linux-kernel - Re: hugetlbfs lockdep spew revisited.

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Date:	Fri, 17 Feb 2012 00:27:26 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Jones <davej@...hat.com>, Josh Boyer <jboyer@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: hugetlbfs lockdep spew revisited.

On Thu, Feb 16, 2012 at 07:08:57PM -0500, Dave Jones wrote:
> Remember this ? https://lkml.org/lkml/2011/4/15/272
> Josh took a stab at fixing it in e096d0c7e2e4e5893792db865dd065ac73cf1f00,
> but it seems to still be there.

> the existing dependency chain (in reverse order) is:

[snip]

... and as bloody usual, that mentioning of readdir in the output is a
red herring; the real problem (and yes, it *is* deadlock-prone) is not
with getdents(2) that cannot happen on anything that could be mmaped;
it's with hugetlbfs_read() (i.e. read(2)) that very definitely *can*.

This is *not* a misannotation and not a false positive; this is a real,
honest deadlock.
Thread A:
	read() on hugetlbfs
	hugetlbfs_read() called
	i_mutex grabbed
	hugetlbfs_read_actor() called
	__copy_to_user() called
	page fault is triggered
Thread B, sharing address space with A:
	mmap() the same file
	->mmap_sem is grabbed on task_B->mm->mmap_sem
	hugetlbfs_file_mmap() is called
	attempt to grab ->i_mutex and block waiting for A to give it up
Thread A:
	pagefault handled blocked on attempt to grab task_A->mm->mmap_sem,
which happens to be the same thing as task_B->mm->mmap_sem.  Block waiting
for B to give it up.

Deadlock.
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