Re: [PATCH v2 12/15] crypto: iaa - Add support for iaa_crypto deflate compression algorithm

[Tom Zanussi]

Hi Herbert,

On Thu, 2023-04-06 at 16:00 +0800, Herbert Xu wrote:
> On Tue, Mar 28, 2023 at 10:35:32AM -0500, Tom Zanussi wrote:
> > 
> > @@ -881,12 +1574,26 @@ static int iaa_crypto_probe(struct idxd_dev
> > *idxd_dev)
> >  
> >         rebalance_wq_table();
> >  
> > +       if (first_wq) {
> > +               iaa_crypto_enabled = true;
> > +               ret = iaa_register_compression_device();
> > +               if (ret != 0) {
> > +                       iaa_crypto_enabled = false;
> > +                       dev_dbg(dev, "IAA compression device
> > registration failed\n");
> > +                       goto err_register;
> > +               }
> > +
> > +               pr_info("iaa_crypto now ENABLED\n");
> > +       }
> > +
> 
> Sorry for picking on your driver but I've got to start somewhere :)

No problem, thanks for reviewing the code. ;-)

> 
> A long standing problem shared by almost all crypto drivers is that
> the hardware removal handling is completely broken.
> 
> This is because hardware can be removed at any time, including during
> a crypto operatin.  So drivers must work carefully around that fact.
> 
> Here is a recipe for dealing with this safely:
> 
> 1) Never unregister your crypto algorithms, even after the last
> piece of hardware has been unplugged.  The algorithms should only
> be unregistered (if they have been registered through the first
> successful probe call) in the module unload function.
> 
> 2) Never free any software state for your hardware without some form
> of synchronisation with oustanding operations.
> 
> Any mechanism would do, for example, you could use a spinlock if the
> critical path isn't very long.  The operational path would take the
> lock, check the hardware state, and if present proceed with the
> operation (but still being prepared to cope if the hardware goes
> AWAL because even if the driver state is still present the actual
> hardware may be gone already).
> 
> Then the removal path would simply take the spinlock, set a flag
> indicating the hardware is gone and then you could safely unlock
> and free your driver states.
> 

OK, yeah, thanks for pointing this out along with the detailed
explanation and remedy.  Will take care of this in the next version.

Tom

> Thanks,