linux-kernel - Re: A better interface, perhaps: a timed signal flag

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Date:	Fri, 28 Jul 2006 10:52:10 -0400
From:	Theodore Tso <tytso@....edu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Neil Horman <nhorman@...driver.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Dave Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org,
	a.zummo@...ertech.it, jg@...edesktop.org
Subject: Re: A better interface, perhaps: a timed signal flag

On Fri, Jul 28, 2006 at 09:33:26AM -0400, Steven Rostedt wrote:
> What you could have is this:
> 
>   volatile int *flag;
> 
>   register_timeout(&time_val, &flag);
>   while (work_to_do()) {
> 	do_a_bit_of_work();
> 	if (*flag)
> 		break;
>   }
> 
> Where the kernel would register a location to set a timeout with, and
> the kernel would setup a flag for you and then map it into userspace.
> Perhaps only allow one flag per task and place it as a field of the task
> structure.  There's no reason that the tasks own task sturct cant be
> mapped read only to user space, is there?

Good point, and limiting this facility to one such timeout per
task_struct seems like a reasonable restriction.  The downsides I can
see about about mapping the tasks' own task struct would be (a) a
potential security leak either now or in the future if some field in
the task_struct shouldn't be visible to a non-privileged userspace
program, and (b) exposing the task_struct might cause some (stupid)
programs to depend on the task_struct layout.  Allocating an otherwise
empty 4k page just for this purpose wouldn't be all that horrible,
though, and would avoid these potential problems.

							- Ted
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