linux-kernel - Re: [RFC] eventfd: add EFD

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Message-ID: <66566792-58a4-bf65-6723-7d2887c84160@redhat.com>
Date:   Wed, 12 Feb 2020 09:31:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel@...r.kernel.org
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Avi Kivity <avi@...lladb.com>,
        Davide Libenzi <davidel@...ilserver.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC] eventfd: add EFD_AUTORESET flag

On 29/01/20 18:20, Stefan Hajnoczi wrote:
> +	/* Semaphore semantics don't make sense when autoreset is enabled */
> +	if ((flags & EFD_SEMAPHORE) && (flags & EFD_AUTORESET))
> +		return -EINVAL;
> +

I think they do, you just want to subtract 1 instead of setting the
count to 0.  This way, writing 1 would be the post operation on the
semaphore, while poll() would be the wait operation.

Paolo