Re: [RFC v8 PATCH 2/5] uprobes: introduce has_uprobes helper

[Yang Shi]

On 8/23/18 8:15 AM, Oleg Nesterov wrote:
> On 08/22, Srikar Dronamraju wrote:
> > * Vlastimil Babka <vbabka@xxxxxxx> [2018-08-22 12:55:59]:
> >
> > > On 08/15/2018 08:49 PM, Yang Shi wrote:
> > > > We need check if mm or vma has uprobes in the following patch to check
> > > > if a vma could be unmapped with holding read mmap_sem.
> Confused... why can't we call uprobe_munmap() under read_lock(mmap_sem) ?

I'm not sure if it is safe or not because it is not recommended and not 
safe to update vma's vm flags with read mmap_sem. uprobe_munmap() may 
update mm flags (MMF_RECALC_UPROBES). So, it sounds safer to not call it 
under read mmap_sem.

> OK, it can race with find_active_uprobe() but I do not see anything really
> wrong, and a false-positive MMF_RECALC_UPROBES is fine.

Thanks for confirming this. If it is ok to have such race, we don't have 
to have has_uprobes() helper anymore since it can be just called under 
read mmap_sem without any special handling.

Yang

> Again, I think we should simply kill uprobe_munmap(), but this needs another
> discussion.
>
> Oleg.