Re: [SMP lock BUG?] Re: Feedback on preemptible kernel patch

[Arjan Filius <iafilius@xs4all.nl>]

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Hi,

On Sun, 9 Sep 2001, george anzinger wrote:

> If the page it is the correct one, when it is found mapped, the code
> should just exit, not BUG() IHMO.


I'll try the ac10 +preempt, see what happens.

>
> George
>
>
> Roger Larsson wrote:
> >
> > Hi,
> >
> > This is interesting. [Assumes UP Athlon - correct]
> > Note that all BUGs out in highmem.h:95 (kmap_atomic)
> > and that test is only on if you have enabled HIGHMEM_DEBUG
> > [my analyze is done with a 2.4.10-pre2 kernel, but I checked with
> > later patches and I do not think they fix it either...]
> >
> > The preemptive kernel puts more SMP stress on the kernel than
> > running with multiple CPUs.
> >
> > So this might be a potential bug in the kernel proper, running with
> > a SMP computer.
> >
> > If I understand the bug correctly, a process gets a page fault.
> > Starts to map in the page. But before the final part it checks -
> > and the page is already there!!! Correct?
> >
> > On Saturday den 8 September 2001 19:33, Arjan Filius wrote:
> > > Hello Robert,
> > >
> > >
> > > I tried 2.4.10-pre4 with patch-rml-2.4.10-pre4-preempt-kernel-1.
> > > But it seems to hit highmem (see below) (i do have 1.5GB ram)
> > > 2.4.10-pre4 plain runs just fine.
> > >
> > > With the kernel option mem=850M the patched kernel boots an seems to run
> > > fine. However i didn't do any stress testing yet, but i still notice
> > > hickups while playing mp3 files at -10 nice level with mpg123 on a 1.1GHz
> > > Athlon, and removing for example a _large_ file (reiser-on-lvm).
> > >
> > > My syslog output with highmem:
> > >
> > > Sep  8 18:10:16 sjoerd kernel: kernel BUG at
> > > /usr/src/linux-2.4.10-pre4/include/asm/highmem.h:95! Sep  8 18:10:16 sjoerd
> > > kernel: invalid operand: 0000
> > > Sep  8 18:10:16 sjoerd kernel: CPU:    0
> > > Sep  8 18:10:16 sjoerd kernel: EIP:    0010:[do_wp_page+636/1088]
> > > [- - -]
> > > sjoerd kernel: Call Trace: [handle_mm_fault+141/224]
> > > [do_page_fault+375/1136] [do_page_fault+0/1136] [__mmdrop+58/64]
> > > [do_exit+595/640] Sep  8 18:10:16 sjoerd kernel:    [error_code+52/64]
> >
> > Lets look at this example. You need to add some inline functions...
> >
> > handle_mm_fault
> >         takes the mm->page_table_lock [this should prevent reschedules]
> >         allocs pmd
> >         allocs pte
> >         handle_pte_fault(...)
> > handle_pte_fault [inline, most likely path]
> >         pte is present
> >         it is a write access
> >         but the pte is not writeable  - call do_wp_page
> > do_wp_page
> >         plays some games with the lock...
> >         finally calls copy_cow_page [inline] with the page_table_lock
> >         UNLOCKED!
> > copy_cow_page
> >         calls clear_user_highpage or copy_user_highpage
> > both clear_user_highpage and copy_user_highpage
> >         calls kmap_atomic
> > kmap_atomic
> >         page is a highmem page
> >         but during the time this process was unlocked some other
> >         thread has allocated the page in question... BUG out.
> >
> > So somewere between the UNLOCK (might be a lot later) and the
> > BUG test in kmap_atomic the process running in kernel got preempted.
> > (most likely during the page copy since it will take some time)
> >
> > Another process (thread) started to run - hit the same page fault
> > but succeeded in its alloc.
> >
> > Back to the first process it continues, finally checks - the page
> > is there... and BUGS.
> >
> > Note that this can happen in a pure SMP kernel.
> >
> > But let the processes (threads) run on two CPUs. And let the
> > first get an interrupt/bh after unlock - the other can pass
> > and add the page before the first one can continue - same
> > result!
> >
> > /RogerL
> >
> > --
> > Roger Larsson
> > Skelleftea
> > Sweden
> > -
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>

-- 
Arjan Filius
mailto:iafilius@xs4all.nl

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