From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 904262DC32E; Tue, 31 Mar 2026 23:26:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774999613; cv=none; b=eTdilz+pxbvq8kGncKepxZL98U1+wN0wi9JQpN0zPB4HhZXYCnEJLsVoq5w25k3wxwclRVT/ewYJObi4AvBvYG43Bu2qBHFxg+bNTSwTBAbHYT7TAhEVR4gbFu33+paTP5M0JeXRb+W+89xiK4gaa/IXlNYM3oJeDxk1ekkryFo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774999613; c=relaxed/simple; bh=WrwAul0cYFQxTwgO6mUBtQ+cwWDF9WYhoT9zS3qubAQ=; h=Date:To:From:Subject:Message-Id; b=cSWkkJRAA9sUKfLf2SG2a1nE05bfBTXiPL4mXXcO2XKsXATZ5JgfAnGHtzo18VPnBsP0StLFYlHAPzIyMI8IpdqZuTr1gZhQo+CUAfx0HSOJyciKh/g7UyMyZ60kFoh7oeYdO30mUQGwF48g4jGrWfiT/jWSRkMCMFpSIEvxYLw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=sunxof9n; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="sunxof9n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 294D8C19423; Tue, 31 Mar 2026 23:26:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1774999613; bh=WrwAul0cYFQxTwgO6mUBtQ+cwWDF9WYhoT9zS3qubAQ=; h=Date:To:From:Subject:From; b=sunxof9nyxZf9K8bTi1jxiu6wTpO/U5SOsxiPN7BX+k8M+6KI4xoYwODA2/tqhL/f K78NL/dk0MjzM+hpORdcd/SAE200H88nm7qaciKGy9nIpP0gznz3sOnQGwapAxnGfL sJme9mdFbblY9FrpXRhNKn1rnm2UGghWoVyYweyQ= Date: Tue, 31 Mar 2026 16:26:52 -0700 To: mm-commits@vger.kernel.org,stable@vger.kernel.org,sj@kernel.org,akpm@linux-foundation.org From: Andrew Morton Subject: + mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch added to mm-hotfixes-unstable branch Message-Id: <20260331232653.294D8C19423@smtp.kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: mm/damon/core: fix damos_walk() vs kdamond_fn() exit race has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: SeongJae Park 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]. Link: https://lkml.kernel.org/r/20260327233319.3528-3-sj@kernel.org Link: https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org [1] Fixes: bf0eaba0ff9c ("mm/damon/core: implement damos_walk()") Signed-off-by: SeongJae Park Cc: # 6.14.x Signed-off-by: Andrew Morton --- include/linux/damon.h | 1 + mm/damon/core.c | 21 ++++++++++++++------- 2 files changed, 15 insertions(+), 7 deletions(-) --- a/include/linux/damon.h~mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race +++ a/include/linux/damon.h @@ -809,6 +809,7 @@ struct damon_ctx { struct mutex call_controls_lock; struct damos_walk_control *walk_control; + bool walk_control_obsolete; struct mutex walk_control_lock; /* --- a/mm/damon/core.c~mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race +++ a/mm/damon/core.c @@ -1528,6 +1528,10 @@ 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) @@ -1535,19 +1539,16 @@ 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; @@ -2731,6 +2732,9 @@ 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); @@ -2839,6 +2843,9 @@ 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 sj@kernel.org are mm-damon-sysfs-dealloc-repeat_call_control-if-damon_call-fails.patch mm-damon-core-fix-damon_call-vs-kdamond_fn-exit-race.patch mm-damon-core-fix-damos_walk-vs-kdamond_fn-exit-race.patch