Re: 3.11.4: kernel BUG at fs/buffer.c:1268

From: Andreas Dilger
Date: Thu Oct 31 2013 - 14:33:52 EST

On Oct 17, 2013, at 4:14 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Oct 17, 2013 at 05:11:43PM -0400, George Spelvin wrote:
>> Well, it happened again (error appended). Can you please clarify what you mean
>> by "such BUG_ON()"; I'm having a hard time following the RCU code and determining
>> all the situations under which __fput() might be called.
> __fput() can be called via task_work_run() or via schedule_work(). That's
> all. And it certainly should never be called with interrupts disabled.
> So stick BUG_ON(irqs_disabled()) in it (WARN_ON() might be better, but
> not by much).
> There are two ways these traces could've happened:
> * exit_task_work() called by do_exit() with irqs disabled.
> Definitely buggy (and would do really nasty things to several functions
> called by do_exit() before that one).
> * __fput() is called with irqs enabled, but somewhere on the
> way into ext4 (dput -> iput -> evict inode -> free blocks, now that
> unlinked file got closed -> ...) we manage to disable irqs and forget
> to enable them.

IMHO the most common case of "BUG: sleeping function called from
invalid context” is due to stack overflow. This corrupts the task
struct, and incorrectly sets the “in_interrupt” bit.

What kind of storage stack is underneath this filesystem? If
it is deep (e.g. DM + LVM + iSCSI) then the stack overflow is
definitely possible.

There were also a discussion by Christoph of page allocation
recursing into the fs again (in "xfs: prevent stack overflows
from page cache allocation”) though I’m not sure if that applies
to ext4 or not.

Cheers, Andreas

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