linux-kernel - Re: [patch/rfc] eventfd semaphore-like behavior

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Date:	Fri, 20 Feb 2009 08:00:22 +0100
From:	Martin Sustrik <sustrik@...tmq.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

Hi,

Speaking of eventfd use cases, we're using it as a bitmap. Everything 
works great, the only problem being that we have to take extra care not 
to set the same bit twice:

fd_counter is 00000000
write (fd, 00001000)
fd_counter is 00001000
write (fd, 00000001)
fd_counter is 00001001
write (fd, 00001000) /* oops! */
fd_counter is 00010001

Would anyone else benefit from a new eventfd flag, say EFD_BITMAP, that 
would cause write(2) to do binary OR rather than + on the counter?

Martin

Martin Sustrik wrote:
>     
> Enter your search termsSubmit search formWeblkml.org
> Date    Wed, 4 Feb 2009 14:58:39 -0800 (PST)
> From    Davide Libenzi <>
> Subject    [patch/rfc] eventfd semaphore-like behavior
> 
> People started using eventfd in scnarios where before where using pipes.
> Many of them use eventfds in a semaphore-like way, like they were before
> with pipes. The problem with eventfd is that a read() on the fd returns
> and wipes the whole counter, making the use of it as semaphore a little
> bit more cumbersome. You can do a read() followed by a write() of
> COUNTER-1, but IMO it's pretty easy and cheap to make this work w/out
> extra steps. This patch introduces a new eventfd flag that tells eventfd
> to only dequeue 1 from the counter, allowing simple read/write to make it
> behave like a semaphore.
> Simple test here:
> 
> http://www.xmailserver.org/eventfd-sem.c
> 
> 
> Signed-off-by: Davide Libenzi <davidel@...ilserver.org>
> 
> 
> - Davide
> 

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