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

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Message-ID: <20190531095535.GA17637@hirez.programming.kicks-ass.net>
Date:   Fri, 31 May 2019 11:55:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     azat@...event.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 00/13] epoll: support pollable epoll from userspace

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();
>  }