* [PATCH 1/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race
2026-03-27 23:33 [PATCH 0/2] mm/damon/core: fix damon_call()/damos_walk() vs kdmond exit race SeongJae Park
@ 2026-03-27 23:33 ` SeongJae Park
2026-03-28 0:43 ` (sashiko review) " SeongJae Park
2026-03-27 23:33 ` [PATCH 2/2] mm/damon/core: fix damos_walk() " SeongJae Park
2026-03-28 0:42 ` (sashiko status) [PATCH 0/2] mm/damon/core: fix damon_call()/damos_walk() vs kdmond " SeongJae Park
2 siblings, 1 reply; 6+ messages in thread
From: SeongJae Park @ 2026-03-27 23:33 UTC (permalink / raw)
To: Andrew Morton; +Cc: SeongJae Park, # 6 . 14 . x, damon, linux-kernel, linux-mm
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 and API functions themselves can know the context is
terminated. damon_call() 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,
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 result 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 shows 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 it 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() requests
registration. Initialize (unset) it in kdamond_fn() before letting
damon_start() returns and set it just before the cancelling of remaining
damon_call() requests is executed. damon_call() 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.
Note that the deadlock will not happen when damon_call() is called for
repeat mode request. In tis case, damon_call() returns instead of
waiting for the handling when the request registration succeeds and it
shows the kdamond is running. However, if the request also has
dealloc_on_cancel, the request memory would be leaked.
The issue is found by sashiko [1].
[1] https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org
Fixes: 42b7491af14c ("mm/damon/core: introduce damon_call()")
Cc: <stable@vger.kernel.org> # 6.14.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
include/linux/damon.h | 1 +
mm/damon/core.c | 45 ++++++++++++++-----------------------------
2 files changed, 15 insertions(+), 31 deletions(-)
diff --git a/include/linux/damon.h b/include/linux/damon.h
index d9a3babbafc1..5129de70e7b7 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 db6c67e52d2b..9bcda2765ac9 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.
@@ -1619,6 +1590,10 @@ static int damon_call_handle_inactive_ctx(
* synchronization. The return value of the function will be saved in
* &damon_call_control->return_code.
*
+ * 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 damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
@@ -1629,10 +1604,12 @@ 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);
@@ -2952,6 +2929,9 @@ static int kdamond_fn(void *data)
pr_debug("kdamond (%d) starts\n", current->pid);
+ mutex_lock(&ctx->call_controls_lock);
+ ctx->call_controls_obsolete = false;
+ mutex_unlock(&ctx->call_controls_lock);
complete(&ctx->kdamond_started);
kdamond_init_ctx(ctx);
@@ -3062,6 +3042,9 @@ 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
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 2/2] mm/damon/core: fix damos_walk() vs kdamond_fn() exit race
2026-03-27 23:33 [PATCH 0/2] mm/damon/core: fix damon_call()/damos_walk() vs kdmond exit race SeongJae Park
2026-03-27 23:33 ` [PATCH 1/2] mm/damon/core: fix damon_call() vs kdamond_fn() " SeongJae Park
@ 2026-03-27 23:33 ` SeongJae Park
2026-03-28 0:42 ` (sashiko status) [PATCH 0/2] mm/damon/core: fix damon_call()/damos_walk() vs kdmond " SeongJae Park
2 siblings, 0 replies; 6+ messages in thread
From: SeongJae Park @ 2026-03-27 23:33 UTC (permalink / raw)
To: Andrew Morton; +Cc: SeongJae Park, # 6 . 14 . x, damon, linux-kernel, linux-mm
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].
[1] https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org
Fixes: bf0eaba0ff9c ("mm/damon/core: implement damos_walk()")
Cc: <stable@vger.kernel.org> # 6.14.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
include/linux/damon.h | 1 +
mm/damon/core.c | 21 ++++++++++++++-------
2 files changed, 15 insertions(+), 7 deletions(-)
diff --git a/include/linux/damon.h b/include/linux/damon.h
index 5129de70e7b7..f2cdb7c3f5e6 100644
--- a/include/linux/damon.h
+++ b/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;
/*
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9bcda2765ac9..ddabb93f2377 100644
--- a/mm/damon/core.c
+++ b/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);
--
2.47.3
^ permalink raw reply related [flat|nested] 6+ messages in thread