mm/damon/core: fix damos_walk() vs kdamond_fn() exit race

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].


Link: https://lore.kernel.org/20260327233319.3528-3-sj@kernel.org
Link: https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org [1]
Fixes: bf0eaba0ff ("mm/damon/core: implement damos_walk()")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.14.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/damon.h

@@ -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;
 
 	/*

mm/damon/core.c

@@ -1637,6 +1637,10 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
  * 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, struct damos_walk_control *control)
 	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 @@ static int kdamond_fn(void *data)
 	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);