Date: Fri, 21 Jan 2005 13:22:12 -0800
From: Brandon Corey <bcorey@engr.sgi.com>
To: linux-kernel@vger.kernel.org
Subject: Pollable Semaphores
Message-ID: <20050121212212.GA453910@firefly.engr.sgi.com>
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I'm trying to find out if there is a pollable semaphore equivalent on Linux.

The main idea of a "pollable semaphore", is a semaphore with a related
file descriptor.  The file descriptor can be used to select() when the
semaphore is acquirable.  This provides a convenient way for users to
implement code synchronization between threads, where multiple file
descriptors are already being selected against.

We have a pollable semaphore implementation on IRIX that provides this
functionality.  The API consists of a handful of calls for creation and
destruction of pollable semaphores, as well as a means to attach them
to a file descriptor.  Beyond that, from the users point of view, they're
just treated as any other file descriptor.

These calls are routed through a library and then passed off to a kernel
driver that handles the events.  If someone selects against a semaphore
when it's unaquirable, the driver sleeps on a synchronization variable.
When the semaphore is subsequently made aquirable, the driver will wake up
any waiters.  Multiple pollable semaphores mixed with other file
descriptors can be selected against, and a wakeup will occur when any of
the semaphores become acquirable.

Is anyone aware of any equivalent functionality?

Brandon
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