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SubjectRe: [PATCH v6] taskstats: fix data-race
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.brauner@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;
}

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