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

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Date:   Mon, 26 Apr 2021 23:15:40 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     outreachy-kernel@...glegroups.com, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        David Kershner <david.kershner@...sys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [Outreachy kernel] [PATCH v3] staging: unisys: visorhba: Convert module from IDR to XArray

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