[Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Fabio M. De Francesco]

Converted visorhba from IDR to XArray. The abstract data type XArray is
more memory-efficient, parallelisable and cache friendly. It takes
advantage of RCU to perform lookups without locking.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---

Changes from v2: Some residual errors from v1 were not fixed in v2. Now
they have been removed.
Changes from v1: After a first review by Matthew Wilcox, who found a
series of errors and gave suggestions on how to fix them, I rewrote a
larger part of the patch.

 .../staging/unisys/visorhba/visorhba_main.c   | 89 ++++++-------------
 1 file changed, 25 insertions(+), 64 deletions(-)

diff --git a/drivers/staging/unisys/visorhba/visorhba_main.c b/drivers/staging/unisys/visorhba/visorhba_main.c
index 4455d26f7c96..938a5081b835 100644
--- a/drivers/staging/unisys/visorhba/visorhba_main.c
+++ b/drivers/staging/unisys/visorhba/visorhba_main.c
@@ -6,10 +6,10 @@
 
 #include <linux/debugfs.h>
 #include <linux/kthread.h>
-#include <linux/idr.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
 #include <linux/visorbus.h>
+#include <linux/xarray.h>
 #include <scsi/scsi.h>
 #include <scsi/scsi_host.h>
 #include <scsi/scsi_cmnd.h>
@@ -82,7 +82,7 @@ struct visorhba_devdata {
 	 * allows us to pass int handles back-and-forth between us and
 	 * iovm, instead of raw pointers
 	 */
-	struct idr idr;
+	struct xarray xa;
 
 	struct dentry *debugfs_dir;
 	struct dentry *debugfs_info;
@@ -182,71 +182,40 @@ static struct uiscmdrsp *get_scsipending_cmdrsp(struct visorhba_devdata *ddata,
 	return NULL;
 }
 
-/*
- * simple_idr_get - Associate a provided pointer with an int value
- *		    1 <= value <= INT_MAX, and return this int value;
- *		    the pointer value can be obtained later by passing
- *		    this int value to idr_find()
- * @idrtable: The data object maintaining the pointer<-->int mappings
- * @p:	      The pointer value to be remembered
- * @lock:     A spinlock used when exclusive access to idrtable is needed
- *
- * Return: The id number mapped to pointer 'p', 0 on failure
- */
-static unsigned int simple_idr_get(struct idr *idrtable, void *p,
-				   spinlock_t *lock)
-{
-	int id;
-	unsigned long flags;
-
-	idr_preload(GFP_KERNEL);
-	spin_lock_irqsave(lock, flags);
-	id = idr_alloc(idrtable, p, 1, INT_MAX, GFP_NOWAIT);
-	spin_unlock_irqrestore(lock, flags);
-	idr_preload_end();
-	/* failure */
-	if (id < 0)
-		return 0;
-	/* idr_alloc() guarantees > 0 */
-	return (unsigned int)(id);
-}
-
 /*
  * setup_scsitaskmgmt_handles - Stash the necessary handles so that the
  *				completion processing logic for a taskmgmt
  *				cmd will be able to find who to wake up
  *				and where to stash the result
- * @idrtable: The data object maintaining the pointer<-->int mappings
- * @lock:     A spinlock used when exclusive access to idrtable is needed
+ * @xa:       The data object maintaining the pointer<-->int mappings
  * @cmdrsp:   Response from the IOVM
  * @event:    The event handle to associate with an id
  * @result:   The location to place the result of the event handle into
  */
-static void setup_scsitaskmgmt_handles(struct idr *idrtable, spinlock_t *lock,
-				       struct uiscmdrsp *cmdrsp,
+static void setup_scsitaskmgmt_handles(struct xarray *xa, struct uiscmdrsp *cmdrsp,
 				       wait_queue_head_t *event, int *result)
 {
-	/* specify the event that has to be triggered when this */
-	/* cmd is complete */
-	cmdrsp->scsitaskmgmt.notify_handle =
-		simple_idr_get(idrtable, event, lock);
-	cmdrsp->scsitaskmgmt.notifyresult_handle =
-		simple_idr_get(idrtable, result, lock);
+	u32 id;
+	int ret;
+
+	/* specify the event that has to be triggered when this cmd is complete */
+	id = (u32)cmdrsp->scsitaskmgmt.notify_handle;
+	ret = xa_alloc_irq(xa, &id, event, XA_LIMIT(1, INT_MAX), GFP_KERNEL);
+	id = (u32)cmdrsp->scsitaskmgmt.notifyresult_handle;
+	ret = xa_alloc_irq(xa, &id, result, XA_LIMIT(1, INT_MAX), GFP_KERNEL);
 }
 
 /*
  * cleanup_scsitaskmgmt_handles - Forget handles created by
  *				  setup_scsitaskmgmt_handles()
- * @idrtable: The data object maintaining the pointer<-->int mappings
+ * @xa: The data object maintaining the pointer<-->int mappings
  * @cmdrsp:   Response from the IOVM
  */
-static void cleanup_scsitaskmgmt_handles(struct idr *idrtable,
+static void cleanup_scsitaskmgmt_handles(struct xarray *xa,
 					 struct uiscmdrsp *cmdrsp)
 {
-	if (cmdrsp->scsitaskmgmt.notify_handle)
-		idr_remove(idrtable, cmdrsp->scsitaskmgmt.notify_handle);
-	if (cmdrsp->scsitaskmgmt.notifyresult_handle)
-		idr_remove(idrtable, cmdrsp->scsitaskmgmt.notifyresult_handle);
+	xa_erase(xa, cmdrsp->scsitaskmgmt.notify_handle);
+	xa_erase(xa, cmdrsp->scsitaskmgmt.notifyresult_handle);
 }
 
 /*
@@ -273,8 +242,7 @@ static int forward_taskmgmt_command(enum task_mgmt_types tasktype,
 	if (devdata->serverdown || devdata->serverchangingstate)
 		return FAILED;
 
-	scsicmd_id = add_scsipending_entry(devdata, CMD_SCSITASKMGMT_TYPE,
-					   NULL);
+	scsicmd_id = add_scsipending_entry(devdata, CMD_SCSITASKMGMT_TYPE, NULL);
 	if (scsicmd_id < 0)
 		return FAILED;
 
@@ -284,8 +252,7 @@ static int forward_taskmgmt_command(enum task_mgmt_types tasktype,
 
 	/* issue TASK_MGMT_ABORT_TASK */
 	cmdrsp->cmdtype = CMD_SCSITASKMGMT_TYPE;
-	setup_scsitaskmgmt_handles(&devdata->idr, &devdata->privlock, cmdrsp,
-				   &notifyevent, &notifyresult);
+	setup_scsitaskmgmt_handles(&devdata->xa, cmdrsp, &notifyevent, &notifyresult);
 
 	/* save destination */
 	cmdrsp->scsitaskmgmt.tasktype = tasktype;
@@ -311,14 +278,14 @@ static int forward_taskmgmt_command(enum task_mgmt_types tasktype,
 	dev_dbg(&scsidev->sdev_gendev,
 		"visorhba: taskmgmt type=%d success; result=0x%x\n",
 		 tasktype, notifyresult);
-	cleanup_scsitaskmgmt_handles(&devdata->idr, cmdrsp);
+	cleanup_scsitaskmgmt_handles(&devdata->xa, cmdrsp);
 	return SUCCESS;
 
 err_del_scsipending_ent:
 	dev_dbg(&scsidev->sdev_gendev,
 		"visorhba: taskmgmt type=%d not executed\n", tasktype);
 	del_scsipending_ent(devdata, scsicmd_id);
-	cleanup_scsitaskmgmt_handles(&devdata->idr, cmdrsp);
+	cleanup_scsitaskmgmt_handles(&devdata->xa, cmdrsp);
 	return FAILED;
 }
 
@@ -654,13 +621,12 @@ DEFINE_SHOW_ATTRIBUTE(info_debugfs);
  * Service Partition returned the result of the task management
  * command. Wake up anyone waiting for it.
  */
-static void complete_taskmgmt_command(struct idr *idrtable,
-				      struct uiscmdrsp *cmdrsp, int result)
+static void complete_taskmgmt_command(struct xarray *xa, struct uiscmdrsp *cmdrsp, int result)
 {
 	wait_queue_head_t *wq =
-		idr_find(idrtable, cmdrsp->scsitaskmgmt.notify_handle);
+		xa_load(xa, cmdrsp->scsitaskmgmt.notify_handle);
 	int *scsi_result_ptr =
-		idr_find(idrtable, cmdrsp->scsitaskmgmt.notifyresult_handle);
+		xa_load(xa, cmdrsp->scsitaskmgmt.notifyresult_handle);
 	if (unlikely(!(wq && scsi_result_ptr))) {
 		pr_err("visorhba: no completion context; cmd will time out\n");
 		return;
@@ -708,8 +674,7 @@ static void visorhba_serverdown_complete(struct visorhba_devdata *devdata)
 			break;
 		case CMD_SCSITASKMGMT_TYPE:
 			cmdrsp = pendingdel->sent;
-			complete_taskmgmt_command(&devdata->idr, cmdrsp,
-						  TASK_MGMT_FAILED);
+			complete_taskmgmt_command(&devdata->xa, cmdrsp, TASK_MGMT_FAILED);
 			break;
 		default:
 			break;
@@ -905,7 +870,7 @@ static void drain_queue(struct uiscmdrsp *cmdrsp,
 			if (!del_scsipending_ent(devdata,
 						 cmdrsp->scsitaskmgmt.handle))
 				break;
-			complete_taskmgmt_command(&devdata->idr, cmdrsp,
+			complete_taskmgmt_command(&devdata->xa, cmdrsp,
 						  cmdrsp->scsitaskmgmt.result);
 		} else if (cmdrsp->cmdtype == CMD_NOTIFYGUEST_TYPE)
 			dev_err_once(&devdata->dev->device,
@@ -1053,8 +1018,6 @@ static int visorhba_probe(struct visor_device *dev)
 	if (err)
 		goto err_debugfs_info;
 
-	idr_init(&devdata->idr);
-
 	devdata->cmdrsp = kmalloc(sizeof(*devdata->cmdrsp), GFP_ATOMIC);
 	visorbus_enable_channel_interrupts(dev);
 
@@ -1096,8 +1059,6 @@ static void visorhba_remove(struct visor_device *dev)
 	scsi_remove_host(scsihost);
 	scsi_host_put(scsihost);
 
-	idr_destroy(&devdata->idr);
-
 	dev_set_drvdata(&dev->device, NULL);
 	debugfs_remove(devdata->debugfs_info);
 	debugfs_remove_recursive(devdata->debugfs_dir);
-- 
2.31.1

Re: [Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Matthew Wilcox]

On Mon, Apr 26, 2021 at 08:42:45PM +0200, Fabio M. De Francesco wrote:
> +static void setup_scsitaskmgmt_handles(struct xarray *xa, struct uiscmdrsp *cmdrsp,
>  				       wait_queue_head_t *event, int *result)
>  {
> -	/* specify the event that has to be triggered when this */
> -	/* cmd is complete */
> -	cmdrsp->scsitaskmgmt.notify_handle =
> -		simple_idr_get(idrtable, event, lock);
> -	cmdrsp->scsitaskmgmt.notifyresult_handle =
> -		simple_idr_get(idrtable, result, lock);
> +	u32 id;
> +	int ret;
> +
> +	/* specify the event that has to be triggered when this cmd is complete */
> +	id = (u32)cmdrsp->scsitaskmgmt.notify_handle;
> +	ret = xa_alloc_irq(xa, &id, event, XA_LIMIT(1, INT_MAX), GFP_KERNEL);

OK, think this one through a bit.  When xa_alloc_irq() stores the ID that
it assigned into 'id', what happens to it next?

> +	id = (u32)cmdrsp->scsitaskmgmt.notifyresult_handle;
> +	ret = xa_alloc_irq(xa, &id, result, XA_LIMIT(1, INT_MAX), GFP_KERNEL);
>  }

> @@ -1053,8 +1018,6 @@ static int visorhba_probe(struct visor_device *dev)
>  	if (err)
>  		goto err_debugfs_info;
>  
> -	idr_init(&devdata->idr);

You still need to initialise the XArray, either with xa_init() or by
using DEFINE_XARRAY.  Since it's dynamically allocated, the former is
correct.

Re: [Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Fabio M. De Francesco]

On Monday, April 26, 2021 9:21:01 PM CEST Matthew Wilcox wrote:
> On Mon, Apr 26, 2021 at 08:42:45PM +0200, Fabio M. De Francesco wrote:
> > +static void setup_scsitaskmgmt_handles(struct xarray *xa, struct 
uiscmdrsp *cmdrsp,
> > 
> >  				       wait_queue_head_t *event, 
int *result)
> >  
> >  {
> > 
> > -	/* specify the event that has to be triggered when this */
> > -	/* cmd is complete */
> > -	cmdrsp->scsitaskmgmt.notify_handle =
> > -		simple_idr_get(idrtable, event, lock);
> > -	cmdrsp->scsitaskmgmt.notifyresult_handle =
> > -		simple_idr_get(idrtable, result, lock);
> > +	u32 id;
> > +	int ret;
> > +
> > +	/* specify the event that has to be triggered when this cmd is 
complete */
> > +	id = (u32)cmdrsp->scsitaskmgmt.notify_handle;
> > +	ret = xa_alloc_irq(xa, &id, event, XA_LIMIT(1, INT_MAX), 
GFP_KERNEL);
> 
> OK, think this one through a bit.  When xa_alloc_irq() stores the ID that
> it assigned into 'id', what happens to it next?
>
Oh, I overlooked that... The ID in 'id' is lost when the function exits and 
the stack frame is unwound.

Now I have another problem: xa_alloc_irq() writes id u32* but I have an u64* 
in 'id'. What happens if I cast 'id' to an u32* when passing it to 
xa_alloc_irq()?

u64 *id;
int ret;
id = &cmdrsp->scsitaskmgmt.notify_handle;
ret = xa_alloc_irq(xa, (u32 *)id, event, XA_LIMIT(1, INT_MAX), GFP_KERNEL);

Do I destroy the information stored in 'id' with that cast?
> 
> > +	id = (u32)cmdrsp->scsitaskmgmt.notifyresult_handle;
> > +	ret = xa_alloc_irq(xa, &id, result, XA_LIMIT(1, INT_MAX), 
GFP_KERNEL);
> > 
> >  }
> > 
> > @@ -1053,8 +1018,6 @@ static int visorhba_probe(struct visor_device *dev)
> > 
> >  	if (err)
> >  	
> >  		goto err_debugfs_info;
> > 
> > -	idr_init(&devdata->idr);
> 
> You still need to initialise the XArray, either with xa_init() or by
> using DEFINE_XARRAY.  Since it's dynamically allocated, the former is
> correct.
>
I had read that XArray must be initialized with xa_init(). However, when I 
deleted the line with the initialization of the IDR, I forgot to replace it 
with a statement that uses xa_init().

Thanks,

Fabio

Re: [Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Matthew Wilcox]

On Mon, Apr 26, 2021 at 11:15:40PM +0200, Fabio M. De Francesco wrote:
> On Monday, April 26, 2021 9:21:01 PM CEST Matthew Wilcox wrote:
> > On Mon, Apr 26, 2021 at 08:42:45PM +0200, Fabio M. De Francesco wrote:
> > > +static void setup_scsitaskmgmt_handles(struct xarray *xa, struct 
> uiscmdrsp *cmdrsp,
> > > 
> > >  				       wait_queue_head_t *event, 
> int *result)
> > >  
> > >  {
> > > 
> > > -	/* specify the event that has to be triggered when this */
> > > -	/* cmd is complete */
> > > -	cmdrsp->scsitaskmgmt.notify_handle =
> > > -		simple_idr_get(idrtable, event, lock);
> > > -	cmdrsp->scsitaskmgmt.notifyresult_handle =
> > > -		simple_idr_get(idrtable, result, lock);
> > > +	u32 id;
> > > +	int ret;
> > > +
> > > +	/* specify the event that has to be triggered when this cmd is 
> complete */
> > > +	id = (u32)cmdrsp->scsitaskmgmt.notify_handle;
> > > +	ret = xa_alloc_irq(xa, &id, event, XA_LIMIT(1, INT_MAX), 
> GFP_KERNEL);
> > 
> > OK, think this one through a bit.  When xa_alloc_irq() stores the ID that
> > it assigned into 'id', what happens to it next?
> >
> Oh, I overlooked that... The ID in 'id' is lost when the function exits and 
> the stack frame is unwound.
> 
> Now I have another problem: xa_alloc_irq() writes id u32* but I have an u64* 
> in 'id'. What happens if I cast 'id' to an u32* when passing it to 
> xa_alloc_irq()?
> 
> u64 *id;
> int ret;
> id = &cmdrsp->scsitaskmgmt.notify_handle;
> ret = xa_alloc_irq(xa, (u32 *)id, event, XA_LIMIT(1, INT_MAX), GFP_KERNEL);
> 
> Do I destroy the information stored in 'id' with that cast?

That is a great question!  That would be a really serious bug because
it behaves differently on big and little endian systems.  That is, on a
little endian system, a pointer to a u64 can be treated as a pointer to a
u32 and it will write to the bottom 32 bits of the u64.  On a big endian
system, treating a pointer to a u64 as if it's a pointer to a u32 means
you write to the _top_ 32 bits of the u64, and things go wrong from there!

Similarly, if you have a u16, you can't pass a pointer to it, because
the called function has no idea that it's only 16 bits, and will do a
32-bit store to it, overwriting the 16 bits after it.

So you need to pass a pointer to a u32 on the stack, and then copy the
id out of it afterwards.

Re: [Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

[Fabio M. De Francesco]

On Tuesday, April 27, 2021 12:03:27 AM CEST Matthew Wilcox wrote:
> On Mon, Apr 26, 2021 at 11:15:40PM +0200, Fabio M. De Francesco wrote:
> > On Monday, April 26, 2021 9:21:01 PM CEST Matthew Wilcox wrote:
> > > On Mon, Apr 26, 2021 at 08:42:45PM +0200, Fabio M. De Francesco wrote:
> > > > +static void setup_scsitaskmgmt_handles(struct xarray *xa, struct
> > 
> > uiscmdrsp *cmdrsp,
> > 
> > > >  				       wait_queue_head_t *event,
> > 
> > int *result)
> > 
> > > >  {
> > > > 
> > > > -	/* specify the event that has to be triggered when this */
> > > > -	/* cmd is complete */
> > > > -	cmdrsp->scsitaskmgmt.notify_handle =
> > > > -		simple_idr_get(idrtable, event, lock);
> > > > -	cmdrsp->scsitaskmgmt.notifyresult_handle =
> > > > -		simple_idr_get(idrtable, result, lock);
> > > > +	u32 id;
> > > > +	int ret;
> > > > +
> > > > +	/* specify the event that has to be triggered when this cmd is
> > 
> > complete */
> > 
> > > > +	id = (u32)cmdrsp->scsitaskmgmt.notify_handle;
> > > > +	ret = xa_alloc_irq(xa, &id, event, XA_LIMIT(1, INT_MAX),
> > 
> > GFP_KERNEL);
> > 
> > > OK, think this one through a bit.  When xa_alloc_irq() stores the ID 
that
> > > it assigned into 'id', what happens to it next?
> > 
> > Oh, I overlooked that... The ID in 'id' is lost when the function exits 
and
> > the stack frame is unwound.
> > 
> > Now I have another problem: xa_alloc_irq() writes id u32* but I have an 
u64*
> > in 'id'. What happens if I cast 'id' to an u32* when passing it to
> > xa_alloc_irq()?
> > 
> > u64 *id;
> > int ret;
> > id = &cmdrsp->scsitaskmgmt.notify_handle;
> > ret = xa_alloc_irq(xa, (u32 *)id, event, XA_LIMIT(1, INT_MAX), 
GFP_KERNEL);
> > 
> > Do I destroy the information stored in 'id' with that cast?
> 
> That is a great question!  That would be a really serious bug because
> it behaves differently on big and little endian systems.  That is, on a
> little endian system, a pointer to a u64 can be treated as a pointer to a
> u32 and it will write to the bottom 32 bits of the u64.  On a big endian
> system, treating a pointer to a u64 as if it's a pointer to a u32 means
> you write to the _top_ 32 bits of the u64, and things go wrong from there!
> 
> Similarly, if you have a u16, you can't pass a pointer to it, because
> the called function has no idea that it's only 16 bits, and will do a
> 32-bit store to it, overwriting the 16 bits after it.
> 
> So you need to pass a pointer to a u32 on the stack, and then copy the
> id out of it afterwards.
>
As far as I understand, in setup_scsitaskmgmt_handles(), the task of  
xa_alloc_irq() is: find two empty entries in 'xa', store the indexes into the 
'id'  pointer (before the first call this is set to &cmdrsp-
>scsitaskmgmt.notify_handle, and before the second call it is changed to 
&cmdrsp->scsitaskmgmt.notifyresult_handle), then store the entries at their 
respective indexes.

Indexes that xa_alloc_* set are of type u32*, so why not just change the type 
of notify_handle and notifyresult_handle from u64 to u32?

Furthermore, I cannot understand why those indexes should be passed in and out 
as arguments of the function . It seems that they are not needed anywhere else 
in that file. Are they?

Maybe that I'm still missing something...

Thanks,

Fabio