[RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

[SeongJae Park]

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

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

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

Fix this by introducing another damon_ctx field, namely
call_controls_obsolete.  It is protected by the
damon_ctx->call_controls_lock, which protects damon_call() registration.
Initialize (unset) it in kdamond_init_ctx() and set just before the
cancelling of remaining damon_call() is executed.  damon_call() reads
the obsolete field under the lock and avoid adding a new request.

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

The issue is found by sashiko [1].

[1] https://lore.kernel.org/20260325141956.87144-1-sj@xxxxxxxxxx

Fixes: 42b7491af14c ("mm/damon/core: introduce damon_call()")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.14.x
Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
---
 include/linux/damon.h |  1 +
 mm/damon/core.c       | 41 ++++++++++-------------------------------
 2 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index d9a3babbafc16..5129de70e7b70 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -818,6 +818,7 @@ struct damon_ctx {
 
 	/* lists of &struct damon_call_control */
 	struct list_head call_controls;
+	bool call_controls_obsolete;
 	struct mutex call_controls_lock;
 
 	struct damos_walk_control *walk_control;
diff --git a/mm/damon/core.c b/mm/damon/core.c
index db6c67e52d2b8..a2b553e2c5a81 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1573,35 +1573,6 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
 	return pid;
 }
 
-/*
- * damon_call_handle_inactive_ctx() - handle DAMON call request that added to
- *				      an inactive context.
- * @ctx:	The inactive DAMON context.
- * @control:	Control variable of the call request.
- *
- * This function is called in a case that @control is added to @ctx but @ctx is
- * not running (inactive).  See if @ctx handled @control or not, and cleanup
- * @control if it was not handled.
- *
- * Returns 0 if @control was handled by @ctx, negative error code otherwise.
- */
-static int damon_call_handle_inactive_ctx(
-		struct damon_ctx *ctx, struct damon_call_control *control)
-{
-	struct damon_call_control *c;
-
-	mutex_lock(&ctx->call_controls_lock);
-	list_for_each_entry(c, &ctx->call_controls, list) {
-		if (c == control) {
-			list_del(&control->list);
-			mutex_unlock(&ctx->call_controls_lock);
-			return -EINVAL;
-		}
-	}
-	mutex_unlock(&ctx->call_controls_lock);
-	return 0;
-}
-
 /**
  * damon_call() - Invoke a given function on DAMON worker thread (kdamond).
  * @ctx:	DAMON context to call the function for.
@@ -1629,10 +1600,12 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
 	INIT_LIST_HEAD(&control->list);
 
 	mutex_lock(&ctx->call_controls_lock);
+	if (ctx->call_controls_obsolete) {
+		mutex_unlock(&ctx->call_controls_lock);
+		return -ECANCELED;
+	}
 	list_add_tail(&control->list, &ctx->call_controls);
 	mutex_unlock(&ctx->call_controls_lock);
-	if (!damon_is_running(ctx))
-		return damon_call_handle_inactive_ctx(ctx, control);
 	if (control->repeat)
 		return 0;
 	wait_for_completion(&control->completion);
@@ -2939,6 +2912,9 @@ static void kdamond_init_ctx(struct damon_ctx *ctx)
 		damos_set_next_apply_sis(scheme, ctx);
 		damos_set_filters_default_reject(scheme);
 	}
+	mutex_lock(&ctx->call_controls_lock);
+	ctx->call_controls_obsolete = false;
+	mutex_unlock(&ctx->call_controls_lock);
 }
 
 /*
@@ -3062,6 +3038,9 @@ static int kdamond_fn(void *data)
 	damon_destroy_targets(ctx);
 
 	kfree(ctx->regions_score_histogram);
+	mutex_lock(&ctx->call_controls_lock);
+	ctx->call_controls_obsolete = true;
+	mutex_unlock(&ctx->call_controls_lock);
 	kdamond_call(ctx, true);
 	damos_walk_cancel(ctx);
 
-- 
2.47.3