On Tue, 2009-06-30 at 10:42 +0200, Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
> > Jan Kiszka wrote:
> >> Hi all,
> >>
> >> seen such loops before? This particular trace is from a 2.6.29.3 kernel
> >> with ipipe-2.3-01 (SMP/PREEMPT_VOLUNTARY), but the same happens with
> >> 2.6.29.5/2.3-03:
> >>
> >> :|   +func                -653    0.084  __ipipe_handle_exception+0x11 
> >> (page_fault+0x26)
> >> :|   +func                -653    0.096  ipipe_check_context+0xd 
> >> (__ipipe_handle_exception+0x71)
> >> :|   #end     0x80000000  -653    0.069  do_page_fault+0x33 
> >> (__ipipe_handle_exception+0x1ff)
> >> :    #func                -653    0.078  __ipipe_unstall_root+0x9 
> >> (do_page_fault+0x3cb)
> >> :|   #begin   0x80000000  -653    0.068  __ipipe_unstall_root+0x34 
> >> (do_page_fault+0x3cb)
> >> :|   +end     0x80000000  -653    0.069  __ipipe_unstall_root+0x59 
> >> (do_page_fault+0x3cb)
> >> :    +func                -653    0.060  down_read_trylock+0x4 
> >> (do_page_fault+0x424)
> >> :    +func                -653    0.068  _spin_lock_irqsave+0x9 
> >> (__down_read_trylock+0x16)
> >> :    +func                -653    0.108  ipipe_check_context+0xd 
> >> (_spin_lock_irqsave+0x1d)
> >> :    #func                -652    0.066  _spin_unlock_irqrestore+0x4 
> >> (__down_read_trylock+0x3f)
> >> :    #func                -652    0.069  __ipipe_restore_root+0x4 
> >> (_spin_unlock_irqrestore+0x21)
> >> :    #func                -652    0.074  __ipipe_unstall_root+0x9 
> >> (__ipipe_restore_root+0x2c)
> >> :|   #begin   0x80000000  -652    0.066  __ipipe_unstall_root+0x34 
> >> (__ipipe_restore_root+0x2c)
> >> :|   +end     0x80000000  -652    0.069  __ipipe_unstall_root+0x59 
> >> (__ipipe_restore_root+0x2c)
> >> :    +func                -652    0.096  find_vma+0x4 (do_page_fault+0x465)
> >> :    +func                -652    0.150  ltt_run_filter_default+0x4 
> >> (_ltt_specialized_trace+0xc1)
> >> :    +func                -652    0.098  handle_mm_fault+0x11 
> >> (do_page_fault+0x537)
> >> :    +func                -652    0.090  _spin_lock+0x4 
> >> (handle_mm_fault+0x680)
> >> :    +func                -652    0.063  ptep_set_access_flags+0x9 
> >> (handle_mm_fault+0x6d1)
> >> :    +func                -652    0.282  flush_tlb_page+0xd 
> >> (handle_mm_fault+0x6e7)
> >> :    +func                -651    0.162  ltt_run_filter_default+0x4 
> >> (_ltt_specialized_trace+0xc1)
> >> :    +func                -651    0.062  up_read+0x4 (do_page_fault+0x5a9)
> >> :    +func                -651    0.072  _spin_lock_irqsave+0x9 
> >> (__up_read+0x1c)
> >> :    +func                -651    0.117  ipipe_check_context+0xd 
> >> (_spin_lock_irqsave+0x1d)
> >> :    #func                -651    0.074  _spin_unlock_irqrestore+0x4 
> >> (__up_read+0x92)
> >> :    #func                -651    0.069  __ipipe_restore_root+0x4 
> >> (_spin_unlock_irqrestore+0x21)
> >> :    #func                -651    0.060  __ipipe_unstall_root+0x9 
> >> (__ipipe_restore_root+0x2c)
> >> :|   #begin   0x80000000  -651    0.056  __ipipe_unstall_root+0x34 
> >> (__ipipe_restore_root+0x2c)
> >> :|   +end     0x80000000  -651    0.420  __ipipe_unstall_root+0x59 
> >> (__ipipe_restore_root+0x2c)
> >> :|   +func                -650    0.084  __ipipe_handle_exception+0x11 
> >> (page_fault+0x26)
> >>
> >> and again and again...
> >>
> >> We are looping over a minor fault here (according to /proc/PID/stat),
> >> the context is a Xenomai task in secondary mode. As the task no longer
> >> processes signals in this state, the whole system is more or less
> >> broken. Tomorrow I will try to find out the faulting address with an
> >> instrumented kernel, but maybe you already have some ideas.
> > 
> > The fault is apparently triggered by __xn_put_user(XNRELAX,
> > thread->u_mode) in xnshadow_relax. thread->u_mode is pointing to an
> > invalid region ATM. The questions are now: Who corrupted this, user
> > space on init (not that likely) or kernel space later on (unpleasant
> > thought)? Moreover: Why can't we recover from a fault on u_mode?
> 
> I already investigated such an issue, and my conclusion was that there
> are some places in the code where we can not cope with a fault.
> xnshadow_relax being such a place, because, if relax faults, then what
> will the fault handler do? Call relax again. Fortunately, mlockall and
> the nocow stuff fixes this.


xnshadow_relax() faulting before the current thread bears the XNRELAX
bit would mean that a creepy issue involving ondemand PTEs in _kernel_
space must have caused this. Having the init_mm mappings known from all
processes seems more relevant to this issue than anything nocow and/or
mlockall could ever do to fix it.

> 
> Another way to implement the u_mode thing would be to use the shared
> heaps we use for fast mutexes.
> 
-- 
Philippe.



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