From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove
Date: Thu, 21 Jan 2010 19:45:38 +0200
Message-ID: <20100121174538.GG16707@redhat.com>
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>
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Cc: Davide Libenzi <davidel@xmailserver.org>, mtosatti@redhat.com,
	kvm@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
To: Avi Kivity <avi@redhat.com>
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On Thu, Jan 21, 2010 at 07:47:40PM +0200, Avi Kivity wrote:
> On 01/21/2010 07:32 PM, Michael S. Tsirkin wrote:
>>
>>> Can't you read from the file?
>>>      
>> IMO no, the read could block.
>>    
>
> 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.

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