Re: WinXPSP2, Intel Core2Duo, Kernel drivers

No, I am referring to a routine normally called at DISPATCH_LEVEL that takes
a lock, and which an individual forgot was for DISPATCH and called from his
ISR. This was a purely unitentional mistake, that happened to work just
fine on an SMP, but not on a uni-processor.


--
Don Burn (MVP, Windows DDK)
Windows 2k/XP/2k3 Filesystem and Driver Consulting
http://www.windrvr.com
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<soviet_bloke@xxxxxxxxxxx> wrote in message
news:1157667798.504670.23420@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi Don

In fact, I thought we were speaking about accidentally forgetting that
the routine, called by ISR, deals with spinlocks. However, what you
have mentioned in your latest post is already from the totally
different field - you mentioned the scenario of synchronizing some
routine with ISR by means of spinlock, rather than of
KeSynchronizeExecution(). This is the classical example of intentional
improper design, rather than of accidental bug .

However, I made it clear that my statement does not cover intentional
"unsupported" strategies - after all, if you intentionally go for
something "unsupported" you have to know what you are doing (and, at
this point, you may ask yourself whether you really want to do it). In
this particular case, the routine that you want to synchronize with ISR
has to check how many CPUs are there, i.e. do something like

if(*KeNumberOfProcessors==1) KeRaiseIrql (IRQL >=the one ISR runs
at,...);
else KeAcquireSpinlock(...);

If you do it it this way, your code will be able to run on SMP machine,
UP one plus UP machine with MP HAL installed. Certainly, this is still
improper design (after all, you make code that runs at DIRQL "wait" for
the one that runs at DISPATCH_LEVEL, which is just stupid thing to do),
but it will work

Anton Bassov



Don Burn wrote:
Because on an SMP machine CPU 1 will get the lock, then an interrupt
occurs
on CPU 2 and it will spin when it calls the routine until CPU 1 is
finished,
but if this is a uni-processor, the lock is taken, the interupt occurs,
the
interupt executes through the code, and then the returns with the
processor
finishing the code under the lock. If this is for instance a lock to
protect insertion into a linked list, you can have a totally messed up
list
with the forward link of A pointing to B whose back link points to C.

This is because KeAcquireSpinLockAtDpcLevel on a uni-processor is a NOP.


--
Don Burn (MVP, Windows DDK)
Windows 2k/XP/2k3 Filesystem and Driver Consulting
http://www.windrvr.com
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<soviet_bloke@xxxxxxxxxxx> wrote in message
news:1157659623.530224.278180@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi Don

Sorry, it was early the instance I was thinking of was
KeAcquireSpinLockAtDpc.

But why should KeAcquireSpinLockAtDpcLevel() pose problems???? After
all, it does not try to modify IRQL

Anton Bassov


Don Burn wrote:
Sorry, it was early the instance I was thinking of was
KeAcquireSpinLockAtDpc. This code was done by a very good developer
who
is
now a respected member of the Windows driver community (and not me).


--
Don Burn (MVP, Windows DDK)
Windows 2k/XP/2k3 Filesystem and Driver Consulting
http://www.windrvr.com
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<soviet_bloke@xxxxxxxxxxx> wrote in message
news:1157640287.046142.16480@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi Don

For instance, if you forget that a routine uses
spinlocks and then accidently call it from your ISR, it may very
well
work
on an SMP system because the spinlock is an actual test and set,
but
it
has
terrible consequences on a UP system where it becomes a NOP since
the
IRQL
is above DISPATCH_LEVEL.

Trying to call the routine that uses spinlocks from ISR is not going
to
work fine on
MP system either. Let's look at how MP HAL obtains spinlocks - the
code
below is implementation of KeAcquireSpinLock() from halmacpi.dll


80012830: mov edx,dword ptr ds:[FFFE0080h]
80012836: mov dword ptr ds:[FFFE0080h],41h
80012840: shr edx,4
80012843: movzx eax,byte ptr [edx+8001D218h]

8001284A: lock bts dword ptr [ecx],0
8001284F: jb 80012854
80012851: ret

As you can see, the very first step KeAcquireSpinLock() does is
UNCONDITIONALLY(!!!)writing 0x41 to the Task Priority Register, i.e.
setting IRQL to DISPATCH_LEVEL, and then proceeds to set and test.
KeAcquireQueuedSpinLock() behaves in similar fashion. Therefore,
calling these functions at IRQL>DISPATCH_LEVEL is quite unwise thing
to
do - even on MP HAL.

Anton Bassov




Don Burn wrote:
<soviet_bloke@xxxxxxxxxxx> wrote in message
Well, you can think of my examples as of ones of the bugs if you
wish -
no matter how you call them, these are the examples of doing
something
you are not supposed to do, which may have undesirable effects
(unless
you are 100% sure you know what you are doing)

Yes they are doing something you are not supposed to do, that is a
pretty
good definition of a bug. For instance, if you forget that a
routine
uses
spinlocks and then accidently call it from your ISR, it may very
well
work
on an SMP system because the spinlock is an actual test and set,
but
it
has
terrible consequences on a UP system where it becomes a NOP since
the
IRQL
is above DISPATCH_LEVEL.



--
Don Burn (MVP, Windows DDK)
Windows 2k/XP/2k3 Filesystem and Driver Consulting
http://www.windrvr.com
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