Re: [PATCH 3/3] uprobes: teach find_active_uprobe() to clear MMF_HAS_UPROBES

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-08-19 18:40:42]:

> The wrong MMF_HAS_UPROBES doesn't really hurt, just it triggers
> the "slow" and unnecessary handle_swbp() path if the task hits
> the non-uprobe breakpoint.
> 
> So this patch changes find_active_uprobe() to check every valid
> vma and clear MMF_HAS_UPROBES if no uprobes were found. This is
> adds the slow O(n) path, but it is only called in unlikely case
> when the task hits the normal breakpoint first time after
> uprobe_unregister().
> 
> Note the "not strictly accurate" comment in mmf_recalc_uprobes().
> We can fix this, we only need to teach vma_has_uprobes() to return
> a bit more more info, but I am not sure this worth the trouble.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  kernel/events/uprobes.c |   22 ++++++++++++++++++++++
>  1 files changed, 22 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 176de8c..0b7918c 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1397,6 +1397,25 @@ static bool can_skip_sstep(struct uprobe *uprobe, struct pt_regs *regs)
>  	return false;
>  }
> 
> +static void mmf_recalc_uprobes(struct mm_struct *mm)
> +{
> +	struct vm_area_struct *vma;
> +
> +	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +		if (!valid_vma(vma, false))
> +			continue;
> +		/*
> +		 * This is not strictly accurate, we can race with
> +		 * uprobe_unregister() and see the already removed
> +		 * uprobe if delete_uprobe() was not yet called.
> +		 */
> +		if (vma_has_uprobes(vma, vma->vm_start, vma->vm_end))

Should we set the MMF_RECALC_UPROBES here?

Its harmless but my thought was if we indeed saw a uprobe that was
already deleted, then the next time we hit a non uprobe breakpoint in
the same process context, we will not come here because
MMF_RECALC_UPROBES is cleared.

The rest looks fine. 


Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>


> +			return;
> +	}
> +
> +	clear_bit(MMF_HAS_UPROBES, &mm->flags);
> +}
> +
>  static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
>  {
>  	struct mm_struct *mm = current->mm;
> @@ -1418,6 +1437,9 @@ static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
>  	} else {
>  		*is_swbp = -EFAULT;
>  	}
> +
> +	if (!uprobe && test_and_clear_bit(MMF_RECALC_UPROBES, &mm->flags))
> +		mmf_recalc_uprobes(mm);
>  	up_read(&mm->mmap_sem);
> 
>  	return uprobe;
> -- 
> 1.5.5.1
>