linux-kernel - Re: [x86] BUG: unable to handle kernel paging request at 00740060

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Message-ID: <20131008143400.GA14721@redhat.com>
Date:	Tue, 8 Oct 2013 16:34:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [x86] BUG: unable to handle kernel paging request at 00740060

I'll try to find other messages to understand what you are talking
about, just one note for now....

On 10/07, Linus Torvalds wrote:
>
> Your oops makes very little sense, it looks like task_work_run() just
> called out to random crap, probably because the work was already
> released, so "work->func()" ends up being bad.

Or task_work_run() can hit work->func == NULL if do_exit() is called
twice if, say, the task does BUG() after exit_task_work().

> participants anyway, just in case there is some race. The comment says
> that it can race with task_work_cancel() playing with *work. Oleg,
> comments?

The comment tries to say that if we are racing with task_work_cancel()
it can't delete the first entry == work, we won the race, its
cmpxchg(task->task_works) should fail.

Howver, task_work_cancel() can delete one of the next entries and
change, say, work->next. And we need to wait anyway if it scans this
list.

I'll try to recheck, but so far I do not see anything wrong.

Oleg.

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