Re: 2.6.24-rc1-gb4f5550 oops

[Oleg Nesterov <oleg@tv-sign.ru>]

On 11/14, Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@tv-sign.ru> wrote:
> 
> > > [18073.371126] Unable to handle kernel NULL pointer dereference at 0000000000000120 RIP:
> > > [18073.371134]  [<ffffffff8023572e>] check_preempt_wakeup+0x6e/0x110
> > > [18073.371144] PGD 81f9067 PUD 81c8067 PMD 0
> > > [18073.371151] Oops: 0000 [1] PREEMPT SMP
> > > [18073.371157] CPU 2
> > > [18073.371161] Modules linked in: vfat fat
> > > [18073.371168] Pid: 4639, comm: kwin Not tainted 2.6.24-rc1 #1
> > > [18073.371171] RIP: 0010:[<ffffffff8023572e>]  [<ffffffff8023572e>] check_preempt_wakeup+0x6e/0x110
> > > [18073.371177] RSP: 0018:ffff810008531a78  EFLAGS: 00010006
> > > [18073.371179] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > > [18073.371183] RDX: ffff810004441bf0 RSI: ffff81000801e860 RDI: ffff81000444ab80
> > > [18073.371186] RBP: ffff810008531aa8 R08: 000000d0d47a4a90 R09: 0000000000000000
> > > [18073.371188] R10: ffff810004441bf0 R11: 0000000000000001 R12: ffff810006520400
> > > [18073.371190] R13: ffff81000801e860 R14: ffff81000a63a000 R15: ffff81000443d8e0
> > > [18073.371193] FS:  00002b7d646a86f0(0000) GS:ffff810004c11780(0000) knlGS:0000000000000000
> > > [18073.371196] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> > > [18073.371199] CR2: 0000000000000120 CR3: 0000000008495000 CR4: 00000000000006e0
> > > [18073.371202] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [18073.371211] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > > [18073.371214] Process kwin (pid: 4639, threadinfo ffff810008530000, task ffff81000840a860)
> > > [18073.371216] Stack:  ffff81000444ab80 0000000000000001 ffff81000801e860 ffff81000444ab80
> > > [18073.371231]  0000000000000002 ffff81000443d8e0 ffff810008531b38 ffffffff8023061e
> > > [18073.371238]  0000000000000000 ffff810004441b80 0000000000000002 0000000100000000
> > > [18073.371245] Call Trace:
> > > [18073.371250]  [<ffffffff8023061e>] try_to_wake_up+0x2fe/0x3a0
> > 
> > I suspect I see the bug in that area, but I am not sure it can explain 
> > this trace completely.
> 
> there's a fix pending from Dmitry - please see below. It took days for 
> Grant to trigger the crash so it needs some time to be confirmed but it 
> could explain the crash in theory.

Yes I agree, it can explain the crash.

However, this patch can't fix the bug I was talking about (of course, unless
I missed something). It is still possible that the "fair_sched_class" task
can have ->se.cfs_rq/parent pointing to the freed memory, no?

Oleg.