linux-kernel - Re: [PATCH 0/2] eventfd: new EFD

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Message-ID: <4A938BD5.1050302@gnu.org>
Date:	Tue, 25 Aug 2009 08:59:33 +0200
From:	Paolo Bonzini <bonzini@....org>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Avi Kivity <avi@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
	gleb@...hat.com, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] eventfd: new EFD_STATE flag


>>> There are userspace libraries that do almost everything, but you hardly
>>> see things like pthread_(EFD_STATE-like)_create() or similar system
>>> interfaces based on such abstraction.
>>
>> It actually seems as close to a condition variable as an eventfd can be.
>
> A pthread condition typical code usage maps to eventfd like:
>
> 	while (read(efd, ...)>  0)
> 		if (CONDITION)
> 			break;
>
> So a pthread condition is really a wakeup gate like eventfd is.
> EFD_STATE has nothing to do with a pthread condition.

No, your code does not work for pthread_cond_broadcast (which arguably 
is the common case) because all the eventfd readers after the first 
would block.

Paolo
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