To: Brandon Corey <bcorey@engr.sgi.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Pollable Semaphores
References: <20050121212212.GA453910@firefly.engr.sgi.com>
From: Roland Dreier <roland@topspin.com>
Date: Fri, 21 Jan 2005 13:37:47 -0800
In-Reply-To: <20050121212212.GA453910@firefly.engr.sgi.com> (Brandon Corey's
 message of "Fri, 21 Jan 2005 13:22:12 -0800")
Message-ID: <521xceqx90.fsf@topspin.com>
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    Brandon> I'm trying to find out if there is a pollable semaphore
    Brandon> equivalent on Linux.  The main idea of a "pollable
    Brandon> semaphore", is a semaphore with a related file
    Brandon> descriptor.  The file descriptor can be used to select()
    Brandon> when the semaphore is acquirable.  This provides a
    Brandon> convenient way for users to implement code
    Brandon> synchronization between threads, where multiple file
    Brandon> descriptors are already being selected against.

Yes, I believe futexes and specifically FUTEX_FD can be used to
implement this.  See http://people.redhat.com/~drepper/futex.pdf for
full details.

 - Roland
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