Re: [PATCH 5/5] crypto: stm32/crc: protect from concurrent accesses

From: Ard Biesheuvel
Date: Mon May 25 2020 - 03:46:15 EST

On Mon, 25 May 2020 at 09:24, Nicolas TOROMANOFF
<nicolas.toromanoff@xxxxxx> wrote:
> Hello,
> > -----Original Message-----
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > Sent: Friday, May 22, 2020 6:12 PM>
> > On Tue, 12 May 2020 at 16:13, Nicolas Toromanoff
> > <nicolas.toromanoff@xxxxxx> wrote:
> > >
> > > Protect STM32 CRC device from concurrent accesses.
> > >
> > > As we create a spinlocked section that increase with buffer size, we
> > > provide a module parameter to release the pressure by splitting
> > > critical section in chunks.
> > >
> > > Size of each chunk is defined in burst_size module parameter.
> > > By default burst_size=0, i.e. don't split incoming buffer.
> > >
> > > Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@xxxxxx>
> >
> > Would you mind explaining the usage model here? It looks like you are sharing a
> > CRC hardware accelerator with a synchronous interface between different users
> > by using spinlocks? You are aware that this will tie up the waiting CPUs
> > completely during this time, right? So it would be much better to use a mutex
> > here. Or perhaps it would make more sense to fall back to a s/w based CRC
> > routine if the h/w is tied up working for another task?
> I know mutex are more acceptable here, but shash _update() and _init() may be call
> from any context, and so I cannot take a mutex.
> And to protect my concurrent HW access I only though about spinlock. Due to possible
> constraint on CPUs, I add a burst_size option to force slitting long buffer into smaller one,
> and so decrease time we take the lock.
> But I didn't though to fallback to software CRC.
> I'll do a patch on top.
> In in the burst_update() function I'll use a spin_trylock_irqsave() and use software CRC32 if HW is already in use.

Right. I didn't even notice that you were keeping interrupts disabled
the whole time when using the h/w block. That means that any serious
use of this h/w block will make IRQ latency go through the roof.

I recommend that you go back to the drawing board on this driver,
rather than papering over the issues with a spin_trylock(). Perhaps it
would be better to model it as a ahash (even though the h/w block
itself is synchronous) and use a kthread to feed in the data.