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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b)
    On Tue, Jan 12, 2010 at 10:38:54AM -0500, Mathieu Desnoyers wrote:
    > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
    > > On Sun, Jan 10, 2010 at 11:30:16PM -0500, Mathieu Desnoyers wrote:
    > > > Here is an implementation of a new system call, sys_membarrier(), which
    > > > executes a memory barrier on all threads of the current process.
    > > >
    > > > It aims at greatly simplifying and enhancing the current signal-based
    > > > liburcu userspace RCU synchronize_rcu() implementation.
    > > > (found at http://lttng.org/urcu)
    > >
    > > I didn't expect quite this comprehensive of an implementation from the
    > > outset, but I guess I cannot complain. ;-)
    > >
    > > Overall, good stuff.
    > >
    > > Interestingly enough, what you have implemented is analogous to
    > > synchronize_rcu_expedited() and friends that have recently been added
    > > to the in-kernel RCU API. By this analogy, my earlier semi-suggestion
    > > of synchronize_rcu(0 would be a candidate non-expedited implementation.
    > > Long latency, but extremely low CPU consumption, full batching of
    > > concurrent requests (even unrelated ones), and so on.
    >
    > Yes, the main different I think is that the sys_membarrier
    > infrastructure focuses on IPI-ing only the current process running
    > threads.

    Which does indeed make sense for the expedited interface. On the other
    hand, if you have a bunch of concurrent non-expedited requests from
    different processes, covering all CPUs efficiently satisfies all of
    the requests in one go. And, if you use synchronize_sched() for the
    non-expedited case, there will be no IPIs in the common case.

    > > A few questions interspersed below.
    > >
    > > > Changelog since v1:
    > > >
    > > > - Only perform the IPI in CONFIG_SMP.
    > > > - Only perform the IPI if the process has more than one thread.
    > > > - Only send IPIs to CPUs involved with threads belonging to our process.
    > > > - Adaptative IPI scheme (single vs many IPI with threshold).
    > > > - Issue smp_mb() at the beginning and end of the system call.
    > > >
    > > > Changelog since v2:
    > > >
    > > > - Iteration on min(num_online_cpus(), nr threads in the process),
    > > > taking runqueue spinlocks, allocating a cpumask, ipi to many to the
    > > > cpumask. Does not allocate the cpumask if only a single IPI is needed.
    > > >
    > > >
    > > > Both the signal-based and the sys_membarrier userspace RCU schemes
    > > > permit us to remove the memory barrier from the userspace RCU
    > > > rcu_read_lock() and rcu_read_unlock() primitives, thus significantly
    > > > accelerating them. These memory barriers are replaced by compiler
    > > > barriers on the read-side, and all matching memory barriers on the
    > > > write-side are turned into an invokation of a memory barrier on all
    > > > active threads in the process. By letting the kernel perform this
    > > > synchronization rather than dumbly sending a signal to every process
    > > > threads (as we currently do), we diminish the number of unnecessary wake
    > > > ups and only issue the memory barriers on active threads. Non-running
    > > > threads do not need to execute such barrier anyway, because these are
    > > > implied by the scheduler context switches.
    > > >
    > > > To explain the benefit of this scheme, let's introduce two example threads:
    > > >
    > > > Thread A (non-frequent, e.g. executing liburcu synchronize_rcu())
    > > > Thread B (frequent, e.g. executing liburcu rcu_read_lock()/rcu_read_unlock())
    > > >
    > > > In a scheme where all smp_mb() in thread A synchronize_rcu() are
    > > > ordering memory accesses with respect to smp_mb() present in
    > > > rcu_read_lock/unlock(), we can change all smp_mb() from
    > > > synchronize_rcu() into calls to sys_membarrier() and all smp_mb() from
    > > > rcu_read_lock/unlock() into compiler barriers "barrier()".
    > > >
    > > > Before the change, we had, for each smp_mb() pairs:
    > > >
    > > > Thread A Thread B
    > > > prev mem accesses prev mem accesses
    > > > smp_mb() smp_mb()
    > > > follow mem accesses follow mem accesses
    > > >
    > > > After the change, these pairs become:
    > > >
    > > > Thread A Thread B
    > > > prev mem accesses prev mem accesses
    > > > sys_membarrier() barrier()
    > > > follow mem accesses follow mem accesses
    > > >
    > > > As we can see, there are two possible scenarios: either Thread B memory
    > > > accesses do not happen concurrently with Thread A accesses (1), or they
    > > > do (2).
    > > >
    > > > 1) Non-concurrent Thread A vs Thread B accesses:
    > > >
    > > > Thread A Thread B
    > > > prev mem accesses
    > > > sys_membarrier()
    > > > follow mem accesses
    > > > prev mem accesses
    > > > barrier()
    > > > follow mem accesses
    > > >
    > > > In this case, thread B accesses will be weakly ordered. This is OK,
    > > > because at that point, thread A is not particularly interested in
    > > > ordering them with respect to its own accesses.
    > > >
    > > > 2) Concurrent Thread A vs Thread B accesses
    > > >
    > > > Thread A Thread B
    > > > prev mem accesses prev mem accesses
    > > > sys_membarrier() barrier()
    > > > follow mem accesses follow mem accesses
    > > >
    > > > In this case, thread B accesses, which are ensured to be in program
    > > > order thanks to the compiler barrier, will be "upgraded" to full
    > > > smp_mb() thanks to the IPIs executing memory barriers on each active
    > > > system threads. Each non-running process threads are intrinsically
    > > > serialized by the scheduler.
    > > >
    > > > Just tried with a cache-hot kernel compilation using 6/8 CPUs.
    > > >
    > > > Normally: real 2m41.852s
    > > > With the sys_membarrier+1 busy-looping thread running: real 5m41.830s
    > > >
    > > > So... 2x slower. That hurts.
    > > >
    > > > So let's try allocating a cpu mask for PeterZ scheme. I prefer to have a
    > > > small allocation overhead and benefit from cpumask broadcast if
    > > > possible so we scale better. But that all depends on how big the
    > > > allocation overhead is.
    > > >
    > > > Impact of allocating a cpumask (time for 10,000,000 sys_membarrier
    > > > calls, one thread is doing the sys_membarrier, the others are busy
    > > > looping)). Given that it costs almost half as much to perform the
    > > > cpumask allocation than to send a single IPI, as we iterate on the CPUs
    > > > until we find more than N match or iterated on all cpus. If we only have
    > > > N match or less, we send single IPIs. If we need more than that, then we
    > > > switch to the cpumask allocation and send a broadcast IPI to the cpumask
    > > > we construct for the matching CPUs. Let's call it the "adaptative IPI
    > > > scheme".
    > > >
    > > > For my Intel Xeon E5405
    > > >
    > > > *This is calibration only, not taking the runqueue locks*
    > > >
    > > > Just doing local mb()+single IPI to T other threads:
    > > >
    > > > T=1: 0m18.801s
    > > > T=2: 0m29.086s
    > > > T=3: 0m46.841s
    > > > T=4: 0m53.758s
    > > > T=5: 1m10.856s
    > > > T=6: 1m21.142s
    > > > T=7: 1m38.362s
    > > >
    > > > Just doing cpumask alloc+IPI-many to T other threads:
    > > >
    > > > T=1: 0m21.778s
    > > > T=2: 0m22.741s
    > > > T=3: 0m22.185s
    > > > T=4: 0m24.660s
    > > > T=5: 0m26.855s
    > > > T=6: 0m30.841s
    > > > T=7: 0m29.551s
    > > >
    > > > So I think the right threshold should be 1 thread (assuming other
    > > > architecture will behave like mine). So starting with 2 threads, we
    > > > allocate the cpumask before sending IPIs.
    > > >
    > > > *end of calibration*
    > > >
    > > > Resulting adaptative scheme, with runqueue locks:
    > > >
    > > > T=1: 0m20.990s
    > > > T=2: 0m22.588s
    > > > T=3: 0m27.028s
    > > > T=4: 0m29.027s
    > > > T=5: 0m32.592s
    > > > T=6: 0m36.556s
    > > > T=7: 0m33.093s
    > > >
    > > > The expected top pattern, when using 1 CPU for a thread doing sys_membarrier()
    > > > in a loop and other threads busy-waiting in user-space on a variable shows that
    > > > the thread doing sys_membarrier is doing mostly system calls, and other threads
    > > > are mostly running in user-space. Side-note, in this test, it's important to
    > > > check that individual threads are not always fully at 100% user-space time (they
    > > > range between ~95% and 100%), because when some thread in the test is always at
    > > > 100% on the same CPU, this means it does not get the IPI at all. (I actually
    > > > found out about a bug in my own code while developing it with this test.)
    > >
    > > The below data is for how many threads in the process?
    >
    > 8 threads: one doing sys_membarrier() in a loop, 7 others waiting on a
    > variable.

    OK, thanks for the info!

    > > Also, is "top"
    > > accurate given that the IPI handler will have interrupts disabled?
    >
    > Probably not. AFAIK. "top" does not really consider interrupts into its
    > accounting. So, better take this top output with a grain of salt or two.

    Might need something like oprofile to get good info?

    > > > Cpu0 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > > Cpu1 : 99.7%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.3%hi, 0.0%si, 0.0%st
    > > > Cpu2 : 99.3%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.7%hi, 0.0%si, 0.0%st
    > > > Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > > Cpu4 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > > Cpu5 : 96.0%us, 1.3%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 2.6%si, 0.0%st
    > > > Cpu6 : 1.3%us, 98.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
    > > > Cpu7 : 96.1%us, 3.3%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.3%hi, 0.3%si, 0.0%st
    > > >
    > > > The system call number is only assigned for x86_64 in this RFC patch.
    > > >
    > > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    > > > CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    > > > CC: mingo@elte.hu
    > > > CC: laijs@cn.fujitsu.com
    > > > CC: dipankar@in.ibm.com
    > > > CC: akpm@linux-foundation.org
    > > > CC: josh@joshtriplett.org
    > > > CC: dvhltc@us.ibm.com
    > > > CC: niv@us.ibm.com
    > > > CC: tglx@linutronix.de
    > > > CC: peterz@infradead.org
    > > > CC: rostedt@goodmis.org
    > > > CC: Valdis.Kletnieks@vt.edu
    > > > CC: dhowells@redhat.com
    > > > ---
    > > > arch/x86/include/asm/unistd_64.h | 2
    > > > kernel/sched.c | 219 +++++++++++++++++++++++++++++++++++++++
    > > > 2 files changed, 221 insertions(+)
    > > >
    > > > Index: linux-2.6-lttng/arch/x86/include/asm/unistd_64.h
    > > > ===================================================================
    > > > --- linux-2.6-lttng.orig/arch/x86/include/asm/unistd_64.h 2010-01-10 22:23:59.000000000 -0500
    > > > +++ linux-2.6-lttng/arch/x86/include/asm/unistd_64.h 2010-01-10 22:29:30.000000000 -0500
    > > > @@ -661,6 +661,8 @@ __SYSCALL(__NR_pwritev, sys_pwritev)
    > > > __SYSCALL(__NR_rt_tgsigqueueinfo, sys_rt_tgsigqueueinfo)
    > > > #define __NR_perf_event_open 298
    > > > __SYSCALL(__NR_perf_event_open, sys_perf_event_open)
    > > > +#define __NR_membarrier 299
    > > > +__SYSCALL(__NR_membarrier, sys_membarrier)
    > > >
    > > > #ifndef __NO_STUBS
    > > > #define __ARCH_WANT_OLD_READDIR
    > > > Index: linux-2.6-lttng/kernel/sched.c
    > > > ===================================================================
    > > > --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-10 22:23:59.000000000 -0500
    > > > +++ linux-2.6-lttng/kernel/sched.c 2010-01-10 23:12:35.000000000 -0500
    > > > @@ -119,6 +119,11 @@
    > > > */
    > > > #define RUNTIME_INF ((u64)~0ULL)
    > > >
    > > > +/*
    > > > + * IPI vs cpumask broadcast threshold. Threshold of 1 IPI.
    > > > + */
    > > > +#define ADAPT_IPI_THRESHOLD 1
    > > > +
    > > > static inline int rt_policy(int policy)
    > > > {
    > > > if (unlikely(policy == SCHED_FIFO || policy == SCHED_RR))
    > > > @@ -10822,6 +10827,220 @@ struct cgroup_subsys cpuacct_subsys = {
    > > > };
    > > > #endif /* CONFIG_CGROUP_CPUACCT */
    > > >
    > > > +/*
    > > > + * Execute a memory barrier on all CPUs on SMP systems.
    > > > + * Do not rely on implicit barriers in smp_call_function(), just in case they
    > > > + * are ever relaxed in the future.
    > > > + */
    > > > +static void membarrier_ipi(void *unused)
    > > > +{
    > > > + smp_mb();
    > > > +}
    > > > +
    > > > +/*
    > > > + * Handle out-of-mem by sending per-cpu IPIs instead.
    > > > + */
    > >
    > > Good handling for out-of-memory errors!
    > >
    > > > +static void membarrier_cpus_retry(int this_cpu)
    > > > +{
    > > > + struct mm_struct *mm;
    > > > + int cpu;
    > > > +
    > > > + for_each_online_cpu(cpu) {
    > > > + if (unlikely(cpu == this_cpu))
    > > > + continue;
    > > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > > + mm = cpu_curr(cpu)->mm;
    > > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > > + if (current->mm == mm)
    > > > + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
    > >
    > > There is of course some possibility of interrupting a real-time task,
    > > as the destination CPU could context-switch once we drop the ->lock.
    > > Not a criticism, just something to keep in mind. After all, the only ways
    > > I can think of to avoid this possibility do so by keeping the CPU from
    > > switching to the real-time task, which sort of defeats the purpose. ;-)
    >
    > Absolutely. And it's of no use to add a check within the IPI handler to
    > verify if it was indeed needed, because all we would skip is a simple
    > smp_mb(), which is relatively minor in terms of overhead compared to the
    > IPI itself.

    Agreed!

    > > > + }
    > > > +}
    > > > +
    > > > +static void membarrier_threads_retry(int this_cpu)
    > > > +{
    > > > + struct mm_struct *mm;
    > > > + struct task_struct *t;
    > > > + struct rq *rq;
    > > > + int cpu;
    > > > +
    > > > + list_for_each_entry_rcu(t, &current->thread_group, thread_group) {
    > > > + local_irq_disable();
    > > > + rq = __task_rq_lock(t);
    > > > + mm = rq->curr->mm;
    > > > + cpu = rq->cpu;
    > > > + __task_rq_unlock(rq);
    > > > + local_irq_enable();
    > > > + if (cpu == this_cpu)
    > > > + continue;
    > > > + if (current->mm == mm)
    > > > + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
    > >
    > > Ditto.
    > >
    > > > + }
    > > > +}
    > > > +
    > > > +static void membarrier_cpus(int this_cpu)
    > > > +{
    > > > + int cpu, i, cpu_ipi[ADAPT_IPI_THRESHOLD], nr_cpus = 0;
    > > > + cpumask_var_t tmpmask;
    > > > + struct mm_struct *mm;
    > > > +
    > > > + /* Get CPU IDs up to threshold */
    > > > + for_each_online_cpu(cpu) {
    > > > + if (unlikely(cpu == this_cpu))
    > > > + continue;
    > >
    > > OK, the above "if" handles the single-threaded-process case.
    > >
    >
    > No. See
    >
    > + if (unlikely(thread_group_empty(current)))
    > + return 0;
    >
    > in the caller below. The if you present here simply ensures that we
    > don't do a superfluous function call on the current thread. It's
    > probably not really worth it for a slow path though.

    OK, got it.

    > > The UP-kernel case is handled by the #ifdef in sys_membarrier(), though
    > > with a bit larger code footprint than the embedded guys would probably
    > > prefer. (Or is the compiler smart enough to omit these function given no
    > > calls to them? If not, recommend putting them under CONFIG_SMP #ifdef.)
    >
    > Hrm, that's a bit odd. I agree that UP systems could simply return
    > -ENOSYS for sys_membarrier, but then I wonder how userland could
    > distinguish between:
    >
    > - an old kernel not supporting sys_membarrier()
    > -> in this case we need to use the smp_mb() fallback on the read-side
    > and in synchronize_rcu().
    > - a recent kernel supporting sys_membarrier(), CONFIG_SMP
    > -> can use the barrier() on read-side, call sys_membarrier upon
    > update.
    > - a recent kernel supporting sys_membarrier, !CONFIG_SMP
    > -> calls to sys_membarrier() are not required, nor is barrier().
    >
    > Or maybe we just postpone the userland smp_mb() question to another
    > thread. This will eventually need to be addressed anyway. Maybe with a
    > vgetmaxcpu() vsyscall.

    [covered in Steve's email]

    > > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > > + mm = cpu_curr(cpu)->mm;
    > > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > > + if (current->mm == mm) {
    > > > + if (nr_cpus == ADAPT_IPI_THRESHOLD) {
    > > > + nr_cpus++;
    > > > + break;
    > > > + }
    > > > + cpu_ipi[nr_cpus++] = cpu;
    > > > + }
    > > > + }
    > > > + if (likely(nr_cpus <= ADAPT_IPI_THRESHOLD)) {
    > > > + for (i = 0; i < nr_cpus; i++) {
    > > > + smp_call_function_single(cpu_ipi[i],
    > > > + membarrier_ipi,
    > > > + NULL, 1);
    > > > + }
    > > > + } else {
    > > > + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) {
    > > > + membarrier_cpus_retry(this_cpu);
    > > > + return;
    > > > + }
    > > > + for (i = 0; i < ADAPT_IPI_THRESHOLD; i++)
    > > > + cpumask_set_cpu(cpu_ipi[i], tmpmask);
    > > > + /* Continue previous online cpu iteration */
    > > > + cpumask_set_cpu(cpu, tmpmask);
    > > > + for (;;) {
    > > > + cpu = cpumask_next(cpu, cpu_online_mask);
    > > > + if (unlikely(cpu == this_cpu))
    > > > + continue;
    > > > + if (unlikely(cpu >= nr_cpu_ids))
    > > > + break;
    > > > + spin_lock_irq(&cpu_rq(cpu)->lock);
    > > > + mm = cpu_curr(cpu)->mm;
    > > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
    > > > + if (current->mm == mm)
    > > > + cpumask_set_cpu(cpu, tmpmask);
    > > > + }
    > > > + smp_call_function_many(tmpmask, membarrier_ipi, NULL, 1);
    > > > + free_cpumask_var(tmpmask);
    > > > + }
    > > > +}
    > > > +
    > > > +static void membarrier_threads(int this_cpu)
    > > > +{
    > > > + int cpu, i, cpu_ipi[ADAPT_IPI_THRESHOLD], nr_cpus = 0;
    > > > + cpumask_var_t tmpmask;
    > > > + struct mm_struct *mm;
    > > > + struct task_struct *t;
    > > > + struct rq *rq;
    > > > +
    > > > + /* Get CPU IDs up to threshold */
    > > > + list_for_each_entry_rcu(t, &current->thread_group,
    > > > + thread_group) {
    > > > + local_irq_disable();
    > > > + rq = __task_rq_lock(t);
    > > > + mm = rq->curr->mm;
    > > > + cpu = rq->cpu;
    > > > + __task_rq_unlock(rq);
    > > > + local_irq_enable();
    > > > + if (cpu == this_cpu)
    > > > + continue;
    > > > + if (current->mm == mm) {
    > > I do not believe that the above test is gaining you anything. It would
    > > fail only if the task switched since the __task_rq_unlock(), but then
    > > again, it could switch immediately after the above test just as well.
    >
    > OK. Anyway I think I'll go the the shorter implementation using the
    > mm_cpumask, and add an additionnal ->mm check with spinlocks.

    Checking the ones not in mm_cpumask? I guess I will find out when I
    see the new patch.

    Thanx, Paul

    > > > + if (nr_cpus == ADAPT_IPI_THRESHOLD) {
    > > > + nr_cpus++;
    > > > + break;
    > > > + }
    > > > + cpu_ipi[nr_cpus++] = cpu;
    > > > + }
    > > > + }
    > > > + if (likely(nr_cpus <= ADAPT_IPI_THRESHOLD)) {
    > > > + for (i = 0; i < nr_cpus; i++) {
    > > > + smp_call_function_single(cpu_ipi[i],
    > > > + membarrier_ipi,
    > > > + NULL, 1);
    > > > + }
    > > > + } else {
    > > > + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) {
    > > > + membarrier_threads_retry(this_cpu);
    > > > + return;
    > > > + }
    > > > + for (i = 0; i < ADAPT_IPI_THRESHOLD; i++)
    > > > + cpumask_set_cpu(cpu_ipi[i], tmpmask);
    > > > + /* Continue previous thread iteration */
    > > > + cpumask_set_cpu(cpu, tmpmask);
    > > > + list_for_each_entry_continue_rcu(t,
    > > > + &current->thread_group,
    > > > + thread_group) {
    > > > + local_irq_disable();
    > > > + rq = __task_rq_lock(t);
    > > > + mm = rq->curr->mm;
    > > > + cpu = rq->cpu;
    > > > + __task_rq_unlock(rq);
    > > > + local_irq_enable();
    > > > + if (cpu == this_cpu)
    > > > + continue;
    > > > + if (current->mm == mm)
    > >
    > > Ditto.
    > >
    > > > + cpumask_set_cpu(cpu, tmpmask);
    > > A> + }
    > > > + smp_call_function_many(tmpmask, membarrier_ipi, NULL, 1);
    > > > + free_cpumask_var(tmpmask);
    > > > + }
    > > > +}
    > > > +
    > > > +/*
    > > > + * sys_membarrier - issue memory barrier on current process running threads
    > > > + *
    > > > + * Execute a memory barrier on all running threads of the current process.
    > > > + * Upon completion, the caller thread is ensured that all process threads
    > > > + * have passed through a state where memory accesses match program order.
    > > > + * (non-running threads are de facto in such a state)
    > > > + *
    > > > + * We do not use mm_cpumask because there is no guarantee that each architecture
    > > > + * switch_mm issues a smp_mb() before and after mm_cpumask modification upon
    > > > + * scheduling change. Furthermore, leave_mm is also modifying the mm_cpumask (at
    > > > + * least on x86) from the TLB flush IPI handler. So rather than playing tricky
    > > > + * games with lazy TLB flush, let's simply iterate on online cpus/thread group,
    > > > + * whichever is the smallest.
    > > > + */
    > > > +SYSCALL_DEFINE0(membarrier)
    > > > +{
    > > > +#ifdef CONFIG_SMP
    > > > + int this_cpu;
    > > > +
    > > > + if (unlikely(thread_group_empty(current)))
    > > > + return 0;
    > > > +
    > > > + rcu_read_lock(); /* protect cpu_curr(cpu)-> and rcu list */
    > > > + preempt_disable();
    > >
    > > Hmmm... You are going to hate me for pointing this out, Mathieu, but
    > > holding preempt_disable() across the whole sys_membarrier() processing
    > > might be hurting real-time latency more than would unconditionally
    > > IPIing all the CPUs. :-/
    >
    > Hehe, I pointed this out myself a few emails ago :) This is why I
    > started by using raw_smp_processor_id(). Well, let's make it simple
    > first, and then we can improve if needed.
    >
    > >
    > > That said, we have no shortage of situations where we scan the CPUs with
    > > preemption disabled, and with interrupts disabled, for that matter.
    >
    > Yep.
    >
    > Thanks,
    >
    > Mathieu
    >
    > >
    > > > + /*
    > > > + * Memory barrier on the caller thread _before_ sending first IPI.
    > > > + */
    > > > + smp_mb();
    > > > + /*
    > > > + * We don't need to include ourself in IPI, as we already
    > > > + * surround our execution with memory barriers.
    > > > + */
    > > > + this_cpu = smp_processor_id();
    > > > + /* Approximate which is fastest: CPU or thread group iteration ? */
    > > > + if (num_online_cpus() <= atomic_read(&current->mm->mm_users))
    > > > + membarrier_cpus(this_cpu);
    > > > + else
    > > > + membarrier_threads(this_cpu);
    > > > + /*
    > > > + * Memory barrier on the caller thread _after_ we finished
    > > > + * waiting for the last IPI.
    > > > + */
    > > > + smp_mb();
    > > > + preempt_enable();
    > > > + rcu_read_unlock();
    > > > +#endif /* #ifdef CONFIG_SMP */
    > > > + return 0;
    > > > +}
    > > > +
    > > > #ifndef CONFIG_SMP
    > > >
    > > > int rcu_expedited_torture_stats(char *page)
    > > > --
    > > > Mathieu Desnoyers
    > > > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
    >
    > --
    > Mathieu Desnoyers
    > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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