Message-ID: <4B589582.9030300@redhat.com>
Date: Thu, 21 Jan 2010 19:57:22 +0200
From: Avi Kivity <avi@redhat.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
CC: Davide Libenzi <davidel@xmailserver.org>, mtosatti@redhat.com,
       kvm@vger.kernel.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove
References: <20100121162648.GA16458@redhat.com> <alpine.DEB.2.00.1001210857310.8548@makko.or.mcafeemobile.com> <4B588B29.2050100@redhat.com> <20100121172336.GA16707@redhat.com> <4B588FCE.7030803@redhat.com> <20100121173259.GC16707@redhat.com> <4B58933C.4090209@redhat.com> <20100121174538.GG16707@redhat.com> <4B589532.9070408@redhat.com>
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On 01/21/2010 07:56 PM, Avi Kivity wrote:
> On 01/21/2010 07:45 PM, Michael S. Tsirkin wrote:
>>
>>> But you're in process context.  An eventfd never blocks.
>> Yes it blocks if counter is 0. And we don't know
>> it's not 0 unless we read :) catch-22.
>
> Ah yes, I forgot.
>

Well, you can poll it and then read it... this introduces a new race (if 
userspace does a read in parallel) but it's limited to kvm and buggy 
userspace.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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