Re: [RFC PATCH 5/6] pci: Protect the enable/disable state of pci_dev using the state mutex

From: Benjamin Herrenschmidt
Date: Fri Aug 17 2018 - 05:39:54 EST

On Fri, 2018-08-17 at 14:30 +0530, Hari Vyas wrote:
> So many mail threads for common issues going so just trying to
> summarize concern from my side.
> 1) HW level
> PCI Config data overwritten (ex: PCI_COMMAND bits etc) was happening
> at lower layer so from my
> perspective, it is best to handle concern at lower level with locking
> mechanism and that is what
> I proposed in my upcoming patch. Without that, I guess, we may end up
> in issues later with some weird scenario
> which may not be known today and we will again be fine tuning stuff
> here and there.

Maybe... at this point I'm not trying to address that specific issue.
That being said, the PCI_COMMAND register should be under control of
the driver at runtime and thus shouldn't be exposed to races like that
except in the usual case of races in configuring bridges. Those races
definitely need to be handled at the higher level.

So I'm rather dubious of adding a whole new layer of "modify" callbacks
to config space accessors for that, especially since they won't do any
good on architectures with lockless config accesses such as ... x86

> 2) SW level(internal data structure):
> About is_added,is_busmaster: These all are bit fields so infact I too
> suggested to remove those bit fields and
> make separate variables in pci_dev structure.
> This will avoid all data-overwritten,concurrency issues but ofcourse
> at the level of space cost.

It's unnecessary to do blanket changes without first understanding the
details of what's going on. A lot of these things are never touched
outside of the overall single threaded environment of
discovery/assignment or under driver control, in which case it's
expectd that the driver doesn't call them in racy ways

That said, I'm happy to move some of them under my proposed

For is_added specifically, the field is simply set at the wrong time as
you can see in my previous patch.

> Other possibility will be either to use atomic

Atomic is almost always wrong. It doesn't synchronize anything other
than the field, so either it's a big waste of it gives a false sense of
security for something that's almost always still racy.

I'd rather keep the indication that a bunch of those fields *are*
unprotected and rely on the outer layers being single threaded.

> or locking mechanism
> for these. My point here is also same, better
> avoid fine tuning later stage.
> Moving is_added up and down doesn't look like good as we are just
> shifting issue up and down.

No, you are wrong. I also originally covered is_added with my new mutex
but in hindsight it's the wrong thing to do, and went back to the
correct fix at this point which is precisely to move it up.

is_added is essentially owned by the part of the PCI probe code that
runs single threaded before the device gets exposed to the device

Pretty much all of the code, including all the corresponding fields
(struct resources etc...) in pci_dev are unprotected and rely on being
accessed in a single threaded manner. is_added is no exception.

It was simply a mistake to set it after device_attach().

Thus moving it back up fixes it *correctly* by making it follow the
same rules as all the related fields.

That said, if we want to start adding protection to all those other
fields, then this is a different matter alltogether. But at this point,
this is the best fix for the problem at hand.

> Please check and decide accordingly. I will hold my to-be-submitted
> modify() patch about handling hw level
> over-written case with locking around read-write operation.

Can you remind us in this thread which specific cases of RMW races of
config space you were trying to address ?