cxl/core/port: Use dedicated lock for decoder target list

Lockdep reports:

 ======================================================
 WARNING: possible circular locking dependency detected
 5.16.0-rc1+ #142 Tainted: G           OE
 ------------------------------------------------------
 cxl/1220 is trying to acquire lock:
 ffff979b85475460 (kn->active#144){++++}-{0:0}, at: __kernfs_remove+0x1ab/0x1e0

 but task is already holding lock:
 ffff979b87ab38e8 (&dev->lockdep_mutex#2/4){+.+.}-{3:3}, at: cxl_remove_ep+0x50c/0x5c0 [cxl_core]

...where cxl_remove_ep() is a helper that wants to delete ports while
holding a lock on the host device for that port. That sets up a lockdep
violation whereby target_list_show() can not rely holding the decoder's
device lock while walking the target_list. Switch to a dedicated seqlock
for this purpose.

Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164367209095.208169.1171673319121271280.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 9285cdb..fc5d862 100644 (file)
--- a/drivers/cxl/core/port.c
+++ b/drivers/cxl/core/port.c
@@ -104,14 +104,11 @@ static ssize_t target_type_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(target_type);
 
-static ssize_t target_list_show(struct device *dev,
-                              struct device_attribute *attr, char *buf)
+static ssize_t emit_target_list(struct cxl_decoder *cxld, char *buf)
 {
-       struct cxl_decoder *cxld = to_cxl_decoder(dev);
        ssize_t offset = 0;
        int i, rc = 0;
 
-       cxl_device_lock(dev);
        for (i = 0; i < cxld->interleave_ways; i++) {
                struct cxl_dport *dport = cxld->target[i];
                struct cxl_dport *next = NULL;
@@ -124,13 +121,29 @@ static ssize_t target_list_show(struct device *dev,
                rc = sysfs_emit_at(buf, offset, "%d%s", dport->port_id,
                                   next ? "," : "");
                if (rc < 0)
-                       break;
+                       return rc;
                offset += rc;
        }
-       cxl_device_unlock(dev);
+
+       return offset;
+}
+
+static ssize_t target_list_show(struct device *dev,
+                               struct device_attribute *attr, char *buf)
+{
+       struct cxl_decoder *cxld = to_cxl_decoder(dev);
+       ssize_t offset;
+       unsigned int seq;
+       int rc;
+
+       do {
+               seq = read_seqbegin(&cxld->target_lock);
+               rc = emit_target_list(cxld, buf);
+       } while (read_seqretry(&cxld->target_lock, seq));
 
        if (rc < 0)
                return rc;
+       offset = rc;
 
        rc = sysfs_emit_at(buf, offset, "\n");
        if (rc < 0)
@@ -494,15 +507,17 @@ static int decoder_populate_targets(struct cxl_decoder *cxld,
                goto out_unlock;
        }
 
+       write_seqlock(&cxld->target_lock);
        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;
+                       break;
                }
                cxld->target[i] = dport;
        }
+       write_sequnlock(&cxld->target_lock);
 
 out_unlock:
        cxl_device_unlock(&port->dev);
@@ -543,6 +558,7 @@ static struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
 
        cxld->id = rc;
        cxld->nr_targets = nr_targets;
+       seqlock_init(&cxld->target_lock);
        dev = &cxld->dev;
        device_initialize(dev);
        device_set_pm_not_required(dev);

diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 6a38d2e..e791629 100644 (file)
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -185,6 +185,7 @@ enum cxl_decoder_type {
  * @interleave_granularity: data stride per dport
  * @target_type: accelerator vs expander (type2 vs type3) selector
  * @flags: memory type capabilities and locking
+ * @target_lock: coordinate coherent reads of the target list
  * @nr_targets: number of elements in @target
  * @target: active ordered target list in current decoder configuration
  */
@@ -199,6 +200,7 @@ struct cxl_decoder {
        int interleave_granularity;
        enum cxl_decoder_type target_type;
        unsigned long flags;
+       seqlock_t target_lock;
        int nr_targets;
        struct cxl_dport *target[];
 };