On Tue, Oct 08, 2019 at 04:20:35PM +0200, Andrea Parri wrote:
On Mon, Oct 07, 2019 at 04:18:26PM +0200, Dmitry Vyukov wrote:
On Mon, Oct 7, 2019 at 4:14 PM Andrea Parri parri.andrea@gmail.com wrote:
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 smp_store_release() below. */stats = READ_ONCE(sig->stats);This pairing suggests that the READ_ONCE() is heading an address dependency, but I fail to identify it: what is the target memory access of such a (putative) dependency?
I would assume callers of this function access *stats. So the dependency is between loading stats and accessing *stats.
AFAICT, the only caller of the function in 5.4-rc2 is taskstats_exit(), which 'casts' the return value to a boolean (so I really don't see how any address dependency could be carried over/relied upon here).
This does not make sense.
But later taskstats_exit does:
memcpy(stats, tsk->signal->stats, sizeof(*stats));
Perhaps it's supposed to use stats returned by taskstats_tgid_alloc?
Seems reasonable to me. If so, replacing the READ_ONCE() in question with an smp_load_acquire() might be the solution. Thoughts?
I've done that already in my tree yesterday. I can resend for another review if you'd prefer.
Christian