linux-kernel - Re: Userfaultfd doesn't seem to break out of poll on fd close

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Message-ID: <20200416144942.GQ5100@ziepe.ca>
Date:   Thu, 16 Apr 2020 11:49:42 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Hillf Danton <hdanton@...a.com>, Peter Xu <peterx@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sonny Rao <sonnyrao@...gle.com>
Subject: Re: Userfaultfd doesn't seem to break out of poll on fd close

On Wed, Apr 15, 2020 at 06:15:26PM -0700, Brian Geffon wrote:

> And all of that I guess makes sense in the case of a non-blocking fd,
> but what about the case of a blocking file descriptor? Both
> userfaultfd and eventfd can seemingly be stuck in a read syscall with
> no way to break them out when the userfaultfd/eventfd has no further
> utility. Here is an example:
> https://gist.github.com/bgaff/607302d86d99ac539efca307ce2dd679

If an application wants to terminate blocking calls it has to rely on
signal delivery or pthread_cancel to end it.

This is very complicated so it is generally better to use poll and non
blocking.

Jason