Re: [PATCH 13/23] cxl/core: Move target population locking to caller

[Ben Widawsky <ben.widawsky@intel.com>]

On 21-11-22 16:33:02, Jonathan Cameron wrote:
> On Fri, 19 Nov 2021 16:02:40 -0800
> Ben Widawsky <ben.widawsky@intel.com> wrote:
> 
> > In preparation for a port driver that enumerates a descendant port +
> > decoder hierarchy, arrange for an unlocked version of cxl_decoder_add().
> > Otherwise a port-driver that adds a child decoder will deadlock on the
> > device_lock() in ->probe().
> > 
> 
> I think this description should call out that the lock was originally taken
> for a much shorter time in decoder_populate_targets() but is moved
> up one layer.

Sounds good.

> 
> One other query inline.  Seems like we the WARN_ON stuff is a bit
> over paranoid given what's visible in this patch.  If there is a
> good reason for that, then add something to the patch description to
> justify it.
>  
> > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > 
> > ---
> > 
> > Changes since RFCv2:
> > - Reword commit message (Dan)
> > - Move decoder API changes into this patch (Dan)
> > ---
> >  drivers/cxl/core/bus.c | 59 +++++++++++++++++++++++++++++++-----------
> >  drivers/cxl/cxl.h      |  1 +
> >  2 files changed, 45 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/cxl/core/bus.c b/drivers/cxl/core/bus.c
> > index 16b15f54fb62..cd6fe7823c69 100644
> > --- a/drivers/cxl/core/bus.c
> > +++ b/drivers/cxl/core/bus.c
> > @@ -487,28 +487,22 @@ static int decoder_populate_targets(struct cxl_decoder *cxld,
> >  {
> >  	int rc = 0, i;
> >  
> > +	device_lock_assert(&port->dev);
> > +
> >  	if (!target_map)
> >  		return 0;
> >  
> > -	device_lock(&port->dev);
> > -	if (list_empty(&port->dports)) {
> > -		rc = -EINVAL;
> > -		goto out_unlock;
> > -	}
> > +	if (list_empty(&port->dports))
> > +		return -EINVAL;
> >  
> >  	for (i = 0; i < cxld->nr_targets; i++) {
> >  		struct cxl_dport *dport = find_dport(port, target_map[i]);
> >  
> > -		if (!dport) {
> > -			rc = -ENXIO;
> > -			goto out_unlock;
> > -		}
> > +		if (!dport)
> > +			return -ENXIO;
> >  		cxld->target[i] = dport;
> >  	}
> >  
> > -out_unlock:
> > -	device_unlock(&port->dev);
> > -
> >  	return rc;
> >  }
> >  
> > @@ -571,7 +565,7 @@ struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
> >  EXPORT_SYMBOL_NS_GPL(cxl_decoder_alloc, CXL);
> >  
> >  /**
> > - * cxl_decoder_add - Add a decoder with targets
> > + * cxl_decoder_add_locked - Add a decoder with targets
> >   * @cxld: The cxl decoder allocated by cxl_decoder_alloc()
> >   * @target_map: A list of downstream ports that this decoder can direct memory
> >   *              traffic to. These numbers should correspond with the port number
> > @@ -581,12 +575,14 @@ EXPORT_SYMBOL_NS_GPL(cxl_decoder_alloc, CXL);
> >   * is an endpoint device. A more awkward example is a hostbridge whose root
> >   * ports get hot added (technically possible, though unlikely).
> >   *
> > - * Context: Process context. Takes and releases the cxld's device lock.
> > + * This is the locked variant of cxl_decoder_add().
> > + *
> > + * Context: Process context. Expects the cxld's device lock to be held.
> >   *
> >   * Return: Negative error code if the decoder wasn't properly configured; else
> >   *	   returns 0.
> >   */
> > -int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map)
> > +int cxl_decoder_add_locked(struct cxl_decoder *cxld, int *target_map)
> >  {
> >  	struct cxl_port *port;
> >  	struct device *dev;
> > @@ -619,6 +615,39 @@ int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map)
> >  
> >  	return device_add(dev);
> >  }
> > +EXPORT_SYMBOL_NS_GPL(cxl_decoder_add_locked, CXL);
> > +
> > +/**
> > + * cxl_decoder_add - Add a decoder with targets
> > + * @cxld: The cxl decoder allocated by cxl_decoder_alloc()
> > + * @target_map: A list of downstream ports that this decoder can direct memory
> > + *              traffic to. These numbers should correspond with the port number
> > + *              in the PCIe Link Capabilities structure.
> > + *
> > + * This is the unlocked variant of cxl_decoder_add_locked().
> > + * See cxl_decoder_add_locked().
> > + *
> > + * Context: Process context. Takes and releases the cxld's device lock.
> > + */
> > +int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map)
> > +{
> > +	struct cxl_port *port;
> > +	int rc;
> > +
> > +	if (WARN_ON_ONCE(!cxld))
> > +		return -EINVAL;
> 
> Why do we now need these protections but didn't before?

I don't quite understand what you're trying to point out.

Prior to this patch, cxl_decoder_add() checks:
- !cxld
- IS_ERR(cxld)
- cxld->interleave_ways != 0

After this patch, cxl_decoder_add() checks:
- !cxld
- IS_ERR(cxld)
- (and then calls cxl_decoder_add_locked())

And cxl_decoder_add_locked() checks:
- !cxld
- IS_ERR(cxld)
- cxld->interleave_ways != 0

Ultimately we want to check all 3, and since cxl_decoder_add() calls
cxl_decoder_add_locked(), we're good there. The problem is to get from a cxld to
a port, you need to make sure you have a valid cxld, and the API previously
allowed !cxld and IS_ERR(cxld). So there are duplicative checks if you call
cxl_decoder_add(), but other than that I don't see any new protections.

> 
> 
> > +
> > +	if (WARN_ON_ONCE(IS_ERR(cxld)))
> > +		return PTR_ERR(cxld);
> > +
> > +	port = to_cxl_port(cxld->dev.parent);
> > +
> > +	device_lock(&port->dev);
> > +	rc = cxl_decoder_add_locked(cxld, target_map);
> > +	device_unlock(&port->dev);
> > +
> > +	return rc;
> > +}
> >  EXPORT_SYMBOL_NS_GPL(cxl_decoder_add, CXL);
> >  
> >  static void cxld_unregister(void *dev)
> > diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> > index b66ed8f241c6..2c5627fa8a34 100644
> > --- a/drivers/cxl/cxl.h
> > +++ b/drivers/cxl/cxl.h
> > @@ -290,6 +290,7 @@ struct cxl_decoder *to_cxl_decoder(struct device *dev);
> >  bool is_root_decoder(struct device *dev);
> >  struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
> >  				      unsigned int nr_targets);
> > +int cxl_decoder_add_locked(struct cxl_decoder *cxld, int *target_map);
> >  int cxl_decoder_add(struct cxl_decoder *cxld, int *target_map);
> >  int cxl_decoder_autoremove(struct device *host, struct cxl_decoder *cxld);
> >  
>