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

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Message-ID: <20131017221450.GU13318@ZenIV.linux.org.uk>
Date:	Thu, 17 Oct 2013 23:14:50 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	George Spelvin <linux@...izon.com>
Cc:	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: 3.11.4: kernel BUG at fs/buffer.c:1268

On Thu, Oct 17, 2013 at 05:11:43PM -0400, George Spelvin wrote:
> Al Viro wrote:
> > Note that do_group_exit() is preceded by
> >                spin_unlock_irq(&sighand->siglock);
> > so no matter what happened in callers, irq is enabled.  I'd suggest sticking
> > such BUG_ON() into __fput() and trying to reproduce that crap...
> 
> 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).  If such BUG_ON() triggers in
__fput(), this is what happens and the next step will be sticking
several BUG_ON(irqs_disabled()) in do_exit() - something like
	BUG_ON(irqs_disabled());
        exit_sem(tsk);
	BUG_ON(irqs_disabled());
        exit_shm(tsk);
	BUG_ON(irqs_disabled());
        exit_files(tsk);
	BUG_ON(irqs_disabled());
        exit_fs(tsk);
	BUG_ON(irqs_disabled());
        exit_task_namespaces(tsk);
	BUG_ON(irqs_disabled());
        exit_task_work(tsk);
and checking which of those triggers; that'll tell us who has disabled it
and forgot to enable.
	* __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.
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