Date:	Sat, 30 Aug 2008 19:04:08 +0200 (MEST)
From:	Mikael Pettersson <mikpe@...uu.se>
To:	fujita.tomonori@....ntt.co.jp
Cc:	dwmw2@...radead.org, gregkh@...e.de, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org
Subject: [REGRESSION 2.6.27-rc4-git7] mtd broken by cmdfilter move

Starting with kernel 2.6.27-rc4-git7 my n2100 arm box throws a bunch
of kobject errors at boot:

Creating 6 MTD partitions on "physmap-flash.0":
0x00000000-0x00040000 : "RedBoot"
0x00040000-0x00d40000 : "ramdisk"
kobject (df8fd510): tried to init an initialized object, something is seriously wrong.
[<c0024c68>] (dump_stack+0x0/0x14) from [<c01136f4>] (kobject_init+0x40/0x78)
[<c01136b4>] (kobject_init+0x0/0x78) from [<c0113c28>] (kobject_init_and_add+0x20/0x44)
 r5:df8fd510 r4:df9ee8b0
[<c0113c0c>] (kobject_init_and_add+0x4/0x44) from [<c010a92c>] (blk_register_filter+0x4c/0x60)
 r5:df8f6d20 r4:df8fd108
[<c010a8e0>] (blk_register_filter+0x0/0x60) from [<c010983c>] (add_disk+0x60/0xb8)
 r4:df9ee800
[<c01097dc>] (add_disk+0x0/0xb8) from [<c0170bb4>] (add_mtd_blktrans_dev+0x248/0x284)
 r5:df8f6d20 r4:00000001
[<c017096c>] (add_mtd_blktrans_dev+0x0/0x284) from [<c01711a8>] (mtdblock_add_mtd+0x5c/0x68)
 r8:00040000 r7:df8604cc r6:c02c5c98 r5:c02c5cc8 r4:df8602a0
[<c017114c>] (mtdblock_add_mtd+0x0/0x68) from [<c017075c>] (blktrans_notify_add+0x38/0x64)
 r5:df8602a0 r4:c02c5cc8
[<c0170724>] (blktrans_notify_add+0x0/0x64) from [<c016daec>] (add_mtd_device+0xb4/0x110)
 r6:c02c5c34 r5:df8602a0 r4:c02c5c88
[<c016da38>] (add_mtd_device+0x0/0x110) from [<c016ee70>] (add_mtd_partitions+0x490/0x54c)
 r6:df844080 r5:00000c00 r4:df8602a0
[<c016e9e0>] (add_mtd_partitions+0x0/0x54c) from [<c01789b8>] (physmap_flash_probe+0x27c/0x2e8)
[<c017873c>] (physmap_flash_probe+0x0/0x2e8) from [<c01468b8>] (platform_drv_probe+0x20/0x24)
[<c0146898>] (platform_drv_probe+0x0/0x24) from [<c0145b20>] (driver_probe_device+0xd8/0x18c)
[<c0145a48>] (driver_probe_device+0x0/0x18c) from [<c0145c20>] (__driver_attach+0x4c/0x70)
 r7:c02c5e1c r6:c02c5e1c r5:c02b8338 r4:c02b828c
[<c0145bd4>] (__driver_attach+0x0/0x70) from [<c0145180>] (bus_for_each_dev+0x54/0x88)
 r6:c0145bd4 r5:df821ec0 r4:00000000
[<c014512c>] (bus_for_each_dev+0x0/0x88) from [<c0145960>] (driver_attach+0x20/0x28)
 r7:df88c620 r6:00000000 r5:c02c5e1c r4:00000000
[<c0145940>] (driver_attach+0x0/0x28) from [<c0145618>] (bus_add_driver+0xa8/0x214)
[<c0145570>] (bus_add_driver+0x0/0x214) from [<c0145e14>] (driver_register+0x98/0x120)
 r8:00000000 r7:00000000 r6:00000000 r5:c02c5e1c r4:c02cd924
[<c0145d7c>] (driver_register+0x0/0x120) from [<c0146a78>] (platform_driver_register+0x78/0x94)
[<c0146a00>] (platform_driver_register+0x0/0x94) from [<c0016ee4>] (physmap_init+0x14/0x1c)
[<c0016ed0>] (physmap_init+0x0/0x1c) from [<c00202b8>] (__exception_text_end+0x50/0x184)
[<c0020268>] (__exception_text_end+0x0/0x184) from [<c000874c>] (kernel_init+0x70/0xd8)
[<c00086dc>] (kernel_init+0x0/0xd8) from [<c003750c>] (do_exit+0x0/0x708)
 r5:00000000 r4:00000000
0x00d40000-0x00ea0000 : "kernel"
kobject (df8fd510): tried to init an initialized object, something is seriously wrong.

and so on for every partition except the first one.

I bisected this to commit abf5439370491dd6fbb4fe1a7939680d2a9bc9d4,
"block: move cmdfilter from gendisk to request_queue".

There is a many-to-one mapping from gendisks to queues, so when this commit
moved the cmd_filter object from the gendisk to the gendisk's queue it
allowed (by design or by accident, I don't know) gendisks to share cmd_filters.
However, the commit didn't update blk_register_filter() to handle shared
cmd_filters: it still unconditionally calls kobject_init_and_add() for the
gendisk's cmd_filter kobject. When queue and thus cmd_filter sharing is present,
reinitialisation of already initialised kobjects occur, resulting in loud
complaints from lib/kobject.c.

As a quick workaround I added a check to blk_register_filter() to avoid
reinitialising already initialised cmd_filters, and to log the addresses
of the disks, queues, and cmd_filters passed to it:

--- linux-2.6.27-rc4-git6/block/cmd-filter.c.~1~	2008-08-30 17:08:58.000000000 +0200
+++ linux-2.6.27-rc4-git6/block/cmd-filter.c	2008-08-30 17:36:36.000000000 +0200
@@ -215,6 +215,13 @@ int blk_register_filter(struct gendisk *
 	if (!parent)
 		return -ENODEV;
 
+	printk("%s: disk %p queue %p filter %p &filter->kobj %p\n",
+	       __func__, disk, disk->queue, filter, &filter->kobj);
+	if (filter->kobj.state_initialized) {
+		printk("%s: kobj %p already inited, bailing out\n",
+		       __func__, &filter->kobj);
+		return 0;
+	}
 	ret = kobject_init_and_add(&filter->kobj, &rcf_ktype, parent,
 				   "%s", "cmd_filter");
 
This eliminated the kobject complaints for me. The log messages also showed
that the mtd gendisks indeed share queue and thus cmd_filter and its kobject:

6 RedBoot partitions found on MTD device physmap-flash.0
Creating 6 MTD partitions on "physmap-flash.0":
0x00000000-0x00040000 : "RedBoot"
blk_register_filter: disk df8f4000 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
0x00040000-0x00d40000 : "ramdisk"
blk_register_filter: disk df9f0800 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
blk_register_filter: kobj df8fd510 already inited, bailing out
0x00d40000-0x00ea0000 : "kernel"
blk_register_filter: disk df9f0000 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
blk_register_filter: kobj df8fd510 already inited, bailing out
0x00ea0000-0x00fc0000 : "user"
blk_register_filter: disk df9ed800 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
blk_register_filter: kobj df8fd510 already inited, bailing out
0x00fc0000-0x00fc1000 : "RedBoot config"
blk_register_filter: disk df9ed000 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
blk_register_filter: kobj df8fd510 already inited, bailing out
0x00fe0000-0x01000000 : "FIS directory"
blk_register_filter: disk df9ea800 queue df8fd108 filter df8fd4d0 &filter->kobj df8fd510
blk_register_filter: kobj df8fd510 already inited, bailing out

I'll leave it to you guys to decide what the proper fix for this mess should be.

/Mikael
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