mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch removed from -mm tree

The quilt patch titled
Subject: mm/damon/core: fix damos_walk() vs kdamond_fn() exit race
has been removed from the -mm tree. Its filename was
mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: SeongJae Park <[email protected]>
Subject: mm/damon/core: fix damos_walk() vs kdamond_fn() exit race
Date: Fri, 27 Mar 2026 16:33:15 -0700

When kdamond_fn() main loop is finished, the function cancels remaining damos_walk() request and unset the damon_ctx->kdamond so that API callers and API functions themselves can show the context is terminated. damos_walk() adds the caller's request to the queue first. After that, it shows if the kdamond of the damon_ctx is still running (damon_ctx->kdamond is set). Only if the kdamond is running, damos_walk() starts waiting for the kdamond's handling of the newly added request.

The damos_walk() requests registration and damon_ctx->kdamond unset are protected by different mutexes, though. Hence, damos_walk() could race with damon_ctx->kdamond unset, and result in deadlocks.

For example, let's suppose kdamond successfully finished the damow_walk() request cancelling. Right after that, damos_walk() is called for the context. It registers the new request, and shows the context is still running, because damon_ctx->kdamond unset is not yet done. Hence the damos_walk() caller starts waiting for the handling of the request. However, the kdamond is already on the termination steps, so it never handles the new request. As a result, the damos_walk() caller thread infinitely waits.

Fix this by introducing another damon_ctx field, namely walk_control_obsolete. It is protected by the damon_ctx->walk_control_lock, which protects damos_walk() request registration. Initialize (unset) it in kdamond_fn() before letting damon_start() returns and set it just before the cancelling of the remaining damos_walk() request is executed. damos_walk() reads the obsolete field under the lock and avoids adding a new request.

After this change, only requests that are guaranteed to be handled or cancelled are registered. Hence the after-registration DAMON context termination check is no longer needed. Remove it together.

The issue is found by sashiko [1].

---

 include/linux/damon.h |    1 +
 mm/damon/core.c       |   21 ++++++++++++++-------
 2 files changed, 15 insertions(+), 7 deletions(-)

@@ -822,6 +822,7 @@ struct damon_ctx {
 	struct mutex call_controls_lock;
 
 	struct damos_walk_control *walk_control;
+	bool walk_control_obsolete;
 	struct mutex walk_control_lock;
 
 	/*

@@ -1637,6 +1637,10 @@ int damon_call(struct damon_ctx *ctx, st
  * passed at least one &damos->apply_interval_us, kdamond marks the request as
  * completed so that damos_walk() can wakeup and return.
  *
+ * Note that this function should be called only after damon_start() with the
+ * @ctx has succeeded.  Otherwise, this function could fall into an indefinite
+ * wait.
+ *
  * Return: 0 on success, negative error code otherwise.
  */
 int damos_walk(struct damon_ctx *ctx, struct damos_walk_control *control)

@@ -1644,19 +1648,16 @@ int damos_walk(struct damon_ctx *ctx, st
 	init_completion(&control->completion);
 	control->canceled = false;
 	mutex_lock(&ctx->walk_control_lock);
+	if (ctx->walk_control_obsolete) {
+		mutex_unlock(&ctx->walk_control_lock);
+		return -ECANCELED;
+	}
 	if (ctx->walk_control) {
 		mutex_unlock(&ctx->walk_control_lock);
 		return -EBUSY;
 	}
 	ctx->walk_control = control;
 	mutex_unlock(&ctx->walk_control_lock);
-	if (!damon_is_running(ctx)) {
-		mutex_lock(&ctx->walk_control_lock);
-		if (ctx->walk_control == control)
-			ctx->walk_control = NULL;
-		mutex_unlock(&ctx->walk_control_lock);
-		return -EINVAL;
-	}
 	wait_for_completion(&control->completion);
 	if (control->canceled)
 		return -ECANCELED;

@@ -2932,6 +2933,9 @@ static int kdamond_fn(void *data)
 	mutex_lock(&ctx->call_controls_lock);
 	ctx->call_controls_obsolete = false;
 	mutex_unlock(&ctx->call_controls_lock);
+	mutex_lock(&ctx->walk_control_lock);
+	ctx->walk_control_obsolete = false;
+	mutex_unlock(&ctx->walk_control_lock);
 	complete(&ctx->kdamond_started);
 	kdamond_init_ctx(ctx);
 

@@ -3046,6 +3050,9 @@ done:
 	ctx->call_controls_obsolete = true;
 	mutex_unlock(&ctx->call_controls_lock);
 	kdamond_call(ctx, true);
+	mutex_lock(&ctx->walk_control_lock);
+	ctx->walk_control_obsolete = true;
+	mutex_unlock(&ctx->walk_control_lock);
 	damos_walk_cancel(ctx);
 
 	pr_debug("kdamond (%d) finishes\n", current->pid);

_

Patches currently in -mm which might be from [email protected] are

mm-damon-core-disallow-time-quota-setting-zero-esz.patch mm-damon-core-disallow-non-power-of-two-min_region_sz-on-damon_start.patch