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Date:      Wed, 30 Jan 2008 20:52:04 +0000
From:      John Birrell <jb@what-creek.com>
To:        John Baldwin <jhb@freebsd.org>
Cc:        Perforce Change Reviews <perforce@freebsd.org>
Subject:   Re: PERFORCE change 134245 for review
Message-ID:  <20080130205204.GA68541@what-creek.com>
In-Reply-To: <200801300913.03782.jhb@freebsd.org>
References:  <200801272148.m0RLmWdF053919@repoman.freebsd.org> <200801300913.03782.jhb@freebsd.org>
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On Wed, Jan 30, 2008 at 09:13:03AM -0500, John Baldwin wrote:
> On Sunday 27 January 2008 04:48:32 pm John Birrell wrote:
> > http://perforce.freebsd.org/chv.cgi?CH=134245
> > 
> > Change 134245 by jb@jb_freebsd1 on 2008/01/27 21:47:41
> > 
> > 	The linker lock here is private to code in this file. It is
> > 	never a problem with it being locked when any other combination
> > 	of locks are already obtained or even if *no* other locks are
> > 	obtained. Use SX_NOWITNESS to avoid bogus LOR reports.
> 
> If witness reports a LOR it is real, it may just not be obvious.

Here is some code that demonstrates the problem:


static struct mtx test_mtx;
MTX_SYSINIT(test, &test_mtx, "Test", MTX_DEF);

static int
test_list(linker_file_t lf, void *arg)
{
	printf("Linker file name: '%s'\n", lf->filename);
	return (0);
}

static void
test_thread_func(void *arg)
{
	printf("\n\nI'm a big bad thread, but I'm going to wait a bit... stay tuned...\n");

	pause("Waiting a bit...", 5000);

	printf("The big bad thread is awake and he wants to list the linker files...\n");

	linker_file_foreach(test_list, NULL);

	printf("Lock my wide scope mutex because I have stuff to protect...\n");

	mtx_lock(&test_mtx);

	printf("I've done some stuff that my wide scope lock protected, but now I need to list some linker files...\n");

	linker_file_foreach(test_list, NULL);

	printf("Unlock my wide scope mutex because I done what I needed to do...\n");

	mtx_unlock(&test_mtx);

	printf("I'm done now.\n");
	printf("====================\n");

	kthread_exit();
}

static int
test_modevent(module_t mod __unused, int type, void *data __unused)
{
	int error = 0;

	switch (type) {
	case MOD_LOAD:
		printf("--------------------\n");
		printf("I am loaded! I can now list the linker files without grabbing any locks...\n");
		linker_file_foreach(test_list, NULL);
		printf("OK, I'm done listing the linker files.\n");
		printf("Now I'll create a thread to do some stuff after I've gone.\n");
		kthread_add(test_thread_func, NULL, NULL, NULL, 0, 0, "testme");
		printf("OK, the thread is created. I'm out of here.\n");
		printf("--------------------\n");
		break;

	case MOD_UNLOAD:
		printf("I am unloading!\n");
		break;

	case MOD_SHUTDOWN:
		break;

	default:
		error = EOPNOTSUPP;
		break;

	}

	return (error);
}

DEV_MODULE(test, test_modevent, NULL);
MODULE_VERSION(test, 1);

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