[PATCH 2/2] Use write_trylock_irqsave in ptrace_attach

[PATCH 2/2] Use write_trylock_irqsave in ptrace_attach

[Sripathi Kodi]

Hi,

This patch makes ptrace_attach use write_trylock_irqsave.

Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>

---
 kernel/ptrace.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

Index: linux-2.6.21/kernel/ptrace.c
===================================================================
--- linux-2.6.21.orig/kernel/ptrace.c
+++ linux-2.6.21/kernel/ptrace.c
@@ -160,6 +160,7 @@ int ptrace_may_attach(struct task_struct
 int ptrace_attach(struct task_struct *task)
 {
 	int retval;
+	unsigned long flags = 0;
 
 	retval = -EPERM;
 	if (task->pid <= 1)
@@ -178,9 +179,7 @@ repeat:
 	 * cpu's that may have task_lock).
 	 */
 	task_lock(task);
-	local_irq_disable();
-	if (!write_trylock(&tasklist_lock)) {
-		local_irq_enable();
+	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
 		task_unlock(task);
 		do {
 			cpu_relax();
@@ -208,7 +207,7 @@ repeat:
 	force_sig_specific(SIGSTOP, task);
 
 bad:
-	write_unlock_irq(&tasklist_lock);
+	write_unlock_irqrestore(&tasklist_lock, flags);
 	task_unlock(task);
 out:
 	return retval;

Re: [PATCH 2/2] Use write_trylock_irqsave in ptrace_attach

[Andrew Morton]

On Wed, 9 May 2007 14:13:27 +0530 Sripathi Kodi <sripathik@in.ibm.com> wrote:

> Hi,
> 
> This patch makes ptrace_attach use write_trylock_irqsave.
> 
> Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>
> 
> ---
>  kernel/ptrace.c |    7 +++----
>  1 files changed, 3 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6.21/kernel/ptrace.c
> ===================================================================
> --- linux-2.6.21.orig/kernel/ptrace.c
> +++ linux-2.6.21/kernel/ptrace.c
> @@ -160,6 +160,7 @@ int ptrace_may_attach(struct task_struct
>  int ptrace_attach(struct task_struct *task)
>  {
>  	int retval;
> +	unsigned long flags = 0;
>  
>  	retval = -EPERM;
>  	if (task->pid <= 1)
> @@ -178,9 +179,7 @@ repeat:
>  	 * cpu's that may have task_lock).
>  	 */
>  	task_lock(task);
> -	local_irq_disable();
> -	if (!write_trylock(&tasklist_lock)) {
> -		local_irq_enable();
> +	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
>  		task_unlock(task);
>  		do {
>  			cpu_relax();
> @@ -208,7 +207,7 @@ repeat:
>  	force_sig_specific(SIGSTOP, task);
>  
>  bad:
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_irqrestore(&tasklist_lock, flags);
>  	task_unlock(task);
>  out:
>  	return retval;

Your changelogs aren't vey logical.  The context for this change is off in
a different patch.  I reproduce it here:

> I am trying to fix the BUG I mentioned here: 
> http://lkml.org/lkml/2007/04/20/41. I noticed that an elegant way to solve 
> this problem is to have a write_trylock_irqsave helper function. Since we 
> don't have this now, the code in ptrace_attach  implements it using 
> local_irq_disable and write_trylock. I wish to add write_trylock_irqsave to 
> mainline kernel and then fix the -rt specific problem using this.

I can't imagine why -rt's write_unlock_irq() doesn't do local_irq_enable().

I have no problem adding write_trylock_irqsave() - it fills a gap in the
API.

Once we have write_trylock_irqsave() it makes sense to use it here.

One the downside, we added a few bytes to the SMP kernel, which I guess we
can live with.

Whether this change is desired in -rt I don't know.  Ingo?

I don't think the initialisation of `flags' there was needed?

Re: [PATCH 2/2] Use write_trylock_irqsave in ptrace_attach

[Sripathi Kodi]

On Thursday 10 May 2007 07:20, Andrew Morton wrote:
> On Wed, 9 May 2007 14:13:27 +0530 Sripathi Kodi <sripathik@in.ibm.com> 
wrote:
> > Hi,
> >
> > This patch makes ptrace_attach use write_trylock_irqsave.
> >
> > Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>
> >
> > ---
> >  kernel/ptrace.c |    7 +++----
> >  1 files changed, 3 insertions(+), 4 deletions(-)
> >
> > Index: linux-2.6.21/kernel/ptrace.c
> > ===================================================================
> > --- linux-2.6.21.orig/kernel/ptrace.c
> > +++ linux-2.6.21/kernel/ptrace.c
> > @@ -160,6 +160,7 @@ int ptrace_may_attach(struct task_struct
> >  int ptrace_attach(struct task_struct *task)
> >  {
> >  	int retval;
> > +	unsigned long flags = 0;
> >
> >  	retval = -EPERM;
> >  	if (task->pid <= 1)
> > @@ -178,9 +179,7 @@ repeat:
> >  	 * cpu's that may have task_lock).
> >  	 */
> >  	task_lock(task);
> > -	local_irq_disable();
> > -	if (!write_trylock(&tasklist_lock)) {
> > -		local_irq_enable();
> > +	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
> >  		task_unlock(task);
> >  		do {
> >  			cpu_relax();
> > @@ -208,7 +207,7 @@ repeat:
> >  	force_sig_specific(SIGSTOP, task);
> >
> >  bad:
> > -	write_unlock_irq(&tasklist_lock);
> > +	write_unlock_irqrestore(&tasklist_lock, flags);
> >  	task_unlock(task);
> >  out:
> >  	return retval;
>
> Your changelogs aren't vey logical.  The context for this change is off in
>
> a different patch.  I reproduce it here:
> > I am trying to fix the BUG I mentioned here:
> > http://lkml.org/lkml/2007/04/20/41. I noticed that an elegant way to
> > solve this problem is to have a write_trylock_irqsave helper function.
> > Since we don't have this now, the code in ptrace_attach  implements it
> > using local_irq_disable and write_trylock. I wish to add
> > write_trylock_irqsave to mainline kernel and then fix the -rt specific
> > problem using this.
>
> I can't imagine why -rt's write_unlock_irq() doesn't do local_irq_enable().

-rt's write_unlock_irq() does local_irq_enable() while dealing with 'raw' 
rwlock_t. However tasklist_lock is a regular rwlock_t and hence -rt doesn't 
save/restore irqs while dealing with it. The probelm in ptrace_attach arises 
because we explicitely call local_irq_disable(), whereas write_unlock_irq() 
doesn't restore them.


>
> I have no problem adding write_trylock_irqsave() - it fills a gap in the
> API.
>
> Once we have write_trylock_irqsave() it makes sense to use it here.
>
> One the downside, we added a few bytes to the SMP kernel, which I guess we
> can live with.
>
> Whether this change is desired in -rt I don't know.  Ingo?

I will send a patch against -rt in a little while.

>
> I don't think the initialisation of `flags' there was needed?

Yes, it was not needed. I will send a patch to remove it.

Thanks,
Sripathi.

Re: [PATCH 2/2] Use write_trylock_irqsave in ptrace_attach

[Sripathi Kodi]

Hi Andrew,

On Thursday 10 May 2007 07:20, you wrote:
> On Wed, 9 May 2007 14:13:27 +0530 Sripathi Kodi <sripathik@in.ibm.com> 
wrote:

<snip old patch>

> Your changelogs aren't vey logical.  The context for this change is off in
>
> a different patch.  I reproduce it here:
> > I am trying to fix the BUG I mentioned here:
> > http://lkml.org/lkml/2007/04/20/41. I noticed that an elegant way to
> > solve this problem is to have a write_trylock_irqsave helper function.
> > Since we don't have this now, the code in ptrace_attach  implements it
> > using local_irq_disable and write_trylock. I wish to add
> > write_trylock_irqsave to mainline kernel and then fix the -rt specific
> > problem using this.
>
> I can't imagine why -rt's write_unlock_irq() doesn't do local_irq_enable().
>
> I have no problem adding write_trylock_irqsave() - it fills a gap in the
> API.
>
> Once we have write_trylock_irqsave() it makes sense to use it here.
>
> One the downside, we added a few bytes to the SMP kernel, which I guess we
> can live with.
>
> Whether this change is desired in -rt I don't know.  Ingo?
>
> I don't think the initialisation of `flags' there was needed?

I removed the initialization of 'flags' in the following patch. Would you like 
to drop the old one and pick up this?

Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>

diff -uprN linux-2.6.21.1_org/kernel/ptrace.c linux-2.6.21.1/kernel/ptrace.c
--- linux-2.6.21.1_org/kernel/ptrace.c	2007-05-09 13:18:39.000000000 +0530
+++ linux-2.6.21.1/kernel/ptrace.c	2007-05-10 17:40:51.000000000 +0530
@@ -160,6 +160,7 @@ int ptrace_may_attach(struct task_struct
 int ptrace_attach(struct task_struct *task)
 {
 	int retval;
+	unsigned long flags;
 
 	retval = -EPERM;
 	if (task->pid <= 1)
@@ -178,9 +179,7 @@ repeat:
 	 * cpu's that may have task_lock).
 	 */
 	task_lock(task);
-	local_irq_disable();
-	if (!write_trylock(&tasklist_lock)) {
-		local_irq_enable();
+	if (!write_trylock_irqsave(&tasklist_lock, flags)) {
 		task_unlock(task);
 		do {
 			cpu_relax();
@@ -208,7 +207,7 @@ repeat:
 	force_sig_specific(SIGSTOP, task);
 
 bad:
-	write_unlock_irq(&tasklist_lock);
+	write_unlock_irqrestore(&tasklist_lock, flags);
 	task_unlock(task);
 out:
 	return retval;