linux-kernel - Re: [PATCH] epoll: Improved support for multi-threaded clients

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Date:	Tue, 19 Jun 2012 20:17:11 +0200
From:	Christof Meerwald <cmeerw@...erw.org>
To:	"Paton J. Lewis" <palewis@...be.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jason Baron <jbaron@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Holland <pholland@...be.com>,
	Davide Libenzi <davidel@...ilserver.org>
Subject: Re: [PATCH] epoll: Improved support for multi-threaded clients

Hi Paton,

On Mon, Jun 18, 2012 at 04:24:35PM -0700, Paton J. Lewis wrote:
> We believe that EPOLLONESHOT is required in order to make any
> sensible use of calling epoll_wait on a single epoll set
> concurrently in multiple threads.

I guess we have to disagree here - though it might be more difficult.

> >On Mon, 11 Jun 2012 15:34:49 -0700, Paton Lewis wrote:
> >> This patch introduces the new epoll_ctl command EPOLL_CTL_DISABLE, which
> >> disables the associated epoll item and returns -EBUSY if the
> >epoll item is not
> >> currently in the epoll ready queue. This allows multiple threads to use a
> >> mutex to determine when it is safe to delete an epoll item and
> >its associated
> >> resources. This allows epoll items to be deleted and closed efficiently and
> >> without error.

Maybe I am missing something here (as I am not really familiar with
the kernel internals), but I don't really understand the logic behind
your patch. Isn't the "expected" case that the item is not on the
ready list and no I/O is being processed for that item?

So I think instead of checking for the item being on the ready list,
checking for the event mask would make more sense for me, e.g.

  if (!(epi->event.events & ~EP_PRIVATE_BITS))

But, taking one step back - wouldn't an alternative approach be to add
some mechanism to allow a thread to post a user-event for an fd? So in
delete_epoll_item you would post a user event (e.g. EPOLLUSER) for the
fd which you can then handle in your epoll_wait processing thread -
with no additional synchronisation necessary.

However, this would still require EPOLLONESHOT to be useful for memory
management.

Christof

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