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Date: Wed, 18 Oct 2017 13:48:47 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: j.alglave@....ac.uk, luc.maranget@...ia.fr,
stern@...land.harvard.edu, dhowells@...hat.com,
peterz@...radead.org, will.deacon@....com, boqun.feng@...il.com,
npiggin@...il.com, linux-kernel@...r.kernel.org
Subject: Re: Memory-ordering recipes
On Wed, Oct 18, 2017 at 03:07:48AM +0200, Andrea Parri wrote:
> On Tue, Oct 17, 2017 at 02:01:37PM -0700, Paul E. McKenney wrote:
> > On Sun, Sep 17, 2017 at 04:05:09PM -0700, Paul E. McKenney wrote:
> > > Hello!
> > >
> > > The topic of memory-ordering recipes came up at the Linux Plumbers
> > > Conference microconference on Friday, so I thought that I should summarize
> > > what is currently "out there":
> >
> > And here is an updated list of potential Linux-kernel examples for a
> > "recipes" document, and thank you for the feedback. Please feel free
> > to counterpropose better examples. In addition, if there is some other
> > pattern that is commonplace and important enough to be included in a
> > recipes document, please point it out.
> >
> > 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. Simple special cases
> >
> > If there is only one CPU on the one hand or only one variable
> > on the other, the code will execute in order. There are (as
> > usual) some things to be careful of:
> >
> > a. There are some aspects of the C language that are
> > unordered. For example, in the expression "f(x) + g(y)",
> > the order in which f and g are called is not defined;
> > the object code is allowed to use either order or even
> > to interleave the computations.
> >
> > b. Compilers are permitted to use the "as-if" rule. That is,
> > a compiler can emit whatever code it likes, as long as
> > the results of a single-threaded execution appear just
> > as if the compiler had followed all the relevant rules.
> > To see this, compile with a high level of optimization
> > and run the debugger on the resulting binary.
> >
> > c. If there is only one variable but multiple CPUs, all
> > that variable must be properly aligned and all accesses
> > to that variable must be full sized. Variables that
> > straddle cachelines or pages void your full-ordering
> > warranty, as do undersized accesses that load from or
> > store to only part of the variable.
> >
> > 1. Another simple case: Locking. [ Assuming you don't think too
> > hard about it, that is! ]
> >
> > Any CPU that has acquired a given lock sees any changes previously
> > made by any CPU prior to having released that same lock.
> >
> > [ Should we discuss chaining back through different locks,
> > sort of like release-acquire chains? ]
> >
> > 2. 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);
> >
> > Alternatively, from the comment in perf_output_put_handle()
> > in kernel/events/ring_buffer.c:
> >
> > * kernel user
> > *
> > * if (LOAD ->data_tail) { LOAD ->data_head
> > * (A) smp_rmb() (C)
> > * STORE $data LOAD $data
> > * smp_wmb() (B) smp_mb() (D)
> > * STORE ->data_head STORE ->data_tail
> > * }
> > *
> > * Where A pairs with D, and B pairs with C.
> >
> > The B/C pairing is MP with smp_wmb() and smp_rmb().
> >
> > d. Replacing either of the above with smp_mb()
> >
> > Holding off on this one for the moment...
> >
> > 3. LB
> >
> > a. LB+ctrl+mb
> >
> > Again, from the comment in perf_output_put_handle()
> > in kernel/events/ring_buffer.c:
> >
> > * kernel user
> > *
> > * if (LOAD ->data_tail) { LOAD ->data_head
> > * (A) smp_rmb() (C)
> > * STORE $data LOAD $data
> > * smp_wmb() (B) smp_mb() (D)
> > * STORE ->data_head STORE ->data_tail
> > * }
> > *
> > * Where A pairs with D, and B pairs with C.
> >
> > The A/D pairing covers this one.
> >
> > 4. 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?
> >
> > 5. 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;
> > }
> >
> > -----------------
> >
> > And here is the generic pattern for the above two examples
> > taken from waitqueue_active() in include/linux/wait.h:
> >
> > * CPU0 - waker CPU1 - waiter
> > *
> > * for (;;) {
> > * @cond = true; prepare_to_wait(&wq_head, &wait, state);
> > * smp_mb(); // smp_mb() from set_current_state()
> > * if (waitqueue_active(wq_head)) if (@cond)
> > * wake_up(wq_head); break;
> > * schedule();
> > * }
> > * finish_wait(&wq_head, &wait);
> >
> > Note that prepare_to_wait() does the both the write
> > and the set_current_state() that contains the smp_mb().
> > The read is the waitqueue_active() on the one hand and
> > the "if (@cond)" on the other.
> >
> > 6. W+RWC+porel+mb+mb
> >
> > See recipes-LKcode-63cae12bce986.txt.
> >
> > Mostly of historical interest -- as far as I know, this commit
> > was the first to contain a litmus test.
> >
> > 7. Context switch and migration. A bit specialized, so might leave
> > this one out.
> >
> > When a thread moves from one CPU to another to another, the
> > scheduler is required to do whatever is necessary for the thread
> > to see any prior accesses that it executed on other CPUs. This
> > includes "interesting" interactions with wake_up() shown in the
> > following comment from try_to_wake_up() in kernel/sched/core.c:
> >
> > * Notes on Program-Order guarantees on SMP systems.
> > *
> > * MIGRATION
> > *
> > * The basic program-order guarantee on SMP systems is that when a task [t]
> > * migrates, all its activity on its old CPU [c0] happens-before any subsequent
> > * execution on its new CPU [c1].
> > *
> > * For migration (of runnable tasks) this is provided by the following means:
> > *
> > * A) UNLOCK of the rq(c0)->lock scheduling out task t
> > * B) migration for t is required to synchronize *both* rq(c0)->lock and
> > * rq(c1)->lock (if not at the same time, then in that order).
> > * C) LOCK of the rq(c1)->lock scheduling in task
> > *
> > * Transitivity guarantees that B happens after A and C after B.
> > * Note: we only require RCpc transitivity.
> > * Note: the CPU doing B need not be c0 or c1
> > *
> > * Example:
> > *
> > * CPU0 CPU1 CPU2
> > *
> > * LOCK rq(0)->lock
> > * sched-out X
> > * sched-in Y
> > * UNLOCK rq(0)->lock
> > *
> > * LOCK rq(0)->lock // orders against CPU0
> > * dequeue X
> > * UNLOCK rq(0)->lock
> > *
> > * LOCK rq(1)->lock
> > * enqueue X
> > * UNLOCK rq(1)->lock
> > *
> > * LOCK rq(1)->lock // orders against CPU2
> > * sched-out Z
> > * sched-in X
> > * UNLOCK rq(1)->lock
>
> We might want to "specialize" this snippet to emphasize (or illustrate)
> a particular _cycle_; one possible way of "closing the chain" would be:
>
> CPU0
>
> smp_store_release(&X->on_cpu, 0); /* from sched-out X */
> UNLOCK rq(0)->lock
>
>
> CPU2
>
> LOCK rq(0)->lock
> UNLOCK rq(1)->lock
>
>
> CPU1
>
> LOCK rq(1)->lock
> X->on_cpu = 1; /* from sched-in X */
>
>
> BUG_ON("each LOCK reads from its UNLOCK" && X->on_cpu == 0);
>
> (c.f., Z6.2 in test6.pdf)
I added this as a summary, pretty much in the form you have above.
> > *
> > *
> > * BLOCKING -- aka. SLEEP + WAKEUP
> > *
> > * For blocking we (obviously) need to provide the same guarantee as for
> > * migration. However the means are completely different as there is no lock
> > * chain to provide order. Instead we do:
> > *
> > * 1) smp_store_release(X->on_cpu, 0)
> > * 2) smp_cond_load_acquire(!X->on_cpu)
> > *
> > * Example:
> > *
> > * CPU0 (schedule) CPU1 (try_to_wake_up) CPU2 (schedule)
> > *
> > * LOCK rq(0)->lock LOCK X->pi_lock
> > * dequeue X
> > * sched-out X
> > * smp_store_release(X->on_cpu, 0);
> > *
> > * smp_cond_load_acquire(&X->on_cpu, !VAL);
> > * X->state = WAKING
> > * set_task_cpu(X,2)
> > *
> > * LOCK rq(2)->lock
> > * enqueue X
> > * X->state = RUNNING
> > * UNLOCK rq(2)->lock
> > *
> > * LOCK rq(2)->lock // orders against CPU1
> > * sched-out Z
> > * sched-in X
> > * UNLOCK rq(2)->lock
> > *
> > * UNLOCK X->pi_lock
> > * UNLOCK rq(0)->lock
>
> Similarly:
>
> CPU0
>
> smp_store_release(&X->on_cpu, 0); /* from sched-out X */
>
>
> CPU1
>
> smp_cond_load_acquire(&X->on_cpu, !VAL);
> UNLOCK rq(2)->lock
>
>
> CPU2
>
> LOCK rq(2)->lock
> X->on_cpu = 1; // from sched-in X
>
>
> BUG_ON("each LOCK reads from its UNLOCK" && X->on_cpu == 0);
>
> (c.f., WWC in test6.pdf)
And same here.
Thanx, Paul
> Andrea
>
>
> > *
> > *
> > * However; for wakeups there is a second guarantee we must provide, namely we
> > * must observe the state that lead to our wakeup. That is, not only must our
> > * task observe its own prior state, it must also observe the stores prior to
> > * its wakeup.
> > *
> > * This means that any means of doing remote wakeups must order the CPU doing
> > * the wakeup against the CPU the task is going to end up running on. This,
> > * however, is already required for the regular Program-Order guarantee above,
> > * since the waking CPU is the one issueing the ACQUIRE (smp_cond_load_acquire).
>
> > commit 63cae12bce9861cec309798d34701cf3da20bc71
> > Author: Peter Zijlstra <peterz@...radead.org>
> > Date: Fri Dec 9 14:59:00 2016 +0100
> >
> > perf/core: Fix sys_perf_event_open() vs. hotplug
> >
> > There is problem with installing an event in a task that is 'stuck' on
> > an offline CPU.
> >
> > Blocked tasks are not dis-assosciated from offlined CPUs, after all, a
> > blocked task doesn't run and doesn't require a CPU etc.. Only on
> > wakeup do we ammend the situation and place the task on a available
> > CPU.
> >
> > If we hit such a task with perf_install_in_context() we'll loop until
> > either that task wakes up or the CPU comes back online, if the task
> > waking depends on the event being installed, we're stuck.
> >
> > While looking into this issue, I also spotted another problem, if we
> > hit a task with perf_install_in_context() that is in the middle of
> > being migrated, that is we observe the old CPU before sending the IPI,
> > but run the IPI (on the old CPU) while the task is already running on
> > the new CPU, things also go sideways.
> >
> > Rework things to rely on task_curr() -- outside of rq->lock -- which
> > is rather tricky. Imagine the following scenario where we're trying to
> > install the first event into our task 't':
> >
> > CPU0 CPU1 CPU2
> >
> > (current == t)
> >
> > t->perf_event_ctxp[] = ctx;
> > smp_mb();
> > cpu = task_cpu(t);
> >
> > switch(t, n);
> > migrate(t, 2);
> > switch(p, t);
> >
> > ctx = t->perf_event_ctxp[]; // must not be NULL
> >
> > smp_function_call(cpu, ..);
> >
> > generic_exec_single()
> > func();
> > spin_lock(ctx->lock);
> > if (task_curr(t)) // false
> >
> > add_event_to_ctx();
> > spin_unlock(ctx->lock);
> >
> > perf_event_context_sched_in();
> > spin_lock(ctx->lock);
> > // sees event
> >
> > So its CPU0's store of t->perf_event_ctxp[] that must not go 'missing'.
> > Because if CPU2's load of that variable were to observe NULL, it would
> > not try to schedule the ctx and we'd have a task running without its
> > counter, which would be 'bad'.
> >
> > As long as we observe !NULL, we'll acquire ctx->lock. If we acquire it
> > first and not see the event yet, then CPU0 must observe task_curr()
> > and retry. If the install happens first, then we must see the event on
> > sched-in and all is well.
> >
> > I think we can translate the first part (until the 'must not be NULL')
> > of the scenario to a litmus test like:
> >
> > C C-peterz
> >
> > {
> > }
> >
> > P0(int *x, int *y)
> > {
> > int r1;
> >
> > WRITE_ONCE(*x, 1);
> > smp_mb();
> > r1 = READ_ONCE(*y);
> > }
> >
> > P1(int *y, int *z)
> > {
> > WRITE_ONCE(*y, 1);
> > smp_store_release(z, 1);
> > }
> >
> > P2(int *x, int *z)
> > {
> > int r1;
> > int r2;
> >
> > r1 = smp_load_acquire(z);
> > smp_mb();
> > r2 = READ_ONCE(*x);
> > }
> >
> > exists
> > (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
> >
> > Where:
> > x is perf_event_ctxp[],
> > y is our tasks's CPU, and
> > z is our task being placed on the rq of CPU2.
> >
> > The P0 smp_mb() is the one added by this patch, ordering the store to
> > perf_event_ctxp[] from find_get_context() and the load of task_cpu()
> > in task_function_call().
> >
> > The smp_store_release/smp_load_acquire model the RCpc locking of the
> > rq->lock and the smp_mb() of P2 is the context switch switching from
> > whatever CPU2 was running to our task 't'.
> >
> > This litmus test evaluates into:
> >
> > Test C-peterz Allowed
> > States 7
> > 0:r1=0; 2:r1=0; 2:r2=0;
> > 0:r1=0; 2:r1=0; 2:r2=1;
> > 0:r1=0; 2:r1=1; 2:r2=1;
> > 0:r1=1; 2:r1=0; 2:r2=0;
> > 0:r1=1; 2:r1=0; 2:r2=1;
> > 0:r1=1; 2:r1=1; 2:r2=0;
> > 0:r1=1; 2:r1=1; 2:r2=1;
> > No
> > Witnesses
> > Positive: 0 Negative: 7
> > Condition exists (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
> > Observation C-peterz Never 0 7
> > Hash=e427f41d9146b2a5445101d3e2fcaa34
> >
> > And the strong and weak model agree.
> >
> > Reported-by: Mark Rutland <mark.rutland@....com>
> > Tested-by: Mark Rutland <mark.rutland@....com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> > Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Cc: Jiri Olsa <jolsa@...hat.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: Stephane Eranian <eranian@...gle.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Vince Weaver <vincent.weaver@...ne.edu>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: jeremy.linton@....com
> > Link: http://lkml.kernel.org/r/20161209135900.GU3174@twins.programming.kicks-ass.net
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index ab15509fab8c..72ce7d63e561 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -2249,7 +2249,7 @@ static int __perf_install_in_context(void *info)
> > struct perf_event_context *ctx = event->ctx;
> > struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
> > struct perf_event_context *task_ctx = cpuctx->task_ctx;
> > - bool activate = true;
> > + bool reprogram = true;
> > int ret = 0;
> >
> > raw_spin_lock(&cpuctx->ctx.lock);
> > @@ -2257,27 +2257,26 @@ static int __perf_install_in_context(void *info)
> > raw_spin_lock(&ctx->lock);
> > task_ctx = ctx;
> >
> > - /* If we're on the wrong CPU, try again */
> > - if (task_cpu(ctx->task) != smp_processor_id()) {
> > - ret = -ESRCH;
> > - goto unlock;
> > - }
> > + reprogram = (ctx->task == current);
> >
> > /*
> > - * If we're on the right CPU, see if the task we target is
> > - * current, if not we don't have to activate the ctx, a future
> > - * context switch will do that for us.
> > + * If the task is running, it must be running on this CPU,
> > + * otherwise we cannot reprogram things.
> > + *
> > + * If its not running, we don't care, ctx->lock will
> > + * serialize against it becoming runnable.
> > */
> > - if (ctx->task != current)
> > - activate = false;
> > - else
> > - WARN_ON_ONCE(cpuctx->task_ctx && cpuctx->task_ctx != ctx);
> > + if (task_curr(ctx->task) && !reprogram) {
> > + ret = -ESRCH;
> > + goto unlock;
> > + }
> >
> > + WARN_ON_ONCE(reprogram && cpuctx->task_ctx && cpuctx->task_ctx != ctx);
> > } else if (task_ctx) {
> > raw_spin_lock(&task_ctx->lock);
> > }
> >
> > - if (activate) {
> > + if (reprogram) {
> > ctx_sched_out(ctx, cpuctx, EVENT_TIME);
> > add_event_to_ctx(event, ctx);
> > ctx_resched(cpuctx, task_ctx);
> > @@ -2328,13 +2327,36 @@ perf_install_in_context(struct perf_event_context *ctx,
> > /*
> > * Installing events is tricky because we cannot rely on ctx->is_active
> > * to be set in case this is the nr_events 0 -> 1 transition.
> > + *
> > + * Instead we use task_curr(), which tells us if the task is running.
> > + * However, since we use task_curr() outside of rq::lock, we can race
> > + * against the actual state. This means the result can be wrong.
> > + *
> > + * If we get a false positive, we retry, this is harmless.
> > + *
> > + * If we get a false negative, things are complicated. If we are after
> > + * perf_event_context_sched_in() ctx::lock will serialize us, and the
> > + * value must be correct. If we're before, it doesn't matter since
> > + * perf_event_context_sched_in() will program the counter.
> > + *
> > + * However, this hinges on the remote context switch having observed
> > + * our task->perf_event_ctxp[] store, such that it will in fact take
> > + * ctx::lock in perf_event_context_sched_in().
> > + *
> > + * We do this by task_function_call(), if the IPI fails to hit the task
> > + * we know any future context switch of task must see the
> > + * perf_event_ctpx[] store.
> > */
> > -again:
> > +
> > /*
> > - * Cannot use task_function_call() because we need to run on the task's
> > - * CPU regardless of whether its current or not.
> > + * This smp_mb() orders the task->perf_event_ctxp[] store with the
> > + * task_cpu() load, such that if the IPI then does not find the task
> > + * running, a future context switch of that task must observe the
> > + * store.
> > */
> > - if (!cpu_function_call(task_cpu(task), __perf_install_in_context, event))
> > + smp_mb();
> > +again:
> > + if (!task_function_call(task, __perf_install_in_context, event))
> > return;
> >
> > raw_spin_lock_irq(&ctx->lock);
> > @@ -2348,12 +2370,16 @@ again:
> > raw_spin_unlock_irq(&ctx->lock);
> > return;
> > }
> > - raw_spin_unlock_irq(&ctx->lock);
> > /*
> > - * Since !ctx->is_active doesn't mean anything, we must IPI
> > - * unconditionally.
> > + * If the task is not running, ctx->lock will avoid it becoming so,
> > + * thus we can safely install the event.
> > */
> > - goto again;
> > + if (task_curr(task)) {
> > + raw_spin_unlock_irq(&ctx->lock);
> > + goto again;
> > + }
> > + add_event_to_ctx(event, ctx);
> > + raw_spin_unlock_irq(&ctx->lock);
> > }
> >
> > /*
>
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