# Re: \[PATCH v3 net-next 3/9\] net: mscc: ocelot: serialize access to the MAC table

- From: Vladimir Oltean \<olteanv@gmail.com\>
- Date: 2021-10-22 20:51:54 
- List: netdev
- Page: https://ratatoskr.run/netdev/2021/10/11214271
- Thread: https://ratatoskr.run/netdev/2021/10/11214210/t.md

```text
On Fri, Oct 22, 2021 at 09:04:47PM +0200, Andrew Lunn wrote:
> On Fri, Oct 22, 2021 at 09:00:52PM +0300, Vladimir Oltean wrote:
> > On Fri, Oct 22, 2021 at 10:34:04AM -0700, Florian Fainelli wrote:
> > > On 10/22/21 10:27 AM, Vladimir Oltean wrote:
> > > > DSA would like to remove the rtnl_lock from its
> > > > SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE handlers, and the felix driver uses
> > > > the same MAC table functions as ocelot.
> > > > 
> > > > This means that the MAC table functions will no longer be implicitly
> > > > serialized with respect to each other by the rtnl_mutex, we need to add
> > > > a dedicated lock in ocelot for the non-atomic operations of selecting a
> > > > MAC table row, reading/writing what we want and polling for completion.
> > > > 
> > > > Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> > > > ---
> > > >  drivers/net/ethernet/mscc/ocelot.c | 53 +++++++++++++++++++++++-------
> > > >  include/soc/mscc/ocelot.h          |  3 ++
> > > >  2 files changed, 44 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/drivers/net/ethernet/mscc/ocelot.c b/drivers/net/ethernet/mscc/ocelot.c
> > > > index 4e5ae687d2e2..72925529b27c 100644
> > > > --- a/drivers/net/ethernet/mscc/ocelot.c
> > > > +++ b/drivers/net/ethernet/mscc/ocelot.c
> > > > @@ -20,11 +20,13 @@ struct ocelot_mact_entry {
> > > >  	enum macaccess_entry_type type;
> > > >  };
> > > >  
> > > > +/* Must be called with &ocelot->mact_lock held */
> > > 
> > > I don't know if the sparse annotations: __must_hold() would work here,
> > > but if they do, they serve as both comment and static verification,
> > > might as well use them?
> > 
> > I've never come across that annotation before, thanks.
> > I'll fix this and the other issue and resend once the build tests for
> > this series finish.
> 
> If sparse cannot figure it out, mv88e6xxx has:
> 
> static void assert_reg_lock(struct mv88e6xxx_chip *chip)
> {
>         if (unlikely(!mutex_is_locked(&chip->reg_lock))) {
>                 dev_err(chip->dev, "Switch registers lock not held!\n");
>                 dump_stack();
>         }
> }
> 
> which is a bit heavier in weight, but the MDIO bus transaction will
> dominate the time for such operations, not checking a mutex.

Yes, and then there's also lockdep_assert_held. I knew about those.

Truth be told, I thought sparse would be smarter. But I tested with this
program:

#include <stdio.h>

# define __must_hold(x)	__attribute__((context(x,1,1)))
# define __acquires(x)	__attribute__((context(x,0,1)))
# define __releases(x)	__attribute__((context(x,1,0)))
# define __acquire(x)	__context__(x,1)
# define __release(x)	__context__(x,-1)

static void __acquires(a) lock(int a)
{
	__acquire(a);
}

static void __releases(a) unlock(int a)
{
	__release(a);
}

static void __must_hold(a) fn(int a)
{
	printf("%s: %d\n", __func__, a);
}

int main(int argc, char **argv)
{
	int a = 0;

	unlock(a);
	fn(a);
	lock(a);

	return 0;
}

and it doesn't see any problem whatsoever. It's only good to detect
context imbalances that aren't annotated.

Then I noticed Johannes Berg's sparse commit 2479d0f7819b ("Revert the
context tracking code"), and this discussion:
https://www.spinics.net/lists/linux-sparse/msg03934.html

so.... yeah. It's just about as useful as a very pretentious comment.

I don't know, I can send another version that replaces the __must_hold
with something else, but I'd rather not sprinkle around lockdep_assert_held()
calls the same I did with a static analyzer attribute like it's nothing...
```