linux-kernel - Why sparse does not support context tracking?

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Date:	Mon, 04 May 2015 13:05:22 +0200
From:	Alexander Riesen <alexander.riesen@...itec.com>
To:	linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org
CC:	Josh Triplett <josh@...edesktop.org>,
	Christopher Li <sparse@...isli.org>,
	Johannes Berg <johannes@...solutions.net>
Subject: Why sparse does not support context tracking?

Hi,

I tried to instrument my code (a kernel driver) with __must_hold hints
and noticed that they don't really work: a definitely broken code caused
no errors from sparse.

A test in the "sparse" tree confirmed. Here is the test I used:

static int ctx;

static void must_hold(void) __attribute__((context(ctx,1,1)))
{
     if (ctx)
         ;
}
static void bad_must_hold(void)
{
     must_hold();
}

After a bit of looking I found a commit which removed context tracking
from the "sparse":

Author: Johannes Berg <johannes@...solutions.net> 2008-12-23 10:59:22

     Revert the context tracking code

     > Do you want to resend your change which revert the context changes?
     > Make it base on Josh's git's tree and I will merge your changes in my
     > branch.

     Below. Or I can give it to you in git if you prefer. I still think we
     should redo this in some form so that annotations with different
     contexts can work properly, but I don't have time to take care of it
     right now.

     johannes

     >From ca95b62edf1600a2b55ed9ca0515d049807a84fc Mon Sep 17 00:00:00 2001
     From: Johannes Berg <johannes@...solutions.net>
     Date: Tue, 23 Dec 2008 10:53:19 +0100
     Subject: [PATCH] Revert context tracking code

So it seems that the code has problems with different contexts. Still, it seems
a bit heavy-handed to remove it completely (assuming it worked, of course).
The kernel even has a bit of use for it (not much, admittedly: DRBD and aoe).

Is it be possible to reintroduce the code at least for the same context?
So that it catches at least some locks not taken?

Regards,
Alex Riesen

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