another CPU before we LOAD(writer_signal)? > > Lets discuss the simple and artificial example. Suppose we have > > int X, Y; > > int func(void) > { > X = 1; // suppose that nobody else can change it > mb(); > return Y; > } > > Now you are saying that we can change it and avoid the costly mb(): > > int func(void) > { > X = 1; > > if (X != 1) > BUG(); > > rmb(); > return Y; > } > > I doubt. rmb() can only guarantee that the preceding LOAD's should be > completed. Without mb() it is possible that this CPU won't write X to > memory at all. > Oh, ok :-( Thanks for correcting me and for the detailed explanation! For a moment, I really thought we had it solved at last! ;-( Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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