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SubjectRe: [PATCH v8 -tip 13/26] kernel/entry: Add support for core-wide protection of kernel-mode
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On 11/10/20 11:42 PM, Joel Fernandes wrote:
> On Tue, Nov 10, 2020 at 10:35:17AM +0100, Alexandre Chartre wrote:
> [..]
>>> ---8<-----------------------
>>>
>>> From b2835a587a28405ffdf8fc801e798129a014a8c8 Mon Sep 17 00:00:00 2001
>>> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
>>> Date: Mon, 27 Jul 2020 17:56:14 -0400
>>> Subject: [PATCH] kernel/entry: Add support for core-wide protection of
>>> kernel-mode
> [..]
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index d38e904dd603..fe6f225bfbf9 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -2071,4 +2071,16 @@ int sched_trace_rq_nr_running(struct rq *rq);
>>> const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
>>> +#ifdef CONFIG_SCHED_CORE
>>> +void sched_core_unsafe_enter(void);
>>> +void sched_core_unsafe_exit(void);
>>> +bool sched_core_wait_till_safe(unsigned long ti_check);
>>> +bool sched_core_kernel_protected(void);
>>> +#else
>>> +#define sched_core_unsafe_enter(ignore) do { } while (0)
>>> +#define sched_core_unsafe_exit(ignore) do { } while (0)
>>> +#define sched_core_wait_till_safe(ignore) do { } while (0)
>>> +#define sched_core_kernel_protected(ignore) do { } while (0)
>>> +#endif
>>> +
>>> #endif
>>> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
>>> index 0a1e20f8d4e8..a18ed60cedea 100644
>>> --- a/kernel/entry/common.c
>>> +++ b/kernel/entry/common.c
>>> @@ -28,6 +28,8 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs)
>>> instrumentation_begin();
>>> trace_hardirqs_off_finish();
>>> + if (_TIF_UNSAFE_RET) /* Kernel protection depends on arch defining the flag. */
>>> + sched_core_unsafe_enter();
>>> instrumentation_end();
>>> }
>>> @@ -137,6 +139,27 @@ static __always_inline void exit_to_user_mode(void)
>>> /* Workaround to allow gradual conversion of architecture code */
>>> void __weak arch_do_signal(struct pt_regs *regs) { }
>>> +unsigned long exit_to_user_get_work(void)
>>
>> Function should be static.
>
> Fixed.
>
>>> +{
>>> + unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
>>> +
>>> + if ((IS_ENABLED(CONFIG_SCHED_CORE) && !sched_core_kernel_protected())
>>> + || !_TIF_UNSAFE_RET)
>>> + return ti_work;
>>> +
>>> +#ifdef CONFIG_SCHED_CORE
>>> + ti_work &= EXIT_TO_USER_MODE_WORK;
>>> + if ((ti_work & _TIF_UNSAFE_RET) == ti_work) {
>>> + sched_core_unsafe_exit();
>>> + if (sched_core_wait_till_safe(EXIT_TO_USER_MODE_WORK)) {
>>> + sched_core_unsafe_enter(); /* not exiting to user yet. */
>>> + }
>>> + }
>>> +
>>> + return READ_ONCE(current_thread_info()->flags);
>>> +#endif
>>> +}
>>> +
>>> static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>>> unsigned long ti_work)
>>> {
>>> @@ -175,7 +198,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>>> * enabled above.
>>> */
>>> local_irq_disable_exit_to_user();
>>> - ti_work = READ_ONCE(current_thread_info()->flags);
>>> + ti_work = exit_to_user_get_work();
>>> }
>>
>> What happen if the task is scheduled out in exit_to_user_mode_loop? (e.g. if it has
>> _TIF_NEED_RESCHED set). It will have call sched_core_unsafe_enter() and force siblings
>> to wait for it. So shouldn't sched_core_unsafe_exit() be called when the task is
>> scheduled out? (because it won't run anymore) And sched_core_unsafe_enter() when
>> the task is scheduled back in?
>
> No, when the task is scheduled out, it will in kernel mode on the task being
> scheduled in. That task (being scheduled-in) would have already done a
> sched_core_unsafe_enter(). When that task returns to user made, it will do a
> sched_core_unsafe_exit(). When all tasks goto sleep, the last task that
> enters the idle loop will do a sched_core_unsafe_exit(). Just to note: the
> "unsafe kernel context" is per-CPU and not per-task. Does that answer your
> question?

Ok, I think I get it: it works because when a task is scheduled out then the
scheduler will schedule in a new tagged task (because we have core scheduling).
So that new task should be accounted for core-wide protection the same way as
the previous one.


>>> +static inline void init_sched_core_irq_work(struct rq *rq)
>>> +{
>>> + init_irq_work(&rq->core_irq_work, sched_core_irq_work);
>>> +}
>>> +
>>> +/*
>>> + * sched_core_wait_till_safe - Pause the caller's hyperthread until the core
>>> + * exits the core-wide unsafe state. Obviously the CPU calling this function
>>> + * should not be responsible for the core being in the core-wide unsafe state
>>> + * otherwise it will deadlock.
>>> + *
>>> + * @ti_check: We spin here with IRQ enabled and preempt disabled. Break out of
>>> + * the loop if TIF flags are set and notify caller about it.
>>> + *
>>> + * IRQs should be disabled.
>>> + */
>>> +bool sched_core_wait_till_safe(unsigned long ti_check)
>>> +{
>>> + bool restart = false;
>>> + struct rq *rq;
>>> + int cpu;
>>> +
>>> + /* We clear the thread flag only at the end, so need to check for it. */
>>
>> Do you mean "no need to check for it" ?
>
> Fixed.
>
>>> +/*
>>> + * Enter the core-wide IRQ state. Sibling will be paused if it is running
>>> + * 'untrusted' code, until sched_core_unsafe_exit() is called. Every attempt to
>>> + * avoid sending useless IPIs is made. Must be called only from hard IRQ
>>> + * context.
>>> + */
>>> +void sched_core_unsafe_enter(void)
>>> +{
>>> + const struct cpumask *smt_mask;
>>> + unsigned long flags;
>>> + struct rq *rq;
>>> + int i, cpu;
>>> +
>>> + if (!static_branch_likely(&sched_core_protect_kernel))
>>> + return;
>>> +
>>> + /* Ensure that on return to user/guest, we check whether to wait. */
>>> + if (current->core_cookie)
>>> + set_tsk_thread_flag(current, TIF_UNSAFE_RET);
>>> +
>>> + local_irq_save(flags);
>>> + cpu = smp_processor_id();
>>> + rq = cpu_rq(cpu);
>>> + if (!sched_core_enabled(rq))
>>> + goto ret;
>>
>> Should we clear TIF_UNSAFE_RET if (!sched_core_enabled(rq))? This would avoid calling
>> sched_core_wait_till_safe().
>
> Ok, or what I'll do is move the set_tsk_thread_flag to after the check for
> sched_core_enabled().
>
>>> +
>>> + /* Count unsafe_enter() calls received without unsafe_exit() on this CPU. */
>>> + rq->core_this_unsafe_nest++;
>>> +
>>> + /* Should not nest: enter() should only pair with exit(). */
>>> + if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 1))
>>> + goto ret;
>>
>> I would be more precise about the nesting comment: we don't nest not only because each
>> enter() is paired with an exit() but because each enter()/exit() is for a user context.
>> We can have nested interrupts but they will be for a kernel context so they won't enter/exit.
>>
>> So I would say something like:
>>
>> /*
>> * Should not nest: each enter() is paired with an exit(), and enter()/exit()
>> * are done when coming from userspace. We can have nested interrupts between
>> * enter()/exit() but they will originate from the kernel so they won't enter()
>> * nor exit().
>> */
>
> Changed it to following, hope its ok with you:
> /*
> * Should not nest: enter() should only pair with exit(). Both are done
> * during the first entry into kernel and the last exit from kernel.
> * Nested kernel entries (such as nested interrupts) will only trigger
> * enter() and exit() on the outer most kernel entry and exit.
> */
>
>>> +
>>> + raw_spin_lock(rq_lockp(rq));
>>> + smt_mask = cpu_smt_mask(cpu);
>>> +
>>> + /* Contribute this CPU's unsafe_enter() to core-wide unsafe_enter() count. */
>>> + WRITE_ONCE(rq->core->core_unsafe_nest, rq->core->core_unsafe_nest + 1);
>>
>> We are protected by the rq_lockp(rq) spinlock, but we still need to use WRITE_ONCE()
>> because sched_core_wait_till_safe() checks core_unsafe_next without taking rq_lockp(rq),
>> right?
>
> Yes.
>
>> Shouldn't we be using smp_store_release() like sched_core_unsafe_exit() does?
>>
>> In any case, it is worth having a comment why WRITE_ONCE() or smp_store_release() is
>> used.
>
> The smp_store_release() in exit() ensures that the write to the nesting
> counter happens *after* all prior reads and write accesses done by this CPU
> are seen by the spinning CPU doing the smp_load_acquire() before that
> spinning CPU returns. I did put a comment there.
>
> But, I think I don't need smp_store_release() at all here. The spin_unlock
> that follows already has the required release semantics. I will demote it to
> a WRITE_ONCE() in enter() as well, and add appropriate comments.
>

I think a WRITE_ONCE() is not even be useful here. The WRITE_ONCE() will only prevent
some possible compiler optimization in the function wrt rq->core->core_unsafe_nest, but
rq->core->core_unsafe_nest is just updated here, and concurrent changes are protected
by the rq_lockp(rq) spinlock, and the memory barrier is ensured by raw_spin_unlock().

So I thing you can just do: rq->core->core_unsafe_nest++;

And in sched_core_unsafe_exit(), you can just do: rq->core->core_unsafe_nest = nest - 1

Also comment in sched_core_wait_till_safe() wrt smp_load_acquire() should be updated,
it should say:

/*
* Wait till the core of this HT is not in an unsafe state.
*
* Pair with raw_spin_unlock(rq_lockp(rq) in sched_core_unsafe_enter/exit()
*/


>>> +
>>> + if (WARN_ON_ONCE(rq->core->core_unsafe_nest == UINT_MAX))
>>> + goto unlock;
>>
>> It might be better checking if (rq->core->core_unsafe_nest >= cpumask_weight(smt_mask))
>> because we shouldn't exceed the number of siblings.
>
> I am a bit concerned with the time complexity of cpumask_weight(). It may be
> better not to add overhead. I am not fully sure how it works but there is a
> loop in bitmask weight that goes through the bits of the bitmap, what is your
> opinion on that?

Yes, it's looping through the bitmap so probably not worth adding this overhead it here.

>
> Can I add your Reviewed-by tag to below updated patch? Thanks for review!

Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>


alex.

>
> ---8<---
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index bd1a5b87a5e2..a36f08d74e09 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4678,6 +4678,15 @@
>
> sbni= [NET] Granch SBNI12 leased line adapter
>
> + sched_core_protect_kernel=
> + [SCHED_CORE] Pause SMT siblings of a core running in
> + user mode, if at least one of the siblings of the core
> + is running in kernel mode. This is to guarantee that
> + kernel data is not leaked to tasks which are not trusted
> + by the kernel. A value of 0 disables protection, 1
> + enables protection. The default is 1. Note that protection
> + depends on the arch defining the _TIF_UNSAFE_RET flag.
> +
> sched_debug [KNL] Enables verbose scheduler debug messages.
>
> schedstats= [KNL,X86] Enable or disable scheduled statistics.
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index 474f29638d2c..62278c5b3b5f 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -33,6 +33,10 @@
> # define _TIF_PATCH_PENDING (0)
> #endif
>
> +#ifndef _TIF_UNSAFE_RET
> +# define _TIF_UNSAFE_RET (0)
> +#endif
> +
> #ifndef _TIF_UPROBE
> # define _TIF_UPROBE (0)
> #endif
> @@ -69,7 +73,7 @@
>
> #define EXIT_TO_USER_MODE_WORK \
> (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
> - _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | \
> + _TIF_NEED_RESCHED | _TIF_PATCH_PENDING | _TIF_UNSAFE_RET | \
> ARCH_EXIT_TO_USER_MODE_WORK)
>
> /**
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d38e904dd603..fe6f225bfbf9 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2071,4 +2071,16 @@ int sched_trace_rq_nr_running(struct rq *rq);
>
> const struct cpumask *sched_trace_rd_span(struct root_domain *rd);
>
> +#ifdef CONFIG_SCHED_CORE
> +void sched_core_unsafe_enter(void);
> +void sched_core_unsafe_exit(void);
> +bool sched_core_wait_till_safe(unsigned long ti_check);
> +bool sched_core_kernel_protected(void);
> +#else
> +#define sched_core_unsafe_enter(ignore) do { } while (0)
> +#define sched_core_unsafe_exit(ignore) do { } while (0)
> +#define sched_core_wait_till_safe(ignore) do { } while (0)
> +#define sched_core_kernel_protected(ignore) do { } while (0)
> +#endif
> +
> #endif
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 2b8366693d5c..d5d88e735d55 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -28,6 +28,8 @@ static __always_inline void enter_from_user_mode(struct pt_regs *regs)
>
> instrumentation_begin();
> trace_hardirqs_off_finish();
> + if (_TIF_UNSAFE_RET) /* Kernel protection depends on arch defining the flag. */
> + sched_core_unsafe_enter();
> instrumentation_end();
> }
>
> @@ -137,6 +139,27 @@ static __always_inline void exit_to_user_mode(void)
> /* Workaround to allow gradual conversion of architecture code */
> void __weak arch_do_signal(struct pt_regs *regs) { }
>
> +static unsigned long exit_to_user_get_work(void)
> +{
> + unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
> +
> + if ((IS_ENABLED(CONFIG_SCHED_CORE) && !sched_core_kernel_protected())
> + || !_TIF_UNSAFE_RET)
> + return ti_work;
> +
> +#ifdef CONFIG_SCHED_CORE
> + ti_work &= EXIT_TO_USER_MODE_WORK;
> + if ((ti_work & _TIF_UNSAFE_RET) == ti_work) {
> + sched_core_unsafe_exit();
> + if (sched_core_wait_till_safe(EXIT_TO_USER_MODE_WORK)) {
> + sched_core_unsafe_enter(); /* not exiting to user yet. */
> + }
> + }
> +
> + return READ_ONCE(current_thread_info()->flags);
> +#endif
> +}
> +
> static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> unsigned long ti_work)
> {
> @@ -174,7 +197,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> * enabled above.
> */
> local_irq_disable_exit_to_user();
> - ti_work = READ_ONCE(current_thread_info()->flags);
> + ti_work = exit_to_user_get_work();
> }
>
> /* Return the latest work state for arch_exit_to_user_mode() */
> @@ -183,9 +206,10 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>
> static void exit_to_user_mode_prepare(struct pt_regs *regs)
> {
> - unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
> + unsigned long ti_work;
>
> lockdep_assert_irqs_disabled();
> + ti_work = exit_to_user_get_work();
>
> if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
> ti_work = exit_to_user_mode_loop(regs, ti_work);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index fa68941998e3..429f9b8ca38e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -76,6 +76,27 @@ __read_mostly int scheduler_running;
>
> #ifdef CONFIG_SCHED_CORE
>
> +DEFINE_STATIC_KEY_TRUE(sched_core_protect_kernel);
> +static int __init set_sched_core_protect_kernel(char *str)
> +{
> + unsigned long val = 0;
> +
> + if (!str)
> + return 0;
> +
> + if (!kstrtoul(str, 0, &val) && !val)
> + static_branch_disable(&sched_core_protect_kernel);
> +
> + return 1;
> +}
> +__setup("sched_core_protect_kernel=", set_sched_core_protect_kernel);
> +
> +/* Is the kernel protected by core scheduling? */
> +bool sched_core_kernel_protected(void)
> +{
> + return static_branch_likely(&sched_core_protect_kernel);
> +}
> +
> DEFINE_STATIC_KEY_FALSE(__sched_core_enabled);
>
> /* kernel prio, less is more */
> @@ -4596,6 +4617,226 @@ static inline bool cookie_match(struct task_struct *a, struct task_struct *b)
> return a->core_cookie == b->core_cookie;
> }
>
> +/*
> + * Handler to attempt to enter kernel. It does nothing because the exit to
> + * usermode or guest mode will do the actual work (of waiting if needed).
> + */
> +static void sched_core_irq_work(struct irq_work *work)
> +{
> + return;
> +}
> +
> +static inline void init_sched_core_irq_work(struct rq *rq)
> +{
> + init_irq_work(&rq->core_irq_work, sched_core_irq_work);
> +}
> +
> +/*
> + * sched_core_wait_till_safe - Pause the caller's hyperthread until the core
> + * exits the core-wide unsafe state. Obviously the CPU calling this function
> + * should not be responsible for the core being in the core-wide unsafe state
> + * otherwise it will deadlock.
> + *
> + * @ti_check: We spin here with IRQ enabled and preempt disabled. Break out of
> + * the loop if TIF flags are set and notify caller about it.
> + *
> + * IRQs should be disabled.
> + */
> +bool sched_core_wait_till_safe(unsigned long ti_check)
> +{
> + bool restart = false;
> + struct rq *rq;
> + int cpu;
> +
> + /* We clear the thread flag only at the end, so no need to check for it. */
> + ti_check &= ~_TIF_UNSAFE_RET;
> +
> + cpu = smp_processor_id();
> + rq = cpu_rq(cpu);
> +
> + if (!sched_core_enabled(rq))
> + goto ret;
> +
> + /* Down grade to allow interrupts to prevent stop_machine lockups.. */
> + preempt_disable();
> + local_irq_enable();
> +
> + /*
> + * Wait till the core of this HT is not in an unsafe state.
> + *
> + * Pair with smp_store_release() in sched_core_unsafe_exit().
> + */
> + while (smp_load_acquire(&rq->core->core_unsafe_nest) > 0) {
> + cpu_relax();
> + if (READ_ONCE(current_thread_info()->flags) & ti_check) {
> + restart = true;
> + break;
> + }
> + }
> +
> + /* Upgrade it back to the expectations of entry code. */
> + local_irq_disable();
> + preempt_enable();
> +
> +ret:
> + if (!restart)
> + clear_tsk_thread_flag(current, TIF_UNSAFE_RET);
> +
> + return restart;
> +}
> +
> +/*
> + * Enter the core-wide IRQ state. Sibling will be paused if it is running
> + * 'untrusted' code, until sched_core_unsafe_exit() is called. Every attempt to
> + * avoid sending useless IPIs is made. Must be called only from hard IRQ
> + * context.
> + */
> +void sched_core_unsafe_enter(void)
> +{
> + const struct cpumask *smt_mask;
> + unsigned long flags;
> + struct rq *rq;
> + int i, cpu;
> +
> + if (!static_branch_likely(&sched_core_protect_kernel))
> + return;
> +
> + local_irq_save(flags);
> + cpu = smp_processor_id();
> + rq = cpu_rq(cpu);
> + if (!sched_core_enabled(rq))
> + goto ret;
> +
> + /* Ensure that on return to user/guest, we check whether to wait. */
> + if (current->core_cookie)
> + set_tsk_thread_flag(current, TIF_UNSAFE_RET);
> +
> + /* Count unsafe_enter() calls received without unsafe_exit() on this CPU. */
> + rq->core_this_unsafe_nest++;
> +
> + /*
> + * Should not nest: enter() should only pair with exit(). Both are done
> + * during the first entry into kernel and the last exit from kernel.
> + * Nested kernel entries (such as nested interrupts) will only trigger
> + * enter() and exit() on the outer most kernel entry and exit.
> + */
> + if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 1))
> + goto ret;
> +
> + raw_spin_lock(rq_lockp(rq));
> + smt_mask = cpu_smt_mask(cpu);
> +
> + /*
> + * Contribute this CPU's unsafe_enter() to the core-wide unsafe_enter()
> + * count. The raw_spin_unlock() release semantics pairs with the nest
> + * counter's smp_load_acquire() in sched_core_wait_till_safe().
> + */
> + WRITE_ONCE(rq->core->core_unsafe_nest, rq->core->core_unsafe_nest + 1);
> +
> + if (WARN_ON_ONCE(rq->core->core_unsafe_nest == UINT_MAX))
> + goto unlock;
> +
> + if (irq_work_is_busy(&rq->core_irq_work)) {
> + /*
> + * Do nothing more since we are in an IPI sent from another
> + * sibling to enforce safety. That sibling would have sent IPIs
> + * to all of the HTs.
> + */
> + goto unlock;
> + }
> +
> + /*
> + * If we are not the first ones on the core to enter core-wide unsafe
> + * state, do nothing.
> + */
> + if (rq->core->core_unsafe_nest > 1)
> + goto unlock;
> +
> + /* Do nothing more if the core is not tagged. */
> + if (!rq->core->core_cookie)
> + goto unlock;
> +
> + for_each_cpu(i, smt_mask) {
> + struct rq *srq = cpu_rq(i);
> +
> + if (i == cpu || cpu_is_offline(i))
> + continue;
> +
> + if (!srq->curr->mm || is_task_rq_idle(srq->curr))
> + continue;
> +
> + /* Skip if HT is not running a tagged task. */
> + if (!srq->curr->core_cookie && !srq->core_pick)
> + continue;
> +
> + /*
> + * Force sibling into the kernel by IPI. If work was already
> + * pending, no new IPIs are sent. This is Ok since the receiver
> + * would already be in the kernel, or on its way to it.
> + */
> + irq_work_queue_on(&srq->core_irq_work, i);
> + }
> +unlock:
> + raw_spin_unlock(rq_lockp(rq));
> +ret:
> + local_irq_restore(flags);
> +}
> +
> +/*
> + * Process any work need for either exiting the core-wide unsafe state, or for
> + * waiting on this hyperthread if the core is still in this state.
> + *
> + * @idle: Are we called from the idle loop?
> + */
> +void sched_core_unsafe_exit(void)
> +{
> + unsigned long flags;
> + unsigned int nest;
> + struct rq *rq;
> + int cpu;
> +
> + if (!static_branch_likely(&sched_core_protect_kernel))
> + return;
> +
> + local_irq_save(flags);
> + cpu = smp_processor_id();
> + rq = cpu_rq(cpu);
> +
> + /* Do nothing if core-sched disabled. */
> + if (!sched_core_enabled(rq))
> + goto ret;
> +
> + /*
> + * Can happen when a process is forked and the first return to user
> + * mode is a syscall exit. Either way, there's nothing to do.
> + */
> + if (rq->core_this_unsafe_nest == 0)
> + goto ret;
> +
> + rq->core_this_unsafe_nest--;
> +
> + /* enter() should be paired with exit() only. */
> + if (WARN_ON_ONCE(rq->core_this_unsafe_nest != 0))
> + goto ret;
> +
> + raw_spin_lock(rq_lockp(rq));
> + /*
> + * Core-wide nesting counter can never be 0 because we are
> + * still in it on this CPU.
> + */
> + nest = rq->core->core_unsafe_nest;
> + WARN_ON_ONCE(!nest);
> +
> + WRITE_ONCE(rq->core->core_unsafe_nest, nest - 1);
> + /*
> + * The raw_spin_unlock release semantics pairs with the nest counter's
> + * smp_load_acquire() in sched_core_wait_till_safe().
> + */
> + raw_spin_unlock(rq_lockp(rq));
> +ret:
> + local_irq_restore(flags);
> +}
> +
> // XXX fairness/fwd progress conditions
> /*
> * Returns
> @@ -4991,6 +5232,7 @@ static inline void sched_core_cpu_starting(unsigned int cpu)
> rq = cpu_rq(i);
> if (rq->core && rq->core == rq)
> core_rq = rq;
> + init_sched_core_irq_work(rq);
> }
>
> if (!core_rq)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 001382bc67f9..20937a5b6272 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1061,6 +1061,8 @@ struct rq {
> unsigned int core_enabled;
> unsigned int core_sched_seq;
> struct rb_root core_tree;
> + struct irq_work core_irq_work; /* To force HT into kernel */
> + unsigned int core_this_unsafe_nest;
>
> /* shared state */
> unsigned int core_task_seq;
> @@ -1068,6 +1070,7 @@ struct rq {
> unsigned long core_cookie;
> unsigned char core_forceidle;
> unsigned int core_forceidle_seq;
> + unsigned int core_unsafe_nest;
> #endif
> };
>
>

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