linux-kernel - kref refcounting breakage in mainline

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Message-Id: <20070302005833.949be737.akpm@linux-foundation.org>
Date:	Fri, 2 Mar 2007 00:58:33 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg KH <greg@...ah.com>, Tejun Heo <htejun@...il.com>,
	Kay Sievers <kay.sievers@...y.org>
Cc:	linux-kernel@...r.kernel.org, Adrian Bunk <bunk@...sta.de>
Subject: kref refcounting breakage in mainline

-mm has a debugging patch which warns when atomic_dec_and_test() takes an
atomic_t negative
(ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.20/2.6.20-mm2/broken-out/detect-atomic-counter-underflows.patch).

When it is applied to current mainline, a simple `rmmod ipw2200' gives:

[   75.825072] BUG: atomic counter underflow at:
[   75.825180]  [<c01c6eb4>] kref_put+0x66/0x82
[   75.825278]  [<c022e4d4>] bus_remove_driver+0x66/0x75
[   75.825383]  [<c022ee2c>] driver_unregister+0x8/0x13
[   75.825484]  [<c01d7add>] pci_unregister_driver+0xc/0x45
[   75.825593]  [<c0132147>] sys_delete_module+0x157/0x17c
[   75.825703]  [<c013c663>] audit_syscall_entry+0x10d/0x137
[   75.825818]  [<c0103b14>] syscall_call+0x7/0xb
[   75.825913]  [<c02d0000>] xfrm4_dst_destroy+0xe/0xd5

This didn't happen in 2.6.20-mm2, so this bug was introduced by a patch
which was not in the -mm lineup twelve days ago.

Presumably the effect of this is a memory leak or a use-after-free.
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