Date:	Wed, 14 Oct 2009 12:13:36 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	"Leonidas ." <leonidas137@...il.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 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.
--
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