On Wed, 2017-09-06 at 14:47 -0400, Steven Rostedt wrote:
> On Tue, 5 Sep 2017 16:57:14 -0500
> Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx> wrote:
>
>
> >
> > diff --git a/kernel/trace/tracing_map.c
> > b/kernel/trace/tracing_map.c
> > index 305039b..437b490 100644
> > --- a/kernel/trace/tracing_map.c
> > +++ b/kernel/trace/tracing_map.c
> > @@ -414,6 +414,7 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > __tracing_map_insert(struct tracing_map *map, void *key, bool
> > lookup_only)
> > {
> > u32 idx, key_hash, test_key;
> > + int dup_try = 0;
> > struct tracing_map_entry *entry;
> >
> > key_hash = jhash(key, map->key_size, 0);
> > @@ -426,10 +427,31 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > entry = TRACING_MAP_ENTRY(map->map, idx);
> > test_key = entry->key;
> >
> > - if (test_key && test_key == key_hash && entry->val
> > &&
> > - keys_match(key, entry->val->key, map-
> > >key_size)) {
> > - atomic64_inc(&map->hits);
> > - return entry->val;
> > + if (test_key && test_key == key_hash) {
> > + if (entry->val &&
> > + keys_match(key, entry->val->key, map-
> > >key_size)) {
> > + atomic64_inc(&map->hits);
> > + return entry->val;
> > + } else if (unlikely(!entry->val)) {
> I'm thinking we need a READ_ONCE() here.
>
> val = READ_ONCE(entry->val);
>
> then use "val" instead of entry->val. Otherwise, wont it be possible
> if two tasks are inserting at the same time, to have this:
>
> (Using reg as when the value is read into a register from memory)
>
> CPU0 CPU1
> ---- ----
> reg = entry->val
> (reg == zero)
>
> entry->val = elt;
>
> keys_match(key, reg)
> (false)
>
> reg = entry->val
> (reg = elt)
>
> if (unlikely(!reg))
>
> Causes the if to fail.
>
> A READ_ONCE(), would make sure the entry->val used to test against
> key
> would also be the same value used to test if it is zero.
>
Hi Steve,
Thanks for the input.
I agree with your change. Adding READ_ONCE will avoid a race condition
which might result in adding duplicates. Will add it in the next
version.
-Vedang
> -- Steve
>
>
>
> >
> > + /*
> > + * The key is present. But, val
> > (pointer to elt
> > + * struct) is still NULL. which
> > means some other
> > + * thread is in the process of
> > inserting an
> > + * element.
> > + *
> > + * On top of that, it's key_hash
> > is same as the
> > + * one being inserted right now.
> > So, it's
> > + * possible that the element has
> > the same
> > + * key as well.
> > + */
> > +
> > + dup_try++;
> > + if (dup_try > map->map_size) {
> > + atomic64_inc(&map->drops);
> > + break;
> > + }
> > + continue;
> > + }
> > }
> >
> > if (!test_key) {
> > @@ -451,6 +473,13 @@ static inline bool keys_match(void *key, void
> > *test_key, unsigned key_size)
> > atomic64_inc(&map->hits);
> >
> > return entry->val;
> > + } else {
> > + /*
> > + * cmpxchg() failed. Loop around
> > once
> > + * more to check what key was
> > inserted.
> > + */
> > + dup_try++;
> > + continue;
> > }
> > }
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