linux-kernel - Re: [PATCH v3] epoll: Support for disabling items, and a self-test app.

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Date:	Mon, 29 Oct 2012 13:42:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.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-kernel@...r.kernel.org>,
	Paul Holland <pholland@...be.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	<libc-alpha@...rceware.org>, <linux-api@...r.kernel.org>,
	<paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] epoll: Support for disabling items, and a self-test
 app.

On Thu, 25 Oct 2012 17:08:32 -0700
"Paton J. Lewis" <palewis@...be.com> wrote:

> It is not currently possible to reliably delete epoll items when using the
> same epoll set from multiple threads. After calling epoll_ctl with
> EPOLL_CTL_DEL, another thread might still be executing code related to an
> event for that epoll item (in response to epoll_wait). Therefore the deleting
> thread does not know when it is safe to delete resources pertaining to the
> associated epoll item because another thread might be using those resources.
> 
> The deleting thread could wait an arbitrary amount of time after calling
> epoll_ctl with EPOLL_CTL_DEL and before deleting the item, but this is
> inefficient and could result in the destruction of resources before another
> thread is done handling an event returned by epoll_wait.
> 
> This patch enhances epoll_ctl to support EPOLL_CTL_DISABLE, which disables an
> epoll item. If epoll_ctl returns -EBUSY in this case, then another thread may
> handling a return from epoll_wait for this item. Otherwise if epoll_ctl
> returns 0, then it is safe to delete the epoll item. This allows multiple
> threads to use a mutex to determine when it is safe to delete an epoll item
> and its associated resources, which allows epoll items to be deleted both
> efficiently and without error in a multi-threaded environment. Note that
> EPOLL_CTL_DISABLE is only useful in conjunction with EPOLLONESHOT, and using
> EPOLL_CTL_DISABLE on an epoll item without EPOLLONESHOT returns -EINVAL.
> 
> This patch also adds a new test_epoll self-test program to both demonstrate
> the need for this feature and test it.

Thanks.  I queued this for testing and for consideration for 3.8.  I
shall send the revert patches in to Linus to remove this interface
change from 3.7.
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