linux-kernel - Re: [kerneloops] regression in 2.6.27 wrt "lock

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Date:	Sat, 4 Oct 2008 21:52:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [kerneloops] regression in 2.6.27 wrt "lock_page" and the
 "hwclock" program

On Sat, 4 Oct 2008 17:44:33 -0700 Arjan van de Ven <arjan@...radead.org> wrote:

> 
> Details: http://www.kerneloops.org/searchweek.php?search=lock_page
> 
> There's quite a few of this BUG, which seems to be an interaction between the "hwclock" program and something in 2.6.27.
> It's new in .27 and is currently the 8th ranked issue.....
> 
> BUG: sleeping function called from invalid context at
> include/linux/pagemap.h:294 in_atomic():0, irqs_disabled():1
> INFO: lockdep is turned off.
> irq event stamp: 0
> hardirqs last  enabled at (0): [<00000000>] 0x0
> hardirqs last disabled at (0): [<c042c3a4>] copy_process+0x2e7/0x115e
> softirqs last  enabled at (0): [<c042c3a4>] copy_process+0x2e7/0x115e
> softirqs last disabled at (0): [<00000000>] 0x0
> Pid: 9591, comm: hwclock Tainted: G        W 2.6.27-0.372.rc8.fc10.i686 #1 
>  [<c0427a53>] __might_sleep+0xd1/0xd6 
>  [<c0479a8b>] lock_page+0x1a/0x34
>  [<c0479cfa>] find_lock_page+0x23/0x48
>  [<c047a215>] filemap_fault+0x9b/0x330
>  [<c0486493>] __do_fault+0x40/0x2e6
>  [<c0487d63>] handle_mm_fault+0x2ec/0x6d2
>  [<c06e8260>] do_page_fault+0x2e5/0x693
> 

Looks like `hwclock' disabled interrupts in userspace with sys_iopl()?

And then it took a pagefault, which is presumably a bug in hwclock.

That's all a bit antisocial of it.  I guess a suitable quickfix is to
remove the might_sleep() from lock_page() (which would be a good thing
from a text size POV anyway).

But there will of course be other sites which do possibly-sleeping
operations on the pagefault path.

Really, it's a bit stupid doing _any_ system calls (and a pagefault is
a syscall in disguise) with interrupts disabled.  The kernel makes no
guarantees that we'll honour it.  We could just enable interrupts on
pagefault entry - that'll teach 'em.

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