linux-kernel - Re: [git pull] scheduler updates for v2.6.24

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Date:	Wed, 17 Oct 2007 01:31:22 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Gabriel C" <nix.or.die@...glemail.com>,
	"Srivatsa Vaddagiri" <vatsa@...ibm.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [git pull] scheduler updates for v2.6.24

[ cc'ed Srivatsa ]

On 17/10/2007, Gabriel C <nix.or.die@...glemail.com> wrote:
> Ingo Molnar wrote:
> [15692.917111] BUG: unable to handle kernel NULL pointer dereference at virtual address 00000044
> ...
> [15692.917629] EFLAGS: 00010046   (2.6.23-g65a6ec0d #330)
> [15692.917661] EIP is at pick_next_task_fair+0x1f/0x2d

Gabriel, could you please post a disassembled code for pick_next_task_fair()?
(objdump -d kernel/sched.o and then search for pick_next_task_fair --
copy_and_past)

anyway, my guess is that it's :

                se = pick_next_entity(cfs_rq);
                cfs_rq = group_cfs_rq(se);

'se' _happens_ to be NULL and group_cf_rq(se) does se->my_q and
(according to my calculations) offset(my_q) == 68 (0x44) for x86 32bit
system with CONFIG_SCHEDSTATS=n and CONFIG_FAIR_GROUP_SCHED=y
(according to the config).

that might take place provided put_prev_task_fair() failed for some
reason to insert 'current' (or its corresponding group element) back
into the tree in schedule()... say, due to some inconsistency in
cfs_rq's data.

Srivatsa, that's somewhat similar to another issue that has been
posted earlier today (crash in put_prev_task_fair() -->
__enqueue_task() --> rb_insert_color()) that you are already aware of
...  (/me will continue tomorrow).

-- 
Best regards,
Dmitry Adamushko
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