Re: [PATCH v3 00/13] epoll: support pollable epoll from userspace

[Peter Zijlstra]

On Thu, May 16, 2019 at 10:57:57AM +0200, Roman Penyaev wrote:
> When new event comes for some epoll item kernel does the following:
> 
>  struct epoll_uitem *uitem;
> 
>  /* Each item has a bit (index in user items array), discussed later */
>  uitem = user_header->items[epi->bit];
> 
>  if (!atomic_fetch_or(uitem->ready_events, pollflags)) {
>      i = atomic_add(&ep->user_header->tail, 1);
> 
>      item_idx = &user_index[i & index_mask];
> 
>      /* Signal with a bit, user spins on index expecting value > 0 */
>      *item_idx = idx + 1;
> 
>     /*
>      * Want index update be flushed from CPU write buffer and
>      * immediately visible on userspace side to avoid long busy
>      * loops.
>      */

That is garbage; smp_wmb() does no such thing.

>      smp_wmb();
>  }