Re: [broken-out-2007-07-20-00-22] kernel bug at kernel/params:570

Michal Piotrowski
2007-07-21T08:11:40Z

On 21/07/07, Satyam Sharma <satyam.sharma@gmail.com> wrote:
> Ok, this worked:
>
> > His .config has CONFIG_PROFILE_LIKELY=y
> > which replaces unlikely() / likely() with do_check_likely() and forces
> > gcc to clobber %eax with the condition itself, which in our case was
> > (ret < 0) == TRUE, and thus, the "1" value we saw in %eax in the
> > register dumps.
>
> The usage of unlikely() in BUG_ON and CONFIG_PROFILE_LIKELY=y
> meant %eax that held `ret' from kobject_add() was being lost.
>
> With that turned off:
>
> 0xc136071b call 0xc1103744 <kobject_add>
> [ ret i.e. %eax = kobject_add() ]
> 0xc1360720 test %eax,%eax
> [ %eax = %eax && %eax = %eax ]
> [ probably gcc thinks this is an efficient < 0 check on i386 :-) ]
> 0xc1360722 jns 0xc1360728
> [ if false jump down below, else: ]
> 0xc1360724 ud2a
> [ invalid opcode exception. /* from BUG */ ]
> 0xc1360726 jmp 0xc1360726
> [ for (;;) ; /* from BUG */ ]
>
> > We should probably document somewhere that CONFIG_PROFILE_LIKELY
> > is not good for debugging.
> >
> > Hmmm ... thinking out aloud here, but probably I don't need to fix that
> > libata breakage at all. I'll just put the BUG_ON(ret < 0) back in the
> > code, deselect PROFILE_LIKELY, and this time we _will_ have the
> > return of kobject_add() in %eax ...
>
> So %eax retained its value. I reproduced Michal's oops (without having
> to select CONFIG_ATA, because param_sysfs_init() is called way
> earlier than searching for root block device during boot -- which is what
> my system didn't like without ATA), and yes, I saw:
>
> %eax == ffffffea == -22 == -EINVAL.
>
> But:
>
> When I replace the BUG_ON() with Greg's (or my modified) patch,
> the kernel ignores that error as we wanted, but _still_ crashes before
> finding the root block device (because ATA is not in kernel and my
> system seems to require it). Unfortunately, lots of dmesg output
> has scrolled up by then, and our "printk" is lost. Because of no
> CONFIG_ATA, nothing gets written to disk either. I don't have a
> serial cable around, netconsole not up yet at that point.

Neither serial nor net console doesn't work, system hangs very early.

>
> Ugh, I'm running out of ideas.
>
> Satyam
>

Regards,
Michal

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