[PATCH v11 13/31] cxl/mem: Add 20 second timeout for stalled DC_ADD_CAPACITY chains

[Anisa Su]

A DC_ADD_CAPACITY event can span multiple event records grouped together
by the CXL_DCD_EVENT_MORE flag. Extents are staged in the pending list until
the last event record ('More'=0) is received, at which point the pending
list is processed. If the device opens such a chain (More=1) but never
sends the closing record, the staged list sits indefinitely.

Add a delayed-work watchdog that, on expiry, refuses the chain with an
empty ADD_DC_RESPONSE and drops the staged list.

The 20s timeout is a conservative upper bound and may be tightened
later. The timeout is purely defensive — the spec does not require it,
but prevents issues from a lost mailbox response or a crashed fabric manager.

The watchdog bounds how long a chain may stall, but a device could still
defeat it by streaming More=1 records faster than the timeout, growing the
staged list without bound. Also cap a runtime chain at
CXL_DC_MAX_PENDING_EXTENTS and refuse it once exceeded; existing-extent
recovery is bounded separately by the device's reported extent count.

Signed-off-by: Anisa Su <anisa.su@xxxxxxxxxxx>

---
Changes:
1. mbox.c: Fix comment in handle_add_event(), before closing the 'More'
   chain and disabling the watchdog. The comment incorrectly claimed
   handle_add_event() runs in system_wq.
2. mbox.c: Drop unnecessary initialization of add_ctx.armed=false in
   cxl_memdev_state_create(), as allocated memory is already zeroed
3. mbox.c: assert add_ctx.lock is held in add_to_pending_list(); it
   serializes access to add_ctx.pending_extents.
4. mbox.c: cap a runtime More=1 chain at CXL_DC_MAX_PENDING_EXTENTS in
   handle_add_event() so a buggy device cannot grow the staged list
   without bound (the watchdog bounds time, not memory).
---
 drivers/cxl/core/mbox.c | 98 ++++++++++++++++++++++++++++++++++++++++-
 drivers/cxl/cxlmem.h    | 24 ++++++++--
 2 files changed, 117 insertions(+), 5 deletions(-)

diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
index 7dd40fb8d613..4e887b5cdc3e 100644
--- a/drivers/cxl/core/mbox.c
+++ b/drivers/cxl/core/mbox.c
@@ -1208,15 +1208,78 @@ static void clear_pending_extents(void *_mds)
 
 	list_for_each_entry_safe(pos, tmp, &mds->add_ctx.pending_extents, list)
 		delete_extent_node(pos);
+	mds->add_ctx.nr_pending = 0;
 	mds->add_ctx.group = NULL;
 }
 
+/*
+ * Defensive cap on extents staged in one runtime More=1 chain: a buggy
+ * device could otherwise grow the list without bound.  Not spec-defined.
+ */
+#define CXL_DC_MAX_PENDING_EXTENTS	100
+
+/*
+ * Bound on how long the host will wait for a device to finish a
+ * multi-record DC_ADD_CAPACITY chain (More=1 ... More=0) before
+ * refusing the chain.
+ * The timeout is not defined in the spec, but added for defensive purposes.
+ * Since there is no spec-defined timeout, 20s is chosen as a generous
+ * upper bound and matches the GPF timeout.
+ */
+#define CXL_DC_ADD_TIMEOUT	(20 * HZ)
+
+static void cxl_dc_add_timeout(struct work_struct *work)
+{
+	struct pending_add_ctx *ctx = container_of(to_delayed_work(work),
+						   struct pending_add_ctx,
+						   timeout_work);
+	struct cxl_memdev_state *mds = container_of(ctx,
+						    struct cxl_memdev_state,
+						    add_ctx);
+	struct device *dev = mds->cxlds.dev;
+
+	guard(mutex)(&ctx->lock);
+
+	/*
+	 * handle_add_event() cancels this work non-synchronously (a sync
+	 * cancel would deadlock on @ctx->lock, which the chain-close path
+	 * holds), so a callback that already started running can reach here
+	 * after its chain has moved on.  Abort only if a chain is still armed
+	 * AND the timer has not been re-armed since this expiry fired: a fresh
+	 * mod_delayed_work() (a later extent in this chain, or a new chain)
+	 * makes delayed_work_pending() true, meaning this expiry belongs to a
+	 * superseded deadline and must not abort the current chain.
+	 */
+	if (!ctx->armed || delayed_work_pending(&ctx->timeout_work))
+		return;
+
+	dev_warn(dev, "DC add chain timed out; refusing staged extents\n");
+
+	if (cxl_send_dc_response(mds, CXL_MBOX_OP_ADD_DC_RESPONSE,
+				 &ctx->pending_extents, 0))
+		dev_dbg(dev, "Failed to send empty ADD_DC_RESPONSE on timeout\n");
+
+	clear_pending_extents(mds);
+	ctx->armed = false;
+}
+
+static void cxl_cancel_dcd_add_chain_work(void *_mds)
+{
+	struct cxl_memdev_state *mds = _mds;
+
+	cancel_delayed_work_sync(&mds->add_ctx.timeout_work);
+}
+
 static int add_to_pending_list(struct list_head *pending_list,
 			       struct cxl_extent *to_add)
 {
+	struct pending_add_ctx *ctx =
+		container_of(pending_list, struct pending_add_ctx, pending_extents);
 	struct cxl_extent_list_node *node = kzalloc(sizeof(*node), GFP_KERNEL);
 	struct cxl_extent *extent;
 
+	lockdep_assert_held(&ctx->lock);
+
 	if (!node)
 		return -ENOMEM;
 	extent = kmemdup(to_add, sizeof(*extent), GFP_KERNEL);
@@ -1227,6 +1290,7 @@ static int add_to_pending_list(struct list_head *pending_list,
 
 	node->extent = extent;
 	list_add_tail(&node->list, pending_list);
+	ctx->nr_pending++;
 	return 0;
 }
 
@@ -1239,10 +1303,20 @@ static int add_to_pending_list(struct list_head *pending_list,
 static int handle_add_event(struct cxl_memdev_state *mds,
 			    struct cxl_event_dcd *event)
 {
+	struct pending_add_ctx *ctx = &mds->add_ctx;
 	struct device *dev = mds->cxlds.dev;
 	int rc;
 
-	rc = add_to_pending_list(&mds->add_ctx.pending_extents, &event->extent);
+	guard(mutex)(&ctx->lock);
+
+	if (ctx->nr_pending >= CXL_DC_MAX_PENDING_EXTENTS) {
+		dev_warn(dev, "DC add chain exceeds %u extents; dropping (firmware bug)\n",
+			 CXL_DC_MAX_PENDING_EXTENTS);
+		clear_pending_extents(mds);
+		return -ENOSPC;
+	}
+
+	rc = add_to_pending_list(&ctx->pending_extents, &event->extent);
 	if (rc) {
 		clear_pending_extents(mds);
 		return rc;
@@ -1250,9 +1324,19 @@ static int handle_add_event(struct cxl_memdev_state *mds,
 
 	if (event->flags & CXL_DCD_EVENT_MORE) {
 		dev_dbg(dev, "more bit set; delay the surfacing of extent\n");
+		mod_delayed_work(system_wq, &ctx->timeout_work,
+						 CXL_DC_ADD_TIMEOUT);
+		ctx->armed = true;
 		return 0;
 	}
 
+	/*
+	 * Chain is closing.  Disarm before flushing so a pending watchdog
+	 * (queued but blocked on @ctx->lock) sees !armed and bails out.
+	 */
+	ctx->armed = false;
+	cancel_delayed_work(&ctx->timeout_work);
+
 	rc = cxl_send_dc_response(mds, CXL_MBOX_OP_ADD_DC_RESPONSE,
 				  &mds->add_ctx.pending_extents, 0);
 	clear_pending_extents(mds);
@@ -2036,11 +2120,23 @@ struct cxl_memdev_state *cxl_memdev_state_create(struct device *dev, u64 serial,
 
 	mutex_init(&mds->event.log_lock);
 	INIT_LIST_HEAD(&mds->add_ctx.pending_extents);
+	mutex_init(&mds->add_ctx.lock);
+	INIT_DELAYED_WORK(&mds->add_ctx.timeout_work,
+			  cxl_dc_add_timeout);
 
 	rc = devm_add_action_or_reset(dev, clear_pending_extents, mds);
 	if (rc)
 		return ERR_PTR(rc);
 
+	/*
+	 * Registered after clear_pending_extents so devm's reverse-order
+	 * unwind cancels (and waits for) the watchdog first, then the list
+	 * cleanup runs with the watchdog guaranteed not to refire.
+	 */
+	rc = devm_add_action_or_reset(dev, cxl_cancel_dcd_add_chain_work, mds);
+	if (rc)
+		return ERR_PTR(rc);
+
 	rc = devm_cxl_register_mce_notifier(dev, &mds->mce_notifier);
 	if (rc == -EOPNOTSUPP)
 		dev_warn(dev, "CXL MCE unsupported\n");
diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
index 4ffa7bd1e5f1..81498d47f309 100644
--- a/drivers/cxl/cxlmem.h
+++ b/drivers/cxl/cxlmem.h
@@ -8,6 +8,8 @@
 #include <linux/uuid.h>
 #include <linux/node.h>
 #include <linux/list.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
 #include <cxl/event.h>
 #include <cxl/mailbox.h>
 #include "cxl.h"
@@ -407,19 +409,33 @@ static inline struct cxl_dev_state *mbox_to_cxlds(struct cxl_mailbox *cxl_mbox)
 
 /**
  * struct pending_add_ctx - Staging state for an in-progress
- *			    DCD_ADD_CAPACITY event chain
+ *							DCD_ADD_CAPACITY event chain
  * @pending_extents: extents received so far in the chain; flushed when
- *		     the chain closes (More=0)
+ *					 the chain closes (More=0)
  * @group: tag group being assembled from the chain
+ * @timeout_work: watchdog that fires if a chain is opened with
+ *				  CXL_DCD_EVENT_MORE but the closing record never arrives
+ * @lock: serialises updates to the chain state against the watchdog
+ * @armed: set when a More=1 chain opens; cleared when the chain closes,
+ *		   either by a More=0 event record or by the watchdog firing.
  *
  * A DCD_ADD_CAPACITY notification can span multiple event records
  * stitched together by the CXL_DCD_EVENT_MORE flag.  Records are staged
- * here until the device clears More, at which point the staged batch is
- * processed and responded to as a single Add_DC_Response.
+ * here until an event record with 'More'=0 is received, at which point the
+ * staged batch is processed and responded to as a single Add_DC_Response.
+ *
+ * If a chain is opened (More=1) but the device never sends the closing
+ * record, the staged list would otherwise sit indefinitely.  @timeout_work
+ * is a defensive watchdog that refuses such a chain with an empty response
+ * and drops the staged list.
  */
 struct pending_add_ctx {
 	struct list_head pending_extents;
 	struct cxl_dc_tag_group *group;
+	struct delayed_work timeout_work;
+	struct mutex lock;
+	unsigned int nr_pending;
+	bool armed;
 };
 
 /**
-- 
2.43.0