[PATCH] copy_process: cosmetic ->ioprio tweak

[PATCH] copy_process: cosmetic ->ioprio tweak

[Oleg Nesterov]

copy_process:
// holds tasklist_lock + ->siglock
       /*
        * inherit ioprio
        */
       p->ioprio = current->ioprio;

Why? ->ioprio was already copied in dup_task_struct(). I guess this is needed
to ensure that the child can't escape sys_ioprio_set(IOPRIO_WHO_{PGRP,USER}),
yes?

In that case we don't need ->siglock held, and the comment should be updated.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.18-rc4/kernel/fork.c~1_fork	2006-08-19 17:50:56.000000000 +0400
+++ 2.6.18-rc4/kernel/fork.c	2006-08-20 18:18:47.000000000 +0400
@@ -1138,7 +1138,6 @@ static struct task_struct *copy_process(
 
 	/* Our parent execution domain becomes current domain
 	   These must match for thread signalling to apply */
-	   
 	p->parent_exec_id = p->self_exec_id;
 
 	/* ok, now we should be set up.. */
@@ -1161,6 +1160,9 @@ static struct task_struct *copy_process(
 	/* Need tasklist lock for parent etc handling! */
 	write_lock_irq(&tasklist_lock);
 
+	/* for sys_ioprio_set(IOPRIO_WHO_PGRP) */
+	p->ioprio = current->ioprio;
+
 	/*
 	 * The task hasn't been attached yet, so its cpus_allowed mask will
 	 * not be changed, nor will its assigned CPU.
@@ -1220,11 +1222,6 @@ static struct task_struct *copy_process(
 		}
 	}
 
-	/*
-	 * inherit ioprio
-	 */
-	p->ioprio = current->ioprio;
-
 	if (likely(p->pid)) {
 		add_parent(p);
 		if (unlikely(p->ptrace & PT_PTRACED))

Re: [PATCH] copy_process: cosmetic ->ioprio tweak

[Andrew Morton]

On Sun, 20 Aug 2006 18:53:21 +0400
Oleg Nesterov <oleg@tv-sign.ru> wrote:

> copy_process:
> // holds tasklist_lock + ->siglock
>        /*
>         * inherit ioprio
>         */
>        p->ioprio = current->ioprio;
> 
> Why? ->ioprio was already copied in dup_task_struct().

It might just be a thinko.

> I guess this is needed
> to ensure that the child can't escape sys_ioprio_set(IOPRIO_WHO_{PGRP,USER}),
> yes?

How could the child escape that if this assignment was not present?

> In that case we don't need ->siglock held, and the comment should be updated.

Surely.

Re: [PATCH] copy_process: cosmetic ->ioprio tweak

[Oleg Nesterov]

On 08/21, Andrew Morton wrote:
>
> On Sun, 20 Aug 2006 18:53:21 +0400
> Oleg Nesterov <oleg@tv-sign.ru> wrote:
> 
> > copy_process:
> > // holds tasklist_lock + ->siglock
> >        /*
> >         * inherit ioprio
> >         */
> >        p->ioprio = current->ioprio;
> > 
> > Why? ->ioprio was already copied in dup_task_struct().
> 
> It might just be a thinko.
> 
> > I guess this is needed
> > to ensure that the child can't escape sys_ioprio_set(IOPRIO_WHO_{PGRP,USER}),
> > yes?
> 
> How could the child escape that if this assignment was not present?

It is possible that sys_ioprio_set(IOPRIO_WHO_PGRP) was called after
copy_process() already did dup_task_struct(), but before it takes
tasklist_lock. Documentation/block/ioprio.txt doesn't say should
ioprio_set() be "atomic" or not. If not, we can kill this line, and
(more importantly) drop tasklist_lock in fs/ioprio.c

Oleg.