Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On 22 Aug, Nick Piggin wrote:
> On Friday 22 August 2008 00:09, Stefan Richter wrote:
>> Nick Piggin wrote:
>> > On Thursday 21 August 2008 22:26, jmerkey@wolfmountaingroup.com wrote:
>> >> It's simple to reproduce.  Take away the volatile declaration for the
>> >> rlock_t structure in mdb-ia32.c (rlock_t debug_lock) in all code
>> >> references and watch the thing lock up in SMP with multiple processors
>> >> in the debugger each stuck with their own local copy of debug_lock.
>> >
>> > You should disable preempt before getting the processor id. Can't see any
>> > other possible bugs, but you should be able to see from the disassembly
>> > pretty easily.
>>
>> debug_lock() is AFAICS only called from contexts which have preemption
>> disabled.  Last time around I recommended to Jeff to document this
>> requirement on the calling context.
> 
> I'm not talking about where debug_lock gets called, I'm talking about
> where the processor id is derived that eventually filters down to
> debug_lock.

You are right, I replied to fast.  debug_unlock() retrieves the
processor itself, but not debug_lock().

>> But even though preemption is disabled, debug_lock() is still incorrect
>> as I mentioned in my other post a minute ago.  It corrupts its .flags
>> and .count members.  (Or maybe it coincidentally doesn't as long as
>> volatile is around.)
> 
> I don't think so. And flags should only be restored by the processor
> that saved it because the spinlock should disable preemption, right?

OK; the .count seems alright due to restrictions of the calling
contexts.  About .flags:  Jeff, can the following happen?

  - Context A on CPU A has interrupts enabled.  Enters debug_lock(),
    thus disables its interrupts.  (Saves its flags in rlock->flags with
    the plan to enable interrupts later when leaving debug_unlock()
    provided it does so as last holder.)

  - Context B on CPU B happens to have interrupts disabled.  Enters
    debug_lock(), overwrites rlock->flags with its different value.
    (Spins on the rlock which is held by CPU A.)

  - Context A on CPU A leaves debug_unlock.  Doesn't re-enable its
    interrupts as planned, since rlock->flags is the one from CPU B.
-- 
Stefan Richter
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