[PATCH v2 0/10] Fixes for common mistakes w/ for_each_process and task->mm

[PATCH v2 0/10] Fixes for common mistakes w/ for_each_process and task->mm

[Anton Vorontsov]

Hi all,

This is a reincarnation of the task->mm fixes. Several architectures
were traverse the tasklist in an unsafe manner, plus there are a
few cases of unsafe access to task->mm.

In v2 I decided to introduce a small helper in cpu.c: most arches
duplicate the same [buggy] code snippet, so it's better to fix it
and move the logic into a common function.

Thanks!

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com

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[PATCH 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Anton Vorontsov]

Many architctures clear tasks' mm_cpumask like this:

	read_lock(&tasklist_lock);
	for_each_process(p) {
		if (p->mm)
			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
	}
	read_unlock(&tasklist_lock);

The code above has several problems, such as:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

This patch implements a small helper function that does things
correctly, i.e.:

1. We take the task's lock while whe handle its mm (we can't use
   get_task_mm()/mmput() pair as mmput() might sleep);

2. To catch exited main thread case, we use find_lock_task_mm(),
   which walks up all threads and returns an appropriate task
   (with task lock held).

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 include/linux/cpu.h |    1 +
 kernel/cpu.c        |   18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 1f65875..941e865 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -171,6 +171,7 @@ extern void put_online_cpus(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
+void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
 
 #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2060c6e..5255936 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -10,6 +10,7 @@
 #include <linux/sched.h>
 #include <linux/unistd.h>
 #include <linux/cpu.h>
+#include <linux/oom.h>
 #include <linux/export.h>
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>
@@ -171,6 +172,23 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
+void clear_tasks_mm_cpumask(int cpu)
+{
+	struct task_struct *p;
+
+	read_lock(&tasklist_lock);
+	for_each_process(p) {
+		struct task_struct *t;
+
+		t = find_lock_task_mm(p);
+		if (!t)
+			continue;
+		cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
+		task_unlock(t);
+	}
+	read_unlock(&tasklist_lock);
+}
+
 static inline void check_for_tasks(int cpu)
 {
 	struct task_struct *p;
-- 
1.7.9.2

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Re: [PATCH 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Peter Zijlstra]

On Sat, 2012-03-24 at 14:27 +0400, Anton Vorontsov wrote:
> +void clear_tasks_mm_cpumask(int cpu)
> +{
> +       struct task_struct *p;
> +
> +       read_lock(&tasklist_lock);
> +       for_each_process(p) {
> +               struct task_struct *t;
> +
> +               t = find_lock_task_mm(p);
> +               if (!t)
> +                       continue;
> +               cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
> +               task_unlock(t);
> +       }
> +       read_unlock(&tasklist_lock);
> +} 

Why bother with the tasklist_lock at all anymore, afaict you could use
rcu_read_lock() here. This all is called after the cpu is taken down and
marked offline, so its not like new tasks will ever get this cpu set in
their mm mask.


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[PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Anton Vorontsov]

Many architctures clear tasks' mm_cpumask like this:

	read_lock(&tasklist_lock);
	for_each_process(p) {
		if (p->mm)
			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
	}
	read_unlock(&tasklist_lock);

The code above has several problems, such as:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

This patch implements a small helper function that does things
correctly, i.e.:

1. We take the task's lock while whe handle its mm (we can't use
   get_task_mm()/mmput() pair as mmput() might sleep);

2. To catch exited main thread case, we use find_lock_task_mm(),
   which walks up all threads and returns an appropriate task
   (with task lock held).

Also, Per Peter Zijlstra's idea, now we don't grab tasklist_lock in
the new helper, instead we take the rcu read lock. We can do this
because the function is called after the cpu is taken down and marked
offline, so no new tasks will get this cpu set in their mm mask.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---

On Sat, Mar 24, 2012 at 01:43:41PM +0100, Peter Zijlstra wrote:
> On Sat, 2012-03-24 at 14:27 +0400, Anton Vorontsov wrote:
> > +       read_unlock(&tasklist_lock);
> > +} 
> 
> Why bother with the tasklist_lock at all anymore, afaict you could use
> rcu_read_lock() here. This all is called after the cpu is taken down and
> marked offline, so its not like new tasks will ever get this cpu set in
> their mm mask.

I guess you're right. Well, this would make the code a bit fragile,
but a comment might help.

How about this version?

 include/linux/cpu.h |    1 +
 kernel/cpu.c        |   26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 1f65875..941e865 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -171,6 +171,7 @@ extern void put_online_cpus(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
+void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
 
 #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2060c6e..ecdf499 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -10,6 +10,8 @@
 #include <linux/sched.h>
 #include <linux/unistd.h>
 #include <linux/cpu.h>
+#include <linux/oom.h>
+#include <linux/rcupdate.h>
 #include <linux/export.h>
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>
@@ -171,6 +173,30 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
+void clear_tasks_mm_cpumask(int cpu)
+{
+	struct task_struct *p;
+
+	/*
+	 * This function is called after the cpu is taken down and marked
+	 * offline, so its not like new tasks will ever get this cpu set in
+	 * their mm mask. -- Peter Zijlstra
+	 * Thus, we may use rcu_read_lock() here, instead of grabbing
+	 * full-fledged tasklist_lock.
+	 */
+	rcu_read_lock();
+	for_each_process(p) {
+		struct task_struct *t;
+
+		t = find_lock_task_mm(p);
+		if (!t)
+			continue;
+		cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
+		task_unlock(t);
+	}
+	rcu_read_unlock();
+}
+
 static inline void check_for_tasks(int cpu)
 {
 	struct task_struct *p;
-- 
1.7.9.2

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Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Oleg Nesterov]

On 03/24, Anton Vorontsov wrote:
>
> Many architctures clear tasks' mm_cpumask like this:
>
> 	read_lock(&tasklist_lock);
> 	for_each_process(p) {
> 		if (p->mm)
> 			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
> 	}
> 	read_unlock(&tasklist_lock);

Namely arm, powerpc, and sh.

> The code above has several problems, such as:
>
> 1. Working with task->mm w/o getting mm or grabing the task lock is
>    dangerous as ->mm might disappear (exit_mm() assigns NULL under
>    task_lock(), so tasklist lock is not enough).

This is not actually true for arm and sh, afaics. They do not even
need tasklist or rcu lock for for_each_process().

__cpu_disable() is called by __stop_machine(), we know that nobody
can preempt us and other CPUs can do nothing.

> 2. Checking for process->mm is not enough because process' main
>    thread may exit or detach its mm via use_mm(), but other threads
>    may still have a valid mm.

Yes,

> Also, Per Peter Zijlstra's idea, now we don't grab tasklist_lock in
> the new helper, instead we take the rcu read lock. We can do this
> because the function is called after the cpu is taken down and marked
> offline, so no new tasks will get this cpu set in their mm mask.

And only powerpc needs rcu_read_lock() and task_lock().

OTOH, I do not understand why powepc does this on CPU_DEAD...
And probably CPU_UP_CANCELED doesn't need to clear mm_cpumask().

That said, personally I think these patches are fine, the common
helper makes sense.

Oleg.

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Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Peter Zijlstra]

On Sun, 2012-03-25 at 19:42 +0200, Oleg Nesterov wrote:
> __cpu_disable() is called by __stop_machine(), we know that nobody
> can preempt us and other CPUs can do nothing. 

It would be very good to not rely on that though, I would love to get
rid of the stop_machine usage in cpu hotplug some day.

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Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Oleg Nesterov]

On 03/26, Peter Zijlstra wrote:
>
> On Sun, 2012-03-25 at 19:42 +0200, Oleg Nesterov wrote:
> > __cpu_disable() is called by __stop_machine(), we know that nobody
> > can preempt us and other CPUs can do nothing.
>
> It would be very good to not rely on that though,

Yes, yes, perhaps I wasn't clear but I think the patches are fine.

> I would love to get
> rid of the stop_machine usage in cpu hotplug some day.

Interesting... Why? I mean, why do you dislike stop_machine() in
_cpu_down() ? Just curious.

Oleg.

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Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Peter Zijlstra]

On Mon, 2012-03-26 at 19:04 +0200, Oleg Nesterov wrote:

> Interesting... Why? I mean, why do you dislike stop_machine() in
> _cpu_down() ? Just curious.

It disturbs all cpus, the -rt people don't like that their FIFO tasks
don't get to run, the trading people don't like their RDMA poll loops to
be interrupted.. etc.

Now arguably, one should simply not do hotplug crap while such things
are running, and mostly that's a perfectly fine constraint. But it
doesn't help that people view cpu hotplug as a power savings or resource
provisioning 'feature' and there's userspace daemons that plug
on-demand.

But my ultimate goal is to completely remove synchronization that is
actively machine wide, since we all know that as long as such stuff
exists people will want to use it.

Now I don't know we'll ever fully get there -- see the BKL saga -- but
its worth trying I think. The module unload and esp. the text_poke usage
of stop_machine are much worse offenders, since both those are
relatively common and much harder to avoid.

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Re: [PATCH v2.1 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

[Benjamin Herrenschmidt]

On Sun, 2012-03-25 at 19:42 +0200, Oleg Nesterov wrote:
> > Also, Per Peter Zijlstra's idea, now we don't grab tasklist_lock in
> > the new helper, instead we take the rcu read lock. We can do this
> > because the function is called after the cpu is taken down and
> marked
> > offline, so no new tasks will get this cpu set in their mm mask.
> 
> And only powerpc needs rcu_read_lock() and task_lock().
> 
> OTOH, I do not understand why powepc does this on CPU_DEAD...
> And probably CPU_UP_CANCELED doesn't need to clear mm_cpumask().
> 
> That said, personally I think these patches are fine, the common
> helper makes sense. 

Not strictly speaking a problem with this patch, but I was wondering...

Do we know for sure that the mmu context has been fully flushed out
before the unplug ? idle_task_exit() will do a context switch but in our
case that may not be enough.

Once the CPU is offline, tlb flushes won't hit it any more so it can get
out of sync (in some cases the offlining process is just some kind of
deep sleep loop that doesn't involve a TLB state loss).

Should we add a flush_tlb_mm of all those bits in that loop ? that would
be a tad expensive... we don't have a flush_tlb_all() as a generic kind
of accessors, but we could add something like that as a requirement for
ppc_md.cpu_die ?

Cheers,
Ben.


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[PATCH 02/10] arm: Use clear_tasks_mm_cpumask()

[Anton Vorontsov]

Current CPU hotplug code has some task->mm handling issues:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we must take the task lock while handle its mm.

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To fix this we would need to use find_lock_task_mm(), which would
   walk up all threads and returns an appropriate task (with task
   lock held).

clear_tasks_mm_cpumask() has all the issues fixed, so let's use it.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/arm/kernel/smp.c |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index cdeb727..880b93a 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -136,7 +136,6 @@ static void percpu_timer_stop(void);
 int __cpu_disable(void)
 {
 	unsigned int cpu = smp_processor_id();
-	struct task_struct *p;
 	int ret;
 
 	ret = platform_cpu_disable(cpu);
@@ -166,12 +165,7 @@ int __cpu_disable(void)
 	flush_cache_all();
 	local_flush_tlb_all();
 
-	read_lock(&tasklist_lock);
-	for_each_process(p) {
-		if (p->mm)
-			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
-	}
-	read_unlock(&tasklist_lock);
+	clear_tasks_mm_cpumask(cpu);
 
 	return 0;
 }
-- 
1.7.9.2

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[PATCH 03/10] powerpc: Use clear_tasks_mm_cpumask()

[Anton Vorontsov]

Current CPU hotplug code has some task->mm handling issues:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we must take the task lock while handle its mm.

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To fix this we would need to use find_lock_task_mm(), which would
   walk up all threads and returns an appropriate task (with task
   lock held).

clear_tasks_mm_cpumask() has all the issues fixed, so let's use it.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/powerpc/mm/mmu_context_nohash.c |   11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/mm/mmu_context_nohash.c b/arch/powerpc/mm/mmu_context_nohash.c
index 5b63bd3..e779642 100644
--- a/arch/powerpc/mm/mmu_context_nohash.c
+++ b/arch/powerpc/mm/mmu_context_nohash.c
@@ -333,9 +333,7 @@ static int __cpuinit mmu_context_cpu_notify(struct notifier_block *self,
 					    unsigned long action, void *hcpu)
 {
 	unsigned int cpu = (unsigned int)(long)hcpu;
-#ifdef CONFIG_HOTPLUG_CPU
-	struct task_struct *p;
-#endif
+
 	/* We don't touch CPU 0 map, it's allocated at aboot and kept
 	 * around forever
 	 */
@@ -358,12 +356,7 @@ static int __cpuinit mmu_context_cpu_notify(struct notifier_block *self,
 		stale_map[cpu] = NULL;
 
 		/* We also clear the cpu_vm_mask bits of CPUs going away */
-		read_lock(&tasklist_lock);
-		for_each_process(p) {
-			if (p->mm)
-				cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
-		}
-		read_unlock(&tasklist_lock);
+		clear_tasks_mm_cpumask(cpu);
 	break;
 #endif /* CONFIG_HOTPLUG_CPU */
 	}
-- 
1.7.9.2

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[PATCH 04/10] sh: Use clear_tasks_mm_cpumask()

[Anton Vorontsov]

Current CPU hotplug code has some task->mm handling issues:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we must take the task lock while handle its mm.

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To fix this we would need to use find_lock_task_mm(), which would
   walk up all threads and returns an appropriate task (with task
   lock held).

clear_tasks_mm_cpumask() has all the issues fixed, so let's use it.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/sh/kernel/smp.c |    7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
index f624174..2470c83 100644
--- a/arch/sh/kernel/smp.c
+++ b/arch/sh/kernel/smp.c
@@ -123,7 +123,6 @@ void native_play_dead(void)
 int __cpu_disable(void)
 {
 	unsigned int cpu = smp_processor_id();
-	struct task_struct *p;
 	int ret;
 
 	ret = mp_ops->cpu_disable(cpu);
@@ -153,11 +152,7 @@ int __cpu_disable(void)
 	flush_cache_all();
 	local_flush_tlb_all();
 
-	read_lock(&tasklist_lock);
-	for_each_process(p)
-		if (p->mm)
-			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
-	read_unlock(&tasklist_lock);
+	clear_tasks_mm_cpumask(cpu);
 
 	return 0;
 }
-- 
1.7.9.2

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[PATCH 05/10] blackfin: A couple of task->mm handling fixes

[Anton Vorontsov]

The patch fixes two problems:

1. Working with task->mm w/o getting mm or grabing the task lock is
   dangerous as ->mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we have to take the task lock while handle its mm.

2. Checking for process->mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To catch this we use find_lock_task_mm(), which walks up all
   threads and returns an appropriate task (with task lock held).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/blackfin/kernel/trace.c |   26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index 050db44..5102cdf 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -10,6 +10,8 @@
 #include <linux/hardirq.h>
 #include <linux/thread_info.h>
 #include <linux/mm.h>
+#include <linux/oom.h>
+#include <linux/sched.h>
 #include <linux/uaccess.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
@@ -27,7 +29,6 @@ void decode_address(char *buf, unsigned long address)
 	struct task_struct *p;
 	struct mm_struct *mm;
 	unsigned long flags, offset;
-	unsigned char in_atomic = (bfin_read_IPEND() & 0x10) || in_atomic();
 	struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -113,15 +114,15 @@ void decode_address(char *buf, unsigned long address)
 	 */
 	write_lock_irqsave(&tasklist_lock, flags);
 	for_each_process(p) {
-		mm = (in_atomic ? p->mm : get_task_mm(p));
-		if (!mm)
-			continue;
+		struct task_struct *t;
 
-		if (!down_read_trylock(&mm->mmap_sem)) {
-			if (!in_atomic)
-				mmput(mm);
+		t = find_lock_task_mm(p);
+		if (!t)
 			continue;
-		}
+
+		mm = t->mm;
+		if (!down_read_trylock(&mm->mmap_sem))
+			goto __continue;
 
 		for (n = rb_first(&mm->mm_rb); n; n = rb_next(n)) {
 			struct vm_area_struct *vma;
@@ -130,7 +131,7 @@ void decode_address(char *buf, unsigned long address)
 
 			if (address >= vma->vm_start && address < vma->vm_end) {
 				char _tmpbuf[256];
-				char *name = p->comm;
+				char *name = t->comm;
 				struct file *file = vma->vm_file;
 
 				if (file) {
@@ -163,8 +164,7 @@ void decode_address(char *buf, unsigned long address)
 						name, vma->vm_start, vma->vm_end);
 
 				up_read(&mm->mmap_sem);
-				if (!in_atomic)
-					mmput(mm);
+				task_unlock(t);
 
 				if (buf[0] == '\0')
 					sprintf(buf, "[ %s ] dynamic memory", name);
@@ -174,8 +174,8 @@ void decode_address(char *buf, unsigned long address)
 		}
 
 		up_read(&mm->mmap_sem);
-		if (!in_atomic)
-			mmput(mm);
+__continue:
+		task_unlock(t);
 	}
 
 	/*
-- 
1.7.9.2

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[PATCH 06/10] blackfin: Fix possible deadlock in decode_address()

[Anton Vorontsov]

Oleg Nesterov found an interesting deadlock possibility:

> sysrq_showregs_othercpus() does smp_call_function(showacpu)
> and showacpu() show_stack()->decode_address(). Now suppose that IPI
> interrupts the task holding read_lock(tasklist).

To fix this, blackfin should not grab the write_ variant of the
tasklist lock, read_ one is enough.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/blackfin/kernel/trace.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index 5102cdf..9cc9302 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -28,7 +28,7 @@ void decode_address(char *buf, unsigned long address)
 {
 	struct task_struct *p;
 	struct mm_struct *mm;
-	unsigned long flags, offset;
+	unsigned long offset;
 	struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -112,7 +112,7 @@ void decode_address(char *buf, unsigned long address)
 	 * mappings of all our processes and see if we can't be a whee
 	 * bit more specific
 	 */
-	write_lock_irqsave(&tasklist_lock, flags);
+	read_lock(&tasklist_lock);
 	for_each_process(p) {
 		struct task_struct *t;
 
@@ -185,7 +185,7 @@ __continue:
 	sprintf(buf, "/* kernel dynamic memory */");
 
 done:
-	write_unlock_irqrestore(&tasklist_lock, flags);
+	read_unlock(&tasklist_lock);
 }
 
 #define EXPAND_LEN ((1 << CONFIG_DEBUG_BFIN_HWTRACE_EXPAND_LEN) * 256 - 1)
-- 
1.7.9.2

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[PATCH 07/10] um: Should hold tasklist_lock while traversing processes

[Anton Vorontsov]

Traversing the tasks requires holding tasklist_lock, otherwise it
is unsafe.

p.s. However, I'm not sure that calling os_kill_ptraced_process()
in the atomic context is correct. It seem to work, but please
take a closer look.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/um/kernel/reboot.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 4d93dff..66d754c 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -4,6 +4,7 @@
  */
 
 #include "linux/sched.h"
+#include "linux/spinlock.h"
 #include "linux/slab.h"
 #include "kern_util.h"
 #include "os.h"
@@ -22,6 +23,7 @@ static void kill_off_processes(void)
 		struct task_struct *p;
 		int pid;
 
+		read_lock(&tasklist_lock);
 		for_each_process(p) {
 			if (p->mm == NULL)
 				continue;
@@ -29,6 +31,7 @@ static void kill_off_processes(void)
 			pid = p->mm->context.id.u.pid;
 			os_kill_ptraced_process(pid, 1);
 		}
+		read_unlock(&tasklist_lock);
 	}
 }
 
-- 
1.7.9.2

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Re: [PATCH 07/10] um: Should hold tasklist_lock while traversing processes

[Richard Weinberger]

[-- Attachment #1: Type: text/plain, Size: 475 bytes --]

Am 24.03.2012 11:30, schrieb Anton Vorontsov:
> Traversing the tasks requires holding tasklist_lock, otherwise it
> is unsafe.
> 
> p.s. However, I'm not sure that calling os_kill_ptraced_process()
> in the atomic context is correct. It seem to work, but please
> take a closer look.

os_kill_ptraced_process() calls a host function.
From UML's point of view nothing sleeps, so this is fine.

Acked-by: Richard Weinberger <richard@nod.at>

Thanks,
//richard


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Re: [PATCH 07/10] um: Should hold tasklist_lock while traversing processes

[Peter Zijlstra]

On Sat, 2012-03-24 at 14:30 +0400, Anton Vorontsov wrote:
> Traversing the tasks requires holding tasklist_lock, otherwise it
> is unsafe. 

No it doesn't, it either requires tasklist_lock or rcu_read_lock().

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Re: [PATCH 07/10] um: Should hold tasklist_lock while traversing processes

[Anton Vorontsov]

On Sat, Mar 24, 2012 at 01:48:23PM +0100, Peter Zijlstra wrote:
> On Sat, 2012-03-24 at 14:30 +0400, Anton Vorontsov wrote:
> > Traversing the tasks requires holding tasklist_lock, otherwise it
> > is unsafe. 
> 
> No it doesn't, it either requires tasklist_lock or rcu_read_lock().

Well, currently the code does neither. :-)

-- 
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Email: cbouatmailru@gmail.com

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[PATCH 08/10] um: Fix possible race on task->mm

[Anton Vorontsov]

Checking for task->mm is dangerous as ->mm might disappear (exit_mm()
assigns NULL under task_lock(), so tasklist lock is not enough).

We can't use get_task_mm()/mmput() pair as mmput() might sleep,
so let's take the task lock while we care about its mm.

Note that we should also use find_lock_task_mm() to check all process'
threads for a valid mm, but for uml we'll do it in a separate patch.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/um/kernel/reboot.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 66d754c..1411f4e 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -25,10 +25,13 @@ static void kill_off_processes(void)
 
 		read_lock(&tasklist_lock);
 		for_each_process(p) {
-			if (p->mm == NULL)
+			task_lock(p);
+			if (!p->mm) {
+				task_unlock(p);
 				continue;
-
+			}
 			pid = p->mm->context.id.u.pid;
+			task_unlock(p);
 			os_kill_ptraced_process(pid, 1);
 		}
 		read_unlock(&tasklist_lock);
-- 
1.7.9.2

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Re: [PATCH 08/10] um: Fix possible race on task->mm

[Richard Weinberger]

[-- Attachment #1: Type: text/plain, Size: 608 bytes --]

Am 24.03.2012 11:30, schrieb Anton Vorontsov:
> Checking for task->mm is dangerous as ->mm might disappear (exit_mm()
> assigns NULL under task_lock(), so tasklist lock is not enough).
> 
> We can't use get_task_mm()/mmput() pair as mmput() might sleep,
> so let's take the task lock while we care about its mm.
> 
> Note that we should also use find_lock_task_mm() to check all process'
> threads for a valid mm, but for uml we'll do it in a separate patch.
> 
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

Acked-by: Richard Weinberger <richard@nod.at>

Thanks,
//richard


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[PATCH 09/10] um: Properly check all process' threads for a live mm

[Anton Vorontsov]

kill_off_processes() might miss a valid process, this is because
checking for process->mm is not enough. Process' main thread may
exit or detach its mm via use_mm(), but other threads may still
have a valid mm.

To catch this we use find_lock_task_mm(), which walks up all
threads and returns an appropriate task (with task lock held).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/um/kernel/reboot.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 1411f4e..3d15243 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -6,6 +6,7 @@
 #include "linux/sched.h"
 #include "linux/spinlock.h"
 #include "linux/slab.h"
+#include "linux/oom.h"
 #include "kern_util.h"
 #include "os.h"
 #include "skas.h"
@@ -25,13 +26,13 @@ static void kill_off_processes(void)
 
 		read_lock(&tasklist_lock);
 		for_each_process(p) {
-			task_lock(p);
-			if (!p->mm) {
-				task_unlock(p);
+			struct task_struct *t;
+
+			t = find_lock_task_mm(p);
+			if (!t)
 				continue;
-			}
-			pid = p->mm->context.id.u.pid;
-			task_unlock(p);
+			pid = t->mm->context.id.u.pid;
+			task_unlock(t);
 			os_kill_ptraced_process(pid, 1);
 		}
 		read_unlock(&tasklist_lock);
-- 
1.7.9.2

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Re: [PATCH 09/10] um: Properly check all process' threads for a live mm

[Richard Weinberger]

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Am 24.03.2012 11:31, schrieb Anton Vorontsov:
> kill_off_processes() might miss a valid process, this is because
> checking for process->mm is not enough. Process' main thread may
> exit or detach its mm via use_mm(), but other threads may still
> have a valid mm.
> 
> To catch this we use find_lock_task_mm(), which walks up all
> threads and returns an appropriate task (with task lock held).
> 
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

Acked-by: Richard Weinberger <richard@nod.at>

Thanks,
//richard


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[PATCH 10/10] oom: Make find_lock_task_mm() sparse-aware

[Anton Vorontsov]

This is needed so that callers would not get 'context imbalance'
warnings from the sparse tool.

As a side effect, this patch fixes the following sparse warnings:

  CHECK   mm/oom_kill.c
  mm/oom_kill.c:201:28: warning: context imbalance in 'oom_badness' -
  unexpected unlock
  include/linux/rcupdate.h:249:30: warning: context imbalance in
  'dump_tasks' - unexpected unlock
  mm/oom_kill.c:453:9: warning: context imbalance in 'oom_kill_task' -
  unexpected unlock
  CHECK   mm/memcontrol.c
  ...
  mm/memcontrol.c:1130:17: warning: context imbalance in
  'task_in_mem_cgroup' - unexpected unlock

p.s. I know Peter Zijlstra detest the __cond_lock() stuff, but untill
     we have anything better in sparse, let's use it. This particular
     patch helped me to detect one bug that I myself made during
     task->mm fixup series. So, it is useful.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 include/linux/oom.h |   12 +++++++++++-
 mm/oom_kill.c       |    2 +-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/oom.h b/include/linux/oom.h
index 552fba9..26cf628 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -21,6 +21,7 @@
 
 #ifdef __KERNEL__
 
+#include <linux/compiler.h>
 #include <linux/sched.h>
 #include <linux/types.h>
 #include <linux/nodemask.h>
@@ -65,7 +66,16 @@ static inline void oom_killer_enable(void)
 	oom_killer_disabled = false;
 }
 
-extern struct task_struct *find_lock_task_mm(struct task_struct *p);
+extern struct task_struct *__find_lock_task_mm(struct task_struct *p);
+
+static inline struct task_struct *find_lock_task_mm(struct task_struct *p)
+{
+	struct task_struct *ret;
+
+	ret = __find_lock_task_mm(p);
+	(void)__cond_lock(&ret->alloc_lock, ret);
+	return ret;
+}
 
 /* sysctls */
 extern int sysctl_oom_dump_tasks;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 2958fd8..0ebb383 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -136,7 +136,7 @@ static bool has_intersects_mems_allowed(struct task_struct *tsk,
  * pointer.  Return p, or any of its subthreads with a valid ->mm, with
  * task_lock() held.
  */
-struct task_struct *find_lock_task_mm(struct task_struct *p)
+struct task_struct *__find_lock_task_mm(struct task_struct *p)
 {
 	struct task_struct *t = p;
 
-- 
1.7.9.2

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Re: [PATCH 10/10] oom: Make find_lock_task_mm() sparse-aware

[Peter Zijlstra]

On Sat, 2012-03-24 at 14:31 +0400, Anton Vorontsov wrote:
> This is needed so that callers would not get 'context imbalance'
> warnings from the sparse tool.
> 
> As a side effect, this patch fixes the following sparse warnings:
> 
>   CHECK   mm/oom_kill.c
>   mm/oom_kill.c:201:28: warning: context imbalance in 'oom_badness' -
>   unexpected unlock
>   include/linux/rcupdate.h:249:30: warning: context imbalance in
>   'dump_tasks' - unexpected unlock
>   mm/oom_kill.c:453:9: warning: context imbalance in 'oom_kill_task' -
>   unexpected unlock
>   CHECK   mm/memcontrol.c
>   ...
>   mm/memcontrol.c:1130:17: warning: context imbalance in
>   'task_in_mem_cgroup' - unexpected unlock
> 
> p.s. I know Peter Zijlstra detest the __cond_lock() stuff, but untill
>      we have anything better in sparse, let's use it. This particular
>      patch helped me to detect one bug that I myself made during
>      task->mm fixup series. So, it is useful.

Yeah, so Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Also, why didn't lockdep catch it?

Fix sparse already instead of smearing ugly all over.

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Re: [PATCH 10/10] oom: Make find_lock_task_mm() sparse-aware

[Anton Vorontsov]

On Sat, Mar 24, 2012 at 01:52:54PM +0100, Peter Zijlstra wrote:
[...]
> > p.s. I know Peter Zijlstra detest the __cond_lock() stuff, but untill
> >      we have anything better in sparse, let's use it. This particular
> >      patch helped me to detect one bug that I myself made during
> >      task->mm fixup series. So, it is useful.
> 
> Yeah, so Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Also, why didn't lockdep catch it?

Because patch authors test their patches on architectures they own
(well, sometimes I do check patches on exotic architectures w/ qemu,
but it is less convenient than just build/sparse-test the patch w/
a cross compiler).

And since lockdep is a runtime checker, it is not very useful.

Sparse is a build-time checker, so it is even better in the sense
that it is able to catch bugs even in code that is executed rarely.

> Fix sparse already instead of smearing ugly all over.

Just wonder how do you see the feature implemented?

Something like this?

#define __ret_cond_locked(l, c)	__attribute__((ret_cond_locked(l, c)))
#define __ret_value		__attribute__((ret_value))
#define __ret_locked_nonnull(l)	__ret_cond_locked(l, __ret_value);

extern struct task_struct *find_lock_task_mm(struct task_struct *p)
	__ret_locked_nonnull(&__ret_value->alloc_lock);

Thanks,

-- 
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Email: cbouatmailru@gmail.com

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Re: [PATCH 10/10] oom: Make find_lock_task_mm() sparse-aware

[Peter Zijlstra]

On Sat, 2012-03-24 at 20:21 +0400, Anton Vorontsov wrote:

> Just wonder how do you see the feature implemented?
> 
> Something like this?
> 
> #define __ret_cond_locked(l, c)	__attribute__((ret_cond_locked(l, c)))
> #define __ret_value		__attribute__((ret_value))
> #define __ret_locked_nonnull(l)	__ret_cond_locked(l, __ret_value);
> 
> extern struct task_struct *find_lock_task_mm(struct task_struct *p)
> 	__ret_locked_nonnull(&__ret_value->alloc_lock);

Yeah, see the email I just CC'ed you on to linux-sparse.

Basically extend __attribute__((context())) to allow things similar to
what you proposed.

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Re: [PATCH 10/10] oom: Make find_lock_task_mm() sparse-aware

[David Rientjes]

On Sat, 24 Mar 2012, Peter Zijlstra wrote:

> Yeah, so Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Also, why didn't lockdep catch it?
> 
> Fix sparse already instead of smearing ugly all over.
> 

Fully agreed, please don't add this to the oom killer.

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