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Date: Fri, 03 Jul 2009 16:54:58 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-core] x86: Endless minor faults
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To: Jan Kiszka <jan.kiszka@domain.hid>
Cc: xenomai-core <xenomai@xenomai.org>

Jan Kiszka wrote:
> Hi again,
> 
> this is now basically the patch which seems to stabilized x86 /wrt mmu
> switches again:
> 
> There were 3 race windows between setting active_mm of the current task
> and actually switching it (that's a noarch issue), there were several
> calls into switch_mm without proper hard interrupt protection (on archs
> that have no preemptible switch_mm, like x86) and there was a race in
> x86's leave_mm (as Gilles already remarked earlier in this thread -
> while I was looking at an old tree where smp_invalidate_interrupt took
> care of this).
> 
> The patch is thought as a basis for further discussions about
> 
>  o how to solve all the issues for all archs technically (ideally
>    without the need to patch noarch parts in an arch-specific way...)
> 
>  o if anyone thinks there could be more spots like these (I've checked
>    the code only for x86 so far)
> diff --git a/fs/aio.c b/fs/aio.c
> index 76da125..0286f0f 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -618,13 +618,16 @@ static void use_mm(struct mm_struct *mm)
>  {
>  	struct mm_struct *active_mm;
>  	struct task_struct *tsk = current;
> +	unsigned long flags;
>  
>  	task_lock(tsk);
>  	active_mm = tsk->active_mm;
>  	atomic_inc(&mm->mm_count);
>  	tsk->mm = mm;
> +	local_irq_save_hw_cond(flags);
>  	tsk->active_mm = mm;
> -	switch_mm(active_mm, mm, tsk);
> +	__switch_mm(active_mm, mm, tsk);
> +	local_irq_restore_hw_cond(flags);
>  	task_unlock(tsk);
>  
>  	mmdrop(active_mm);
> diff --git a/fs/exec.c b/fs/exec.c
> index 3b36c69..06591ac 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -718,6 +718,7 @@ static int exec_mmap(struct mm_struct *mm)
>  {
>  	struct task_struct *tsk;
>  	struct mm_struct * old_mm, *active_mm;
> +	unsigned long flags;
>  
>  	/* Notify parent that we're no longer interested in the old VM */
>  	tsk = current;
> @@ -740,8 +741,10 @@ static int exec_mmap(struct mm_struct *mm)
>  	task_lock(tsk);
>  	active_mm = tsk->active_mm;
>  	tsk->mm = mm;
> +	local_irq_save_hw_cond(flags);
>  	tsk->active_mm = mm;
>  	activate_mm(active_mm, mm);
> +	local_irq_restore_hw_cond(flags);
>  	task_unlock(tsk);
>  	arch_pick_mmap_layout(mm);
>  	if (old_mm) {
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 01a836b..cf3b68a 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1665,11 +1665,14 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
>  		}
>  
>  		if (new_mm) {
> +			unsigned long flags;
>  			mm = current->mm;
>  			active_mm = current->active_mm;
>  			current->mm = new_mm;
> +			local_irq_save_hw_cond(flags);
>  			current->active_mm = new_mm;
>  			activate_mm(active_mm, new_mm);
> +			local_irq_restore_hw_cond(flags);
>  			new_mm = mm;
>  		}

Ok. These are patches of the noarch code which are dependent on x86
implementation.

I think we need something like

mm_change_enter(flags);
mm_change_leave(flags);

which would resolve to ipipe_active_mm = NULL on architectures with
unlocked context switches, and to local_irq_save_hw/local_irq_restore on
x86?

>   
> 


-- 
					    Gilles.