Date: Tue, 13 Dec 2005 20:13:17 -0600
From: Robert Hancock <hancockr@shaw.ca>
Subject: Re: Strange delay on PCI-DMA-transfer completion by
 wait_event_interruptible()
In-reply-to: <5jnfV-2xy-577@gated-at.bofh.it>
To: linux-kernel <linux-kernel@vger.kernel.org>
Message-id: <439F7FBD.60300@shaw.ca>
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References: <5jeNG-740-1@gated-at.bofh.it> <5jnfV-2xy-577@gated-at.bofh.it>
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linux-os (Dick Johnson) wrote:
> Every time somebody wants to rewrite a macro, they declare that the
> previous one had some race condition. If, in fact, you have only
> one DMA occurring from your device then no race is possible with
> interruptible_sleep_on(). wait_event_interruptible() is the same thing
> but with an additional access to some memory variable, possibly
> causing a cache refill which means it might take more time.

This is not correct. Using interruptible_sleep_on, there is no way to 
prevent the race where the condition being waited on happens between the 
test to see if it has become true and calling interruptible_sleep_on. 
wait_event_interruptible puts the caller into the wait queue before 
testing the condition, which prevents the race.

interruptible_sleep_on is, with good reason, no longer recommended for use.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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