Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

["Satyam Sharma" <satyam.sharma@gmail.com>]

On 7/27/07, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > Maybe I should resurrect it & send it out...

Hmm, something that hooks in not only at do_IRQ time (as the present
in-mainline stackoverflow check thing)?

> > (FWIW I think I recall that the warning itself sometimes tipped the
> > scales enough on 4k stacks to bring the box down)
>
> You can always switch stack for the printk and it probably should panic
> at that point and give a trace then die as that is what we are trying to
> prove does not occur

Yes, only yesterday I saw exactly this happening DEBUG_STACKOVERFLOW
when doing a udf -> pktcdvd -> cdrom -> ide_cd thing. It's one of those
reproducible will-crash-4k-stacks tests, especially if you have debug stuff
enabled in your build that would make on-stack structures (where such
exist on the codepath) a bit heavier.

Admittedly, what seems to have happened is a bit pathological:

[  481.836378] cdrom: entering cdrom_count_tracks
[  481.844266] BUG: sleeping function called from invalid context at
include/asm/semaphore.h:98
[  481.844434] do_IRQ: stack overflow: 164
[  481.844540]  [<c0405cfe>] show_trace_log_lvl+0x19/0x2e
[  481.844707]  [<c0405dfe>] show_trace+0x12/0x14
[  481.844867]  [<c0405e14>] dump_stack+0x14/0x16
[  481.845027]  [<c0406ff6>] do_IRQ+0x7b/0xe1
[  481.845186]  [<c040583e>] common_interrupt+0x2e/0x34
[  481.845348]  [<c042b8e7>] printk+0x1b/0x1d
[  481.845507]  [<c0422c05>] __might_sleep+0x81/0xdc
[  481.845668]  [<c066d869>] __reacquire_kernel_lock+0x2d/0x4f
[  481.845833]  [<c066b09b>] schedule+0x78a/0x7a4
[  481.845996]  [<c066b538>] wait_for_completion+0x72/0x97
[  481.846160]  [<c05937a6>] ide_do_drive_cmd+0xeb/0x109
[  481.846324]  [<f89172a2>] cdrom_queue_packet_command+0x40/0xc5 [ide_cd]
[  481.846503]  [<f89175b7>] ide_cdrom_packet+0x86/0xa4 [ide_cd]
[  481.846669]  [<f8854dc1>] cdrom_get_disc_info+0x48/0x87 [cdrom]
[  481.846839]  [<f8854ec6>] cdrom_get_last_written+0x2a/0xfe [cdrom]
[  481.847009]  [<f891831b>] cdrom_read_toc+0x39d/0x3f3 [ide_cd]
[  481.847231]  [<f8918e7e>] ide_cdrom_audio_ioctl+0x130/0x1ce [ide_cd]
[  481.847414]  [<f8854123>] cdrom_count_tracks+0x5c/0x126 [cdrom]
[  481.847583]  [<f8855688>] cdrom_open+0x147/0x79c [cdrom]
[  481.847748]  [<f891799a>] idecd_open+0x75/0x8a [ide_cd]
[  481.847912]  [<c04aac0e>] do_open+0x1d1/0x284
[  481.848079]  [<c04aad89>] __blkdev_get+0x73/0x7e
[  481.848242]  [<c04aada9>] blkdev_get+0x15/0x17
[  481.848411]  [<f8b34b6b>] pkt_open+0x99/0xc6e [pktcdvd]
[  481.848583]  [<c04aaad3>] do_open+0x96/0x284
[  481.848745]  [<c04aad89>] __blkdev_get+0x73/0x7e
[  481.848910]  [<c04aada9>] blkdev_get+0x15/0x17

(... the trace cut off there, and then the box froze hard, no sysrq ...)

The mount(2) hit the wait_for_completion() in ide_do_drive_cmd(),
little stack was left at this point. But then I have no idea why the
__reacquire_kernel_lock() from schedule() gave a might_sleep() there,
the code in sched.c and kernel_lock.c looks obviously correct -- the
down(&kernel_sem) only happens with both irqs and preemption on.

Anyway, the second line of printk() in __might_sleep (the one that
tells us in_atomic() and irqs_disabled()) was about to be printed when
an interrupt decided to join the fun. do_IRQ() comes in, with debug
stackoverflows on, it notices that only 164 bytes worth of stack is left
and decides to dump_stack ... and while we were doing just that,
we died. (this was 2.6.23-rc1-mm1)
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