Re: [PATCH 1/1] eventfd: implementation of EFD_MASK flag

["Michał Mirosław" <mirqus@gmail.com>]

2013/2/8 Martin Sustrik <sustrik@250bpm.com>:
> When implementing network protocols in user space, one has to implement
> fake user-space file descriptors to represent the sockets for the protocol.
[...]
> This patch implements new EFD_MASK flag which attempts to solve this problem.
[...]
> @@ -55,6 +64,9 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
>  {
>         unsigned long flags;
>
> +       /* This function should never be used with eventfd in the mask mode. */
> +       BUG_ON(ctx->flags & EFD_MASK);
> +
>         spin_lock_irqsave(&ctx->wqh.lock, flags);
>         if (ULLONG_MAX - ctx->count < n)
>                 n = ULLONG_MAX - ctx->count;
> @@ -123,12 +135,16 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait)
>         poll_wait(file, &ctx->wqh, wait);
>
>         spin_lock_irqsave(&ctx->wqh.lock, flags);
> -       if (ctx->count > 0)
> -               events |= POLLIN;
> -       if (ctx->count == ULLONG_MAX)
> -               events |= POLLERR;
> -       if (ULLONG_MAX - 1 > ctx->count)
> -               events |= POLLOUT;
> +       if (ctx->flags & EFD_MASK) {
> +               events = ctx->mask.events;
> +       } else {
> +               if (ctx->count > 0)
> +                       events |= POLLIN;
> +               if (ctx->count == ULLONG_MAX)
> +                       events |= POLLERR;
> +               if (ULLONG_MAX - 1 > ctx->count)
> +                       events |= POLLOUT;
> +       }
>         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
>
>         return events;
[...]
> @@ -412,7 +464,12 @@ struct file *eventfd_file_create(unsigned int count, int flags)
>
>         kref_init(&ctx->kref);
>         init_waitqueue_head(&ctx->wqh);
> -       ctx->count = count;
> +       if (flags & EFD_MASK) {
> +               ctx->mask.events = 0;
> +               ctx->mask.ptr = NULL;
> +       } else {
> +               ctx->count = count;
> +       }
>         ctx->flags = flags;
>
>         file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,

Since EFD_MASK is a persistent flag for a fd's lifetime, maybe you
could instead of all those if/elses and BUG_ON()s use another
file_operations struct for this feature?

Best Regards,
Michał Mirosław
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