Re: [PATCH v6] taskstats: fix data-race

[Christian Brauner]

On Mon, Oct 21, 2019 at 02:19:01PM +0200, Rasmus Villemoes wrote:
> On 21/10/2019 13.33, Christian Brauner wrote:
> > The first approach used smp_load_acquire() and smp_store_release().
> > However, after having discussed this it seems that the data dependency
> > for kmem_cache_alloc() would be fixed by WRITE_ONCE().
> > Furthermore, the smp_load_acquire() would only manage to order the stats
> > check before the thread_group_empty() check. So it seems just using
> > READ_ONCE() and WRITE_ONCE() will do the job and I wanted to bring this
> > up for discussion at least.
> > 
> > /* v6 */
> > - Christian Brauner <christian.brau...@ubuntu.com>:
> >   - bring up READ_ONCE()/WRITE_ONCE() approach for discussion
> > ---
> >  kernel/taskstats.c | 26 +++++++++++++++-----------
> >  1 file changed, 15 insertions(+), 11 deletions(-)
> > 
> > diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> > index 13a0f2e6ebc2..111bb4139aa2 100644
> > --- a/kernel/taskstats.c
> > +++ b/kernel/taskstats.c
> > @@ -554,25 +554,29 @@ static int taskstats_user_cmd(struct sk_buff *skb, 
> > struct genl_info *info)
> >  static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
> >  {
> >     struct signal_struct *sig = tsk->signal;
> > -   struct taskstats *stats;
> > +   struct taskstats *stats_new, *stats;
> >  
> > -   if (sig->stats || thread_group_empty(tsk))
> > -           goto ret;
> > +   /* Pairs with WRITE_ONCE() below. */
> > +   stats = READ_ONCE(sig->stats);
> > +   if (stats || thread_group_empty(tsk))
> > +           return stats;
> >  
> >     /* No problem if kmem_cache_zalloc() fails */
> > -   stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> > +   stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> >  
> >     spin_lock_irq(&tsk->sighand->siglock);
> > -   if (!sig->stats) {
> > -           sig->stats = stats;
> > -           stats = NULL;
> > +   if (!stats) {
> > +           stats = stats_new;
> > +           /* Pairs with READ_ONCE() above. */
> > +           WRITE_ONCE(sig->stats, stats_new);
> > +           stats_new = NULL;
> 
> No idea about the memory ordering issues, but don't you need to
> load/check sig->stats again? Otherwise it seems that two threads might
> both see !sig->stats, both allocate a stats_new, and both
> unconditionally in turn assign their stats_new to sig->stats. Then the
> first assignment ends up becoming a memory leak (and any writes through
> that pointer done by the caller end up in /dev/null...)

Trigger hand too fast. I guess you're thinking sm like:

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 13a0f2e6ebc2..c4e1ed11e785 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -554,25 +554,27 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct 
genl_info *info)
 static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
 {
        struct signal_struct *sig = tsk->signal;
-       struct taskstats *stats;
+       struct taskstats *stats_new, *stats;
 
-       if (sig->stats || thread_group_empty(tsk))
-               goto ret;
+       stats = READ_ONCE(sig->stats);
+       if (stats || thread_group_empty(tsk))
+               return stats;
 
-       /* No problem if kmem_cache_zalloc() fails */
-       stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
+       stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
 
        spin_lock_irq(&tsk->sighand->siglock);
-       if (!sig->stats) {
-               sig->stats = stats;
-               stats = NULL;
+       stats = READ_ONCE(sig->stats);
+       if (!stats) {
+               stats = stats_new;
+               WRITE_ONCE(sig->stats, stats_new);
+               stats_new = NULL;
        }
        spin_unlock_irq(&tsk->sighand->siglock);
 
-       if (stats)
-               kmem_cache_free(taskstats_cache, stats);
-ret:
-       return sig->stats;
+       if (stats_new)
+               kmem_cache_free(taskstats_cache, stats_new);
+
+       return stats;
 }