Re: [PATCH v2 8/8] cxl: Convert to ACQUIRE() for conditional rwsem locking

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Wed, 18 Jun 2025 22:04:16 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> Use ACQUIRE() to cleanup conditional locking paths in the CXL driver
> The ACQUIRE() macro and its associated ACQUIRE_ERR() helpers, like
> scoped_cond_guard(), arrange for scoped-based conditional locking. Unlike
> scoped_cond_guard(), these macros arrange for an ERR_PTR() to be retrieved
> representing the state of the conditional lock.
> 
> The goal of this conversion is to complete the removal of all explicit
> unlock calls in the subsystem. I.e. the methods to acquire a lock are
> solely via guard(), scoped_guard() (for limited cases), or ACQUIRE(). All
> unlock is implicit / scope-based. In order to make sure all lock sites are
> converted, the existing rwsem's are consolidated and renamed in 'struct
> cxl_rwsem'. While that makes the patch noisier it gives a clean cut-off
> between old-world (explicit unlock allowed), and new world (explicit unlock
> deleted).
> 
> Cc: David Lechner <dlechner@baylibre.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: "Fabio M. De Francesco" <fabio.maria.de.francesco@linux.intel.com>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Alison Schofield <alison.schofield@intel.com>
> Cc: Vishal Verma <vishal.l.verma@intel.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Shiju Jose <shiju.jose@huawei.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
A few comments inline.




> index 010964aa5489..a2ba19151d4f 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c


>  static DEVICE_ATTR_RW(interleave_ways);
> @@ -561,15 +537,11 @@ static ssize_t interleave_granularity_show(struct device *dev,
>  {
>  	struct cxl_region *cxlr = to_cxl_region(dev);
>  	struct cxl_region_params *p = &cxlr->params;
> -	ssize_t rc;
> -
> -	rc = down_read_interruptible(&cxl_region_rwsem);
> -	if (rc)
> -		return rc;
> -	rc = sysfs_emit(buf, "%d\n", p->interleave_granularity);
> -	up_read(&cxl_region_rwsem);
>  
> -	return rc;
> +	ACQUIRE(rwsem_read_intr, rwsem)(&cxl_rwsem.region);
> +	if (ACQUIRE_ERR(rwsem_read_intr, &rwsem))
> +		return ACQUIRE_ERR(rwsem_read_intr, &rwsem);

Local variable?

> +	return sysfs_emit(buf, "%d\n", p->interleave_granularity);
>  }

> @@ -2212,19 +2167,19 @@ static int attach_target(struct cxl_region *cxlr,
>  			 struct cxl_endpoint_decoder *cxled, int pos,
>  			 unsigned int state)
>  {
> -	int rc = 0;
> -
> -	if (state == TASK_INTERRUPTIBLE)
> -		rc = down_write_killable(&cxl_region_rwsem);
> -	else
> -		down_write(&cxl_region_rwsem);
> -	if (rc)
> -		return rc;
> +	int rc;
>  
> -	down_read(&cxl_dpa_rwsem);
> -	rc = cxl_region_attach(cxlr, cxled, pos);
> -	up_read(&cxl_dpa_rwsem);
> -	up_write(&cxl_region_rwsem);
> +	if (state == TASK_INTERRUPTIBLE) {
> +		ACQUIRE(rwsem_write_kill, rwsem)(&cxl_rwsem.region);
> +		if ((rc = ACQUIRE_ERR(rwsem_write_kill, &rwsem)) == 0) {

I'd lift the warning print to a wrapper function so that you can
return early in error case and avoid this rather messy line.
e.g.

static int do_attach_target(struct cxl_region *cxlr,
			    struct cxl_endpoint_decoder *cxled, int pos,
			    unsigned int state)

	if (state == TASK_INTERRUPTIBLE) {
		ACQUIRE(rwsem_write_kill, rwsem)(&cxl_rwsem.region);
		rc = ACQUIRE_ERR(rwsem_write_kill, &rwsem);
		if (rc)
			return rc;

		guard(rwsem_read)(&cxl_rwsem.dpa);
		return cxl_region_attach(cxlr, cxled, pos);
	}
	
	guard(rwsem_write)(&cxl_rwsem.region);
	guard(rwsem_read)(&cxl_rwsem.dpa);
	return cxl_region_attach(cxlr, cxled, pos);		
}

static int attach_target(struct cxl_region *cxlr,
			 struct cxl_endpoint_decoder *cxled, int pos,
			 unsigned int state)
{
	int rc = do_attach_target(cxlr, cxled, pos, state);

	if (rc)
		dev_warn();
	
	return rc;
}

> +			guard(rwsem_read)(&cxl_rwsem.dpa);
> +			rc = cxl_region_attach(cxlr, cxled, pos);
> +		}
> +	} else {
> +		guard(rwsem_write)(&cxl_rwsem.region);
> +		guard(rwsem_read)(&cxl_rwsem.dpa);
> +		rc = cxl_region_attach(cxlr, cxled, pos);
> +	}
>  
>  	if (rc)
>  		dev_warn(cxled->cxld.dev.parent,


> @@ -3569,30 +3520,23 @@ static int cxl_region_can_probe(struct cxl_region *cxlr)
>  	struct cxl_region_params *p = &cxlr->params;
>  	int rc;
>  
> -	rc = down_read_interruptible(&cxl_region_rwsem);
> -	if (rc) {
> +	ACQUIRE(rwsem_read_intr, rwsem)(&cxl_rwsem.region);

Similar to earlier - I'd do this next bit in two lines for slightly
better readability.  Same for the other cases. I don't care that strongly
though.

> +	if ((rc = ACQUIRE_ERR(rwsem_read_intr, &rwsem))) {
>  		dev_dbg(&cxlr->dev, "probe interrupted\n");
>  		return rc;
>  	}