Re: [PATCH] mm/vmalloc: Fix unlock order in s_stop()

[Matthew Wilcox]

On Sun, Dec 13, 2020 at 07:39:36PM +0100, Uladzislau Rezki wrote:
> On Sun, Dec 13, 2020 at 01:08:43PM -0500, Waiman Long wrote:
> > When multiple locks are acquired, they should be released in reverse
> > order. For s_start() and s_stop() in mm/vmalloc.c, that is not the
> > case.
> > 
> >   s_start: mutex_lock(&vmap_purge_lock); spin_lock(&vmap_area_lock);
> >   s_stop : mutex_unlock(&vmap_purge_lock); spin_unlock(&vmap_area_lock);
> > 
> > This unlock sequence, though allowed, is not optimal. If a waiter is
> > present, mutex_unlock() will need to go through the slowpath of waking
> > up the waiter with preemption disabled. Fix that by releasing the
> > spinlock first before the mutex.
> > 
> > Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >  mm/vmalloc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 6ae491a8b210..75913f685c71 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3448,11 +3448,11 @@ static void *s_next(struct seq_file *m, void *p, loff_t *pos)
> >  }
> >  
> >  static void s_stop(struct seq_file *m, void *p)
> > -	__releases(&vmap_purge_lock)
> >  	__releases(&vmap_area_lock)
> > +	__releases(&vmap_purge_lock)
> >  {
> > -	mutex_unlock(&vmap_purge_lock);
> >  	spin_unlock(&vmap_area_lock);
> > +	mutex_unlock(&vmap_purge_lock);
> >  }
> >  
> >  static void show_numa_info(struct seq_file *m, struct vm_struct *v)
> BTW, if navigation over both list is an issue, for example when there
> are multiple heavy readers of /proc/vmallocinfo, i think, it make sense
> to implement RCU safe lists iteration and get rid of both locks.

If we need to iterate the list efficiently, i'd suggest getting rid of
the list and using an xarray instead.  maybe a maple tree, once that code
is better exercised.