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SubjectRe: Linux-kernel examples for LKMM recipes
Hi Paul,

On Wed, Oct 11, 2017 at 03:32:30PM -0700, Paul E. McKenney wrote:
> Hello!
>
> At Linux Plumbers Conference, we got requests for a recipes document,
> and a further request to point to actual code in the Linux kernel.
> I have pulled together some examples for various litmus-test families,
> as shown below. The decoder ring for the abbreviations (ISA2, LB, SB,
> MP, ...) is here:
>
> https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf
>
> This document is also checked into the memory-models git archive:
>
> https://github.com/aparri/memory-model.git
>
> I would be especially interested in simpler examples in general, and
> of course any example at all for the cases where I was unable to find
> any. Thoughts?

Below are some examples we did discuss (at some point):

The comment in kernel/events/ring_buffer.c:perf_output_put_handle()
describes instances of MP+wmb+rmb and LB+ctrl+mb.

The comments in kernel/sched/core.c:try_to_wake_up() describes more
instances of MP ("plus locking") and LB (see finish_lock_switch()).

The comment in kernel/sched/core.c:task_rq_lock() describes an ins-
tance of MP+wmb+addr-acqpo.

The comment in include/linux/wait.h:waitqueue_active() describes an
instance of SB+mb+mb.

63cae12bce986 ("perf/core: Fix sys_perf_event_open() vs. hotplug")
describes an instance of W+RWC+porel+mb+mb.

[...]

I wish we could say "any barrier (explicit or implicit) in sources
is accompanied by a comment mentioning the interested pattern...",
but life is not always this simple. ;-)

Andrea


>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> This document lists the litmus-test patterns that we have been discussing,
> along with examples from the Linux kernel. This is intended to feed into
> the recipes document. All examples are from v4.13.
>
> 0. Single-variable SC.
>
> a. Within a single CPU, the use of the ->dynticks_nmi_nesting
> counter by rcu_nmi_enter() and rcu_nmi_exit() qualifies
> (see kernel/rcu/tree.c). The counter is accessed by
> interrupts and NMIs as well as by process-level code.
> This counter can be accessed by other CPUs, but only
> for debug output.
>
> b. Between CPUs, I would put forward the ->dflags
> updates, but this is anything but simple. But maybe
> OK for an illustration?
>
> 1. MP (see test6.pdf for nickname translation)
>
> a. smp_store_release() / smp_load_acquire()
>
> init_stack_slab() in lib/stackdepot.c uses release-acquire
> to handle initialization of a slab of the stack. Working
> out the mutual-exclusion design is left as an exercise for
> the reader.
>
> b. rcu_assign_pointer() / rcu_dereference()
>
> expand_to_next_prime() does the rcu_assign_pointer(),
> and next_prime_number() does the rcu_dereference().
> This mediates access to a bit vector that is expanded
> as additional primes are needed. These two functions
> are in lib/prime_numbers.c.
>
> c. smp_wmb() / smp_rmb()
>
> xlog_state_switch_iclogs() contains the following:
>
> log->l_curr_block -= log->l_logBBsize;
> ASSERT(log->l_curr_block >= 0);
> smp_wmb();
> log->l_curr_cycle++;
>
> And xlog_valid_lsn() contains the following:
>
> cur_cycle = ACCESS_ONCE(log->l_curr_cycle);
> smp_rmb();
> cur_block = ACCESS_ONCE(log->l_curr_block);
>
> d. Replacing either of the above with smp_mb()
>
> Holding off on this one for the moment...
>
> 2. Release-acquire chains, AKA ISA2, Z6.2, LB, and 3.LB
>
> Lots of variety here, can in some cases substitute:
>
> a. READ_ONCE() for smp_load_acquire()
> b. WRITE_ONCE() for smp_store_release()
> c. Dependencies for both smp_load_acquire() and
> smp_store_release().
> d. smp_wmb() for smp_store_release() in first thread
> of ISA2 and Z6.2.
> e. smp_rmb() for smp_load_acquire() in last thread of ISA2.
>
> The canonical illustration of LB involves the various memory
> allocators, where you don't want a load from about-to-be-freed
> memory to see a store initializing a later incarnation of that
> same memory area. But the per-CPU caches make this a very
> long and complicated example.
>
> I am not aware of any three-CPU release-acquire chains in the
> Linux kernel. There are three-CPU lock-based chains in RCU,
> but these are not at all simple, either.
>
> Thoughts?
>
> 3. SB
>
> a. smp_mb(), as in lockless wait-wakeup coordination.
> And as in sys_membarrier()-scheduler coordination,
> for that matter.
>
> Examples seem to be lacking. Most cases use locking.
> Here is one rather strange one from RCU:
>
> void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
> {
> unsigned long flags;
> bool needwake;
> bool havetask = READ_ONCE(rcu_tasks_kthread_ptr);
>
> rhp->next = NULL;
> rhp->func = func;
> raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> needwake = !rcu_tasks_cbs_head;
> *rcu_tasks_cbs_tail = rhp;
> rcu_tasks_cbs_tail = &rhp->next;
> raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> /* We can't create the thread unless interrupts are enabled. */
> if ((needwake && havetask) ||
> (!havetask && !irqs_disabled_flags(flags))) {
> rcu_spawn_tasks_kthread();
> wake_up(&rcu_tasks_cbs_wq);
> }
> }
>
> And for the wait side, using synchronize_sched() to supply
> the barrier for both ends, with the preemption disabling
> due to raw_spin_lock_irqsave() serving as the read-side
> critical section:
>
> if (!list) {
> wait_event_interruptible(rcu_tasks_cbs_wq,
> rcu_tasks_cbs_head);
> if (!rcu_tasks_cbs_head) {
> WARN_ON(signal_pending(current));
> schedule_timeout_interruptible(HZ/10);
> }
> continue;
> }
> synchronize_sched();
>
> -----------------
>
> Here is another one that uses atomic_cmpxchg() as a
> full memory barrier:
>
> if (!wait_event_timeout(*wait, !atomic_read(stopping),
> msecs_to_jiffies(1000))) {
> atomic_set(stopping, 0);
> smp_mb();
> return -ETIMEDOUT;
> }
>
> int omap3isp_module_sync_is_stopping(wait_queue_head_t *wait,
> atomic_t *stopping)
> {
> if (atomic_cmpxchg(stopping, 1, 0)) {
> wake_up(wait);
> return 1;
> }
>
> return 0;
> }
>

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