linux-kernel - Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod

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Message-ID: <20150120230352.GB14475@cloud>
Date:	Tue, 20 Jan 2015 15:03:52 -0800
From:	josh@...htriplett.org
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Fam Zheng <famz@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	David Drysdale <drysdale@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	Theodore Ts'o <tytso@....edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Rashika Kheria <rashika.kheria@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

On Tue, Jan 20, 2015 at 02:40:32PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 20, 2015 at 1:57 AM, Fam Zheng <famz@...hat.com> wrote:
> > This adds a new system call, epoll_mod_wait. It's described as below:
> >
> > NAME
> >        epoll_mod_wait - modify and wait for I/O events on an epoll file
> >                         descriptor
> >
> > SYNOPSIS
> >
> >        int epoll_mod_wait(int epfd, int flags,
> >                           int ncmds, struct epoll_mod_cmd *cmds,
> >                           struct epoll_wait_spec *spec);
> >
> > DESCRIPTION
> >
> >        The epoll_mod_wait() system call can be seen as an enhanced combination
> >        of several epoll_ctl(2) calls, which are followed by an epoll_pwait(2)
> >        call. It is superior in two cases:
> >
> >        1) When epoll_ctl(2) are followed by epoll_wait(2), using epoll_mod_wait
> >        will save context switches between user mode and kernel mode;
> >
> >        2) When you need higher precision than microsecond for wait timeout.
> >
> >        The epoll_ctl(2) operations are embedded into this call by with ncmds
> >        and cmds. The latter is an array of command structs:
> >
> >            struct epoll_mod_cmd {
> >
> >                   /* Reserved flags for future extension, must be 0 for now. */
> >                   int flags;
> >
> >                   /* The same as epoll_ctl() op parameter. */
> >                   int op;
> >
> >                   /* The same as epoll_ctl() fd parameter. */
> >                   int fd;
> >
> >                   /* The same as the "events" field in struct epoll_event. */
> >                   uint32_t events;
> >
> >                   /* The same as the "data" field in struct epoll_event. */
> >                   uint64_t data;
> >
> >                   /* Output field, will be set to the return code once this
> >                    * command is executed by kernel */
> >                   int error;
> >            };
> 
> I would add an extra u32 at the end so that the structure size will be
> a multiple of 8 bytes on all platforms.

*shrug*, but if you do so, enforce that it has a value of 0 or return
-EINVAL, just like a flags field.  Alternatively, move the last field
earlier and make flags a uint64_t.

- Josh Triplett
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