Re: [PATCH v5 01/15] cxl/region: Add region creation ABI

From: Ben Widawsky <ben.widawsky@intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-cxl@vger.kernel.org, patches@lists.linux.dev,
	Alison Schofield <alison.schofield@intel.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Bjorn Helgaas <helgaas@kernel.org>,
	Linux NVDIMM <nvdimm@lists.linux.dev>,
	Linux PCI <linux-pci@vger.kernel.org>
Subject: Re: [PATCH v5 01/15] cxl/region: Add region creation ABI
Date: Thu, 17 Feb 2022 10:58:11 -0800	[thread overview]
Message-ID: <20220217185811.qjct4dlnupgah7lh@intel.com> (raw)
In-Reply-To: <CAPcyv4i83TxCN_-Y3a5CuM2ng9bCAyLm53=wcHWutASd434gkg@mail.gmail.com>

On 22-02-17 09:58:04, Dan Williams wrote:
> On Thu, Feb 17, 2022 at 9:19 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
> >
> > Regions are created as a child of the decoder that encompasses an
> > address space with constraints. Regions have a number of attributes that
> > must be configured before the region can be activated.
> >
> > The ABI is not meant to be secure, but is meant to avoid accidental
> > races. As a result, a buggy process may create a region by name that was
> > allocated by a different process. However, multiple processes which are
> > trying not to race with each other shouldn't need special
> > synchronization to do so.
> >
> > // Allocate a new region name
> > region=$(cat /sys/bus/cxl/devices/decoder0.0/create_region)
> >
> > // Create a new region by name
> > while
> > region=$(cat /sys/bus/cxl/devices/decoder0.0/create_region)
> > ! echo $region > /sys/bus/cxl/devices/decoder0.0/create_region
> > do true; done
> >
> > // Region now exists in sysfs
> > stat -t /sys/bus/cxl/devices/decoder0.0/$region
> >
> > // Delete the region, and name
> > echo $region > /sys/bus/cxl/devices/decoder0.0/delete_region
> >
> > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> 
> Looking good, a few more fixes and cleanups identified below.
> 
> >
> > ---
> > Changes since v4:
> > - Add the missed base attributes addition
> >
> > ---
> >  Documentation/ABI/testing/sysfs-bus-cxl       |  23 ++
> >  .../driver-api/cxl/memory-devices.rst         |  11 +
> >  drivers/cxl/core/Makefile                     |   1 +
> >  drivers/cxl/core/core.h                       |   3 +
> >  drivers/cxl/core/port.c                       |  11 +
> >  drivers/cxl/core/region.c                     | 213 ++++++++++++++++++
> >  drivers/cxl/cxl.h                             |   5 +
> >  drivers/cxl/region.h                          |  23 ++
> >  tools/testing/cxl/Kbuild                      |   1 +
> >  9 files changed, 291 insertions(+)
> >  create mode 100644 drivers/cxl/core/region.c
> >  create mode 100644 drivers/cxl/region.h
> >
> > diff --git a/Documentation/ABI/testing/sysfs-bus-cxl b/Documentation/ABI/testing/sysfs-bus-cxl
> > index 7c2b846521f3..e5db45ea70ad 100644
> > --- a/Documentation/ABI/testing/sysfs-bus-cxl
> > +++ b/Documentation/ABI/testing/sysfs-bus-cxl
> > @@ -163,3 +163,26 @@ Description:
> >                 memory (type-3). The 'target_type' attribute indicates the
> >                 current setting which may dynamically change based on what
> >                 memory regions are activated in this decode hierarchy.
> > +
> > +What:          /sys/bus/cxl/devices/decoderX.Y/create_region
> > +Date:          January, 2022
> > +KernelVersion: v5.18
> > +Contact:       linux-cxl@vger.kernel.org
> > +Description:
> > +               Write a value of the form 'regionX.Y:Z' to instantiate a new
> > +               region within the decode range bounded by decoderX.Y. The value
> > +               written must match the current value returned from reading this
> > +               attribute. This behavior lets the kernel arbitrate racing
> > +               attempts to create a region. The thread that fails to write
> > +               loops and tries the next value. Regions must be created for root
> > +               decoders, and must subsequently configured and bound to a region
> > +               driver before they can be used.
> > +
> > +What:          /sys/bus/cxl/devices/decoderX.Y/delete_region
> > +Date:          January, 2022
> > +KernelVersion: v5.18
> > +Contact:       linux-cxl@vger.kernel.org
> > +Description:
> > +               Deletes the named region.  The attribute expects a region in the
> > +               form "regionX.Y:Z". The region's name, allocated by reading
> > +               create_region, will also be released.
> > diff --git a/Documentation/driver-api/cxl/memory-devices.rst b/Documentation/driver-api/cxl/memory-devices.rst
> > index db476bb170b6..66ddc58a21b1 100644
> > --- a/Documentation/driver-api/cxl/memory-devices.rst
> > +++ b/Documentation/driver-api/cxl/memory-devices.rst
> > @@ -362,6 +362,17 @@ CXL Core
> >  .. kernel-doc:: drivers/cxl/core/mbox.c
> >     :doc: cxl mbox
> >
> > +CXL Regions
> > +-----------
> > +.. kernel-doc:: drivers/cxl/region.h
> > +   :identifiers:
> > +
> > +.. kernel-doc:: drivers/cxl/core/region.c
> > +   :doc: cxl core region
> > +
> > +.. kernel-doc:: drivers/cxl/core/region.c
> > +   :identifiers:
> > +
> >  External Interfaces
> >  ===================
> >
> > diff --git a/drivers/cxl/core/Makefile b/drivers/cxl/core/Makefile
> > index 6d37cd78b151..39ce8f2f2373 100644
> > --- a/drivers/cxl/core/Makefile
> > +++ b/drivers/cxl/core/Makefile
> > @@ -4,6 +4,7 @@ obj-$(CONFIG_CXL_BUS) += cxl_core.o
> >  ccflags-y += -I$(srctree)/drivers/cxl
> >  cxl_core-y := port.o
> >  cxl_core-y += pmem.o
> > +cxl_core-y += region.o
> >  cxl_core-y += regs.o
> >  cxl_core-y += memdev.o
> >  cxl_core-y += mbox.o
> > diff --git a/drivers/cxl/core/core.h b/drivers/cxl/core/core.h
> > index 1a50c0fc399c..adfd42370b28 100644
> > --- a/drivers/cxl/core/core.h
> > +++ b/drivers/cxl/core/core.h
> > @@ -9,6 +9,9 @@ extern const struct device_type cxl_nvdimm_type;
> >
> >  extern struct attribute_group cxl_base_attribute_group;
> >
> > +extern struct device_attribute dev_attr_create_region;
> > +extern struct device_attribute dev_attr_delete_region;
> > +
> >  struct cxl_send_command;
> >  struct cxl_mem_query_commands;
> >  int cxl_query_cmd(struct cxl_memdev *cxlmd,
> > diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
> > index 1e785a3affaa..860e91cae29b 100644
> > --- a/drivers/cxl/core/port.c
> > +++ b/drivers/cxl/core/port.c
> > @@ -213,6 +213,8 @@ static struct attribute_group cxl_decoder_base_attribute_group = {
> >  };
> >
> >  static struct attribute *cxl_decoder_root_attrs[] = {
> > +       &dev_attr_create_region.attr,
> > +       &dev_attr_delete_region.attr,
> >         &dev_attr_cap_pmem.attr,
> >         &dev_attr_cap_ram.attr,
> >         &dev_attr_cap_type2.attr,
> > @@ -270,6 +272,8 @@ static void cxl_decoder_release(struct device *dev)
> >         struct cxl_decoder *cxld = to_cxl_decoder(dev);
> >         struct cxl_port *port = to_cxl_port(dev->parent);
> >
> > +       ida_free(&cxld->region_ida, cxld->next_region_id);
> > +       ida_destroy(&cxld->region_ida);
> >         ida_free(&port->decoder_ida, cxld->id);
> >         kfree(cxld);
> >  }
> > @@ -1244,6 +1248,13 @@ static struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
> >         cxld->target_type = CXL_DECODER_EXPANDER;
> >         cxld->platform_res = (struct resource)DEFINE_RES_MEM(0, 0);
> >
> > +       mutex_init(&cxld->id_lock);
> > +       ida_init(&cxld->region_ida);
> > +       rc = ida_alloc(&cxld->region_ida, GFP_KERNEL);
> > +       if (rc < 0)
> > +               goto err;
> > +
> > +       cxld->next_region_id = rc;
> >         return cxld;
> >  err:
> >         kfree(cxld);
> > diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> > new file mode 100644
> > index 000000000000..5576952e4aa1
> > --- /dev/null
> > +++ b/drivers/cxl/core/region.c
> > @@ -0,0 +1,213 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Copyright(c) 2022 Intel Corporation. All rights reserved. */
> > +#include <linux/device.h>
> > +#include <linux/module.h>
> > +#include <linux/slab.h>
> > +#include <linux/idr.h>
> > +#include <region.h>
> > +#include <cxl.h>
> > +#include "core.h"
> > +
> > +/**
> > + * DOC: cxl core region
> > + *
> > + * CXL Regions represent mapped memory capacity in system physical address
> > + * space. Whereas the CXL Root Decoders identify the bounds of potential CXL
> > + * Memory ranges, Regions represent the active mapped capacity by the HDM
> > + * Decoder Capability structures throughout the Host Bridges, Switches, and
> > + * Endpoints in the topology.
> > + */
> > +
> > +static void cxl_region_release(struct device *dev);
> 
> Why forward declare this versus move cxl_region_type after the definition?
> 
> No other CXL object release functions are forward declared.
> 

I liked having the device_type declared at the top. I will change it to match
the other code.

> > +
> > +static const struct device_type cxl_region_type = {
> > +       .name = "cxl_region",
> > +       .release = cxl_region_release,
> > +};
> > +
> > +static struct cxl_region *to_cxl_region(struct device *dev)
> > +{
> > +       if (dev_WARN_ONCE(dev, dev->type != &cxl_region_type,
> > +                         "not a cxl_region device\n"))
> > +               return NULL;
> > +
> > +       return container_of(dev, struct cxl_region, dev);
> > +}
> > +
> > +static struct cxl_region *cxl_region_alloc(struct cxl_decoder *cxld)
> > +{
> > +       struct cxl_region *cxlr;
> > +       struct device *dev;
> > +
> > +       cxlr = kzalloc(sizeof(*cxlr), GFP_KERNEL);
> > +       if (!cxlr)
> > +               return ERR_PTR(-ENOMEM);
> > +
> > +       dev = &cxlr->dev;
> > +       device_initialize(dev);
> > +       dev->parent = &cxld->dev;
> > +       device_set_pm_not_required(dev);
> > +       dev->bus = &cxl_bus_type;
> > +       dev->type = &cxl_region_type;
> > +
> > +       return cxlr;
> > +}
> > +
> > +static void unregister_region(void *_cxlr)
> > +{
> > +       struct cxl_region *cxlr = _cxlr;
> > +
> > +       if (!test_and_set_bit(REGION_DEAD, &cxlr->flags))
> > +               device_unregister(&cxlr->dev);
> 
> I thought REGION_DEAD was needed to prevent double
> devm_release_action(), not double unregister?
> 

I believe that's correct, repeating what you said on our internal list:

On 22-02-14 14:11:41, Dan Williams wrote:
  True, you do need to solve the race between multiple writers racing to
  do the unregistration, but that could be done with something like:

  if (!test_and_set_bit(REGION_DEAD, &cxlr->flags))
      device_unregister(&cxlr->dev);

So I was just trying to implement what you said. Remainder of the discussion
below...

> > +}
> > +
> > +/**
> > + * devm_cxl_add_region - Adds a region to a decoder
> > + * @cxld: Parent decoder.
> > + * @cxlr: Region to be added to the decoder.
> > + *
> > + * This is the second step of region initialization. Regions exist within an
> > + * address space which is mapped by a @cxld. That @cxld must be a root decoder,
> > + * and it enforces constraints upon the region as it is configured.
> > + *
> > + * Return: 0 if the region was added to the @cxld, else returns negative error
> > + * code. The region will be named "regionX.Y.Z" where X is the port, Y is the
> > + * decoder id, and Z is the region number.
> > + */
> > +static struct cxl_region *devm_cxl_add_region(struct cxl_decoder *cxld)
> > +{
> > +       struct cxl_port *port = to_cxl_port(cxld->dev.parent);
> > +       struct cxl_region *cxlr;
> > +       struct device *dev;
> > +       int rc;
> > +
> > +       cxlr = cxl_region_alloc(cxld);
> > +       if (IS_ERR(cxlr))
> > +               return cxlr;
> > +
> > +       dev = &cxlr->dev;
> > +
> > +       cxlr->id = cxld->next_region_id;
> > +       rc = dev_set_name(dev, "region%d.%d:%d", port->id, cxld->id, cxlr->id);
> > +       if (rc)
> > +               goto err_out;
> > +
> > +       /* affirm that release will have access to the decoder's region ida  */
> > +       get_device(&cxld->dev);
> > +
> > +       rc = device_add(dev);
> > +       if (!rc)
> > +               rc = devm_add_action_or_reset(port->uport, unregister_region,
> > +                                             cxlr);
> > +       if (rc)
> > +               goto err_out;
> 
> All the other usages in device_add() in the subsystem follow the style of:
> 
> rc = device_add(dev);
> if (rc)
>     goto err;
> 
> ...any reason to be unique here and indent the success case?
> 

It allowed skipping a goto. I can change it.

> 
> > +
> > +       return cxlr;
> > +
> > +err_out:
> > +       put_device(dev);
> > +       kfree(cxlr);
> 
> This is a double-free of cxlr;
> 

Because of release()? How does release get called if the region device wasn't
added? Or is there something else?

> > +       return ERR_PTR(rc);
> > +}
> > +
> > +static ssize_t create_region_show(struct device *dev,
> > +                                 struct device_attribute *attr, char *buf)
> > +{
> > +       struct cxl_port *port = to_cxl_port(dev->parent);
> > +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> > +
> > +       return sysfs_emit(buf, "region%d.%d:%d\n", port->id, cxld->id,
> > +                         cxld->next_region_id);
> > +}
> > +
> > +static ssize_t create_region_store(struct device *dev,
> > +                                  struct device_attribute *attr,
> > +                                  const char *buf, size_t len)
> > +{
> > +       struct cxl_port *port = to_cxl_port(dev->parent);
> > +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> > +       struct cxl_region *cxlr;
> > +       int d, p, r, rc = 0;
> > +
> > +       if (sscanf(buf, "region%d.%d:%d", &p, &d, &r) != 3)
> > +               return -EINVAL;
> > +
> > +       if (port->id != p || cxld->id != d)
> > +               return -EINVAL;
> > +
> > +       rc = mutex_lock_interruptible(&cxld->id_lock);
> > +       if (rc)
> > +               return rc;
> > +
> > +       if (cxld->next_region_id != r) {
> > +               rc = -EINVAL;
> > +               goto out;
> > +       }
> > +
> > +       rc = ida_alloc(&cxld->region_ida, GFP_KERNEL);
> > +       if (rc < 0) {
> > +               dev_dbg(dev, "Failed to get next cached id (%d)\n", rc);
> > +               goto out;
> > +       }
> > +
> > +       cxlr = devm_cxl_add_region(cxld);
> > +       if (IS_ERR(cxlr)) {
> > +               rc = PTR_ERR(cxlr);
> > +               goto out;
> > +       }
> > +
> > +       cxld->next_region_id = rc;
> 
> This looks like a leak in the case when devm_cxl_add_region() fails,
> so just move it above that call.
> 
> > +       dev_dbg(dev, "Created %s\n", dev_name(&cxlr->dev));
> > +
> > +out:
> > +       mutex_unlock(&cxld->id_lock);
> > +       return rc ? rc : len;
> 
> if (rc)
>     return rc;
> return len;
> 

Yeah. This was a bug I pointed out already. I have it fixed locally.

> > +}
> > +DEVICE_ATTR_RW(create_region);
> > +
> > +static struct cxl_region *cxl_find_region_by_name(struct cxl_decoder *cxld,
> > +                                                 const char *name)
> > +{
> > +       struct device *region_dev;
> > +
> > +       region_dev = device_find_child_by_name(&cxld->dev, name);
> > +       if (!region_dev)
> > +               return ERR_PTR(-ENOENT);
> > +
> > +       return to_cxl_region(region_dev);
> > +}
> > +
> > +static ssize_t delete_region_store(struct device *dev,
> > +                                  struct device_attribute *attr,
> > +                                  const char *buf, size_t len)
> > +{
> > +       struct cxl_port *port = to_cxl_port(dev->parent);
> > +       struct cxl_decoder *cxld = to_cxl_decoder(dev);
> > +       struct cxl_region *cxlr;
> > +
> > +       cxlr = cxl_find_region_by_name(cxld, buf);
> > +       if (IS_ERR(cxlr))
> > +               return PTR_ERR(cxlr);
> > +
> > +       /* After this, the region is no longer a child of the decoder. */
> > +       devm_release_action(port->uport, unregister_region, cxlr);
> 
> This may trigger a WARN in the case where 2 threads race to trigger
> the release action. I think the DEAD check is needed to gate this
> call, not device_unregister().

Continuing from above use of REGION_DEAD....

Dang. I actually had added a mutex to protect this and I cherry-picked the wrong
patch.

This looks good to me though:

cxlr = cxl_find_region_by_name(cxld, buf);
if (IS_ERR(cxlr))
	return PTR_ERR(cxlr);

if (!test_and_set_bit(REGION_DEAD, &cxlr->flags)) {
	devm_release_action(...)
}

put_device(dev);
return len;

> 
> > +
> > +       /* Release is likely called here, so cxlr is not safe to reference. */
> 
> This is always the case with any put_device(), so no need for this comment.
> 
> > +       put_device(&cxlr->dev);
> > +       cxlr = NULL;
> 
> This NULL assignment has no value.

It was meant to prevent future me from trying to use cxlr after this point,
which I've done. I can remove it.

> 
> > +
> > +       dev_dbg(dev, "Deleted %s\n", buf);
> 
> Not sure a debug statement is needed for something userspace can
> directly view itself with the result code from the sysfs write.
> 

It was helpful for my driver development, but I can remove it.

> > +       return len;
> > +}
> > +DEVICE_ATTR_WO(delete_region);
> > +
> > +static void cxl_region_release(struct device *dev)
> > +{
> > +       struct cxl_decoder *cxld = to_cxl_decoder(dev->parent);
> > +       struct cxl_region *cxlr = to_cxl_region(dev);
> > +
> > +       dev_dbg(&cxld->dev, "Releasing %s\n", dev_name(dev));
> > +       ida_free(&cxld->region_ida, cxlr->id);
> > +       kfree(cxlr);
> > +       put_device(&cxld->dev);
> > +}
> > diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> > index b4047a310340..d5397f7dfcf4 100644
> > --- a/drivers/cxl/cxl.h
> > +++ b/drivers/cxl/cxl.h
> > @@ -221,6 +221,8 @@ enum cxl_decoder_type {
> >   * @target_type: accelerator vs expander (type2 vs type3) selector
> >   * @flags: memory type capabilities and locking
> >   * @target_lock: coordinate coherent reads of the target list
> > + * @region_ida: allocator for region ids.
> > + * @next_region_id: Cached region id for next region.
> >   * @nr_targets: number of elements in @target
> >   * @target: active ordered target list in current decoder configuration
> >   */
> > @@ -236,6 +238,9 @@ struct cxl_decoder {
> >         enum cxl_decoder_type target_type;
> >         unsigned long flags;
> >         seqlock_t target_lock;
> > +       struct mutex id_lock;
> > +       struct ida region_ida;
> > +       int next_region_id;
> >         int nr_targets;
> >         struct cxl_dport *target[];
> >  };
> > diff --git a/drivers/cxl/region.h b/drivers/cxl/region.h
> > new file mode 100644
> > index 000000000000..0016f83bbdfd
> > --- /dev/null
> > +++ b/drivers/cxl/region.h
> > @@ -0,0 +1,23 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/* Copyright(c) 2021 Intel Corporation. */
> > +#ifndef __CXL_REGION_H__
> > +#define __CXL_REGION_H__
> > +
> > +#include <linux/uuid.h>
> > +
> > +#include "cxl.h"
> > +
> > +/**
> > + * struct cxl_region - CXL region
> > + * @dev: This region's device.
> > + * @id: This region's id. Id is globally unique across all regions.
> > + * @flags: Flags representing the current state of the region.
> > + */
> > +struct cxl_region {
> > +       struct device dev;
> > +       int id;
> > +       unsigned long flags;
> > +#define REGION_DEAD 0
> > +};
> > +
> > +#endif
> > diff --git a/tools/testing/cxl/Kbuild b/tools/testing/cxl/Kbuild
> > index 82e49ab0937d..3fe6d34e6d59 100644
> > --- a/tools/testing/cxl/Kbuild
> > +++ b/tools/testing/cxl/Kbuild
> > @@ -46,6 +46,7 @@ cxl_core-y += $(CXL_CORE_SRC)/memdev.o
> >  cxl_core-y += $(CXL_CORE_SRC)/mbox.o
> >  cxl_core-y += $(CXL_CORE_SRC)/pci.o
> >  cxl_core-y += $(CXL_CORE_SRC)/hdm.o
> > +cxl_core-y += $(CXL_CORE_SRC)/region.o
> >  cxl_core-y += config_check.o
> >
> >  obj-m += test/
> > --
> > 2.35.1
> >