On Mon, Oct 21, 2019 at 03:04:18PM +0200, Christian Brauner wrote:
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);
This probably wants to be an acquire, since both the memcpy() later on in taskstats_exit() and the accesses in {b,x}acct_add_tsk() appear to read from the taskstats structure without the sighand->siglock held and therefore may miss zeroed allocation from the zalloc() below, I think.
- 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);
You hold the spinlock here, so I don't think you need the READ_ONCE().
- if (!stats) {
stats = stats_new;WRITE_ONCE(sig->stats, stats_new);
You probably want a release here to publish the zeroes from the zalloc() (back to my first comment). With those changes:
Reviewed-by: Will Deacon will@kernel.org
However, this caused me to look at do_group_exit() and we appear to have racy accesses on sig->flags there thanks to signal_group_exit(). I worry that might run quite deep, and can probably be looked at separately.
Will