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Re: [PATCH v3] epoll: Support for disabling items, and a self-testapp.
- From: Andrew Morton <akpm at linux-foundation dot org>
- To: "Paton J. Lewis" <palewis at adobe dot com>
- Cc: Alexander Viro <viro at zeniv dot linux dot org dot uk>, Jason Baron<jbaron at redhat dot com>, <linux-fsdevel at vger dot kernel dot org>,<linux-kernel at vger dot kernel dot org>, Paul Holland <pholland at adobe dot com>, DavideLibenzi <davidel at xmailserver dot org>, Michael Kerrisk<mtk dot manpages at gmail dot com>, <libc-alpha at sourceware dot org>,<linux-api at vger dot kernel dot org>, <paulmck at linux dot vnet dot ibm dot com>
- Date: Mon, 29 Oct 2012 13:42:23 -0700
- Subject: Re: [PATCH v3] epoll: Support for disabling items, and a self-testapp.
- References: <1351210112-23238-1-git-send-email-palewis@adobe.com>
On Thu, 25 Oct 2012 17:08:32 -0700
"Paton J. Lewis" <palewis@adobe.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.