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Message-ID: <f22d86810910140324l7b78ed57v7e7c774d5f8a1bd3@mail.gmail.com>
Date: Wed, 14 Oct 2009 03:24:49 -0700
From: "Leonidas ." <leonidas137@...il.com>
To: Gleb Natapov <gleb@...hat.com>
Cc: linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: How to check whether executing in atomic context?
On Wed, Oct 14, 2009 at 3:13 AM, Gleb Natapov <gleb@...hat.com> wrote:
> On Wed, Oct 14, 2009 at 02:21:22AM -0700, Leonidas . wrote:
>> On Tue, Oct 13, 2009 at 11:36 PM, Leonidas . <leonidas137@...il.com> wrote:
>> > Hi List,
>> >
>> > I am working on a profiler kind of module, the exported apis of my
>> > module can be
>> > called from process context and interrupt context as well. Depending on the
>> > context I am called in, I need to call sleepable/nonsleepable variants
>> > of my internal
>> > bookkeeping functions.
>> >
>> > I am aware of in_interrupt() call which can be used to check current
>> > context and take action
>> > accordingly.
>> >
>> > Is there any api which can help figure out whether we are executing
>> > while hold a spinlock? I.e
>> > an api which can help figure out sleepable/nonsleepable context? If it
>> > is not there, what can
>> > be done for writing the same? Any pointers will be helpful.
>> >
>> > -Leo.
>> >
>>
>> While searching through the sources, I found this,
>>
>> 97/*
>> 98 * Are we running in atomic context? WARNING: this macro cannot
>> 99 * always detect atomic context; in particular, it cannot know about
>> 100 * held spinlocks in non-preemptible kernels. Thus it should not be
>> 101 * used in the general case to determine whether sleeping is possible.
>> 102 * Do not use in_atomic() in driver code.
>> 103 */
>> 104#define in_atomic() ((preempt_count() & ~PREEMPT_ACTIVE) !=
>> PREEMPT_INATOMIC_BASE)
>> 105
>>
>> this just complicates the matter, right? This does not work in general
>> case but I think this
>> will always work if the kernel is preemptible.
>>
>> Is there no way to write a generic macro?
>>
>>
> Attached patch make in_atomic() to work for non-preemptable kernels too.
> Doesn't look to big or scary.
>
> Disclaimer: tested only inside kvm guest 64bit, haven't measured overhead.
>
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index 6d527ee..a6b6040 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -92,12 +92,11 @@
> */
> #define in_nmi() (preempt_count() & NMI_MASK)
>
> +#define PREEMPT_CHECK_OFFSET 1
> #if defined(CONFIG_PREEMPT)
> # define PREEMPT_INATOMIC_BASE kernel_locked()
> -# define PREEMPT_CHECK_OFFSET 1
> #else
> # define PREEMPT_INATOMIC_BASE 0
> -# define PREEMPT_CHECK_OFFSET 0
> #endif
>
> /*
> @@ -116,12 +115,11 @@
> #define in_atomic_preempt_off() \
> ((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_CHECK_OFFSET)
>
> +#define IRQ_EXIT_OFFSET (HARDIRQ_OFFSET-1)
> #ifdef CONFIG_PREEMPT
> # define preemptible() (preempt_count() == 0 && !irqs_disabled())
> -# define IRQ_EXIT_OFFSET (HARDIRQ_OFFSET-1)
> #else
> # define preemptible() 0
> -# define IRQ_EXIT_OFFSET HARDIRQ_OFFSET
> #endif
>
> #if defined(CONFIG_SMP) || defined(CONFIG_GENERIC_HARDIRQS)
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 72b1a10..7d039ca 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -82,14 +82,24 @@ do { \
>
> #else
>
> -#define preempt_disable() do { } while (0)
> -#define preempt_enable_no_resched() do { } while (0)
> -#define preempt_enable() do { } while (0)
> +#define preempt_disable() \
> +do { \
> + inc_preempt_count(); \
> + barrier(); \
> +} while (0)
> +
> +#define preempt_enable() \
> +do { \
> + barrier(); \
> + dec_preempt_count(); \
> +} while (0)
> +
> +#define preempt_enable_no_resched() preempt_enable()
> #define preempt_check_resched() do { } while (0)
>
> -#define preempt_disable_notrace() do { } while (0)
> -#define preempt_enable_no_resched_notrace() do { } while (0)
> -#define preempt_enable_notrace() do { } while (0)
> +#define preempt_disable_notrace() preempt_disable()
> +#define preempt_enable_no_resched_notrace() preempt_enable()
> +#define preempt_enable_notrace() preempt_enable()
>
> #endif
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 1535f38..841e0d0 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2556,10 +2556,8 @@ void sched_fork(struct task_struct *p, int clone_flags)
> #if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
> p->oncpu = 0;
> #endif
> -#ifdef CONFIG_PREEMPT
> /* Want to start with kernel preemption disabled. */
> task_thread_info(p)->preempt_count = 1;
> -#endif
> plist_node_init(&p->pushable_tasks, MAX_PRIO);
>
> put_cpu();
> @@ -6943,11 +6941,7 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
> spin_unlock_irqrestore(&rq->lock, flags);
>
> /* Set the preempt count _outside_ the spinlocks! */
> -#if defined(CONFIG_PREEMPT)
> task_thread_info(idle)->preempt_count = (idle->lock_depth >= 0);
> -#else
> - task_thread_info(idle)->preempt_count = 0;
> -#endif
> /*
> * The idle tasks have their own, simple scheduling class:
> */
> diff --git a/lib/kernel_lock.c b/lib/kernel_lock.c
> index 39f1029..6e2659d 100644
> --- a/lib/kernel_lock.c
> +++ b/lib/kernel_lock.c
> @@ -93,6 +93,7 @@ static inline void __lock_kernel(void)
> */
> static inline void __lock_kernel(void)
> {
> + preempt_disable();
> _raw_spin_lock(&kernel_flag);
> }
> #endif
> --
> Gleb.
>
Unbelievable! I was just thinking about the logic to achieve the same, and
someone has already done this. Thanks for the patch.
Actually, in my case, I dont think I will be able to patch and rebuild
the kernel.
Also, my use case is also much simpler, I just want to call kmalloc
depending on
the context I am running in, by giving appropriate flags.
So, I am doing something like this:
if(preemptible())
flags = (in_atomic() == 0) ? GFP_KERNEL : GFP_ATOMIC;
else
/*Cant afford failing, dont care abt emergency pool depleting etc*/
flags = GFP_ATOMIC
kmalloc(size, flags);
I am aware that, in_atomic() should not be used by drivers/modules
except core kernel,
but there is no other provision, I cant change the interfaces of the
function calls or have
two copies of functions i.e. sleepable/nonsleepable since the profiler
I am porting is
originally written for BSD.
-Leo.
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