linux-kernel - Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]

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Date:   Sun, 27 Oct 2019 08:22:23 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     torvalds@...ux-foundation.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]

On Thu, Oct 24, 2019 at 05:57:32PM +0100, David Howells wrote:
> pipe: Add fsync() support
> 
> The keyrings testsuite needs the ability to wait for all the outstanding
> notifications in the queue to have been processed so that it can then go
> through them to find out whether the notifications it expected have been
> emitted.
> 
> Implement fsync() support for pipes to provide this.  The tailmost buffer
> at the point of calling is marked and fsync adds itself to the list of
> waiters, noting the tail position to be waited for and marking the buffer
> as no longer mergeable.  Then when the buffer is consumed, if the flag is
> set, any matching waiters are woken up.

I am _really_ worried about overloading fsync for this behavior.  fsync
hasn't done anything for 50 years, and suddenly adding any action
is not helpful.  If you can't use FIONREAD please add a new ioctls
instead, and document it properly.