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 CFB5035A38C for ; Fri, 1 May 2026 11:01:43 +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=1777633303; cv=none; b=eG+86sB9ahxbbZsz/J7GMSNvi/zqh+MuifjkyKrn3M9NrvD7Iy5X7qL6zIFWK8hfgA3mHSWaeUWuu1Fz2ljrvzi1GXW0+4d0z9z55bSLjfgaVwbLXmAuoUmwKRxjt8JCIrlGxGnr9DFdQjROjcmia66easKQIPJr5LEiAUM1Q6k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777633303; c=relaxed/simple; bh=Ulfat3p/LEnQTOLXpNx1N73FOujzK1KI3fphlz4Xais=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=PST/ZOisTDKJ4hPZkWfqLjNL34IvKlyUXK+czAtrSE4BAb6iGd5u4oJplf/ZCA+R02F7Y/OrRH+fA4Gc7QZslEaJEQz0vmiMWfj2XpRSAxeRVnnLTDU7gcCyDl/r4rzShpfqT5XjWf1U9n+UcgSkz9ycx9iyc52V44TnJRUrMGE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=ARXC4MCM; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="ARXC4MCM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6C266C2BCB4; Fri, 1 May 2026 11:01:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1777633303; bh=Ulfat3p/LEnQTOLXpNx1N73FOujzK1KI3fphlz4Xais=; h=Subject:To:Cc:From:Date:From; b=ARXC4MCMZeCBjlVGC3PDv6vT/b+EVO6xuFAjxm/FLSewN8cV7pNKWr6p/Pl5Jkb4K jbW9oMPH+I1vNuc83SxcnJUApT7NFBQfRhp7+BkrbcjVJ/5iyDuBI9M7b17+h0kWe9 Waa4535rxrBQu36rAvb3IGDL94ub8lHt9bBdOxqQ= Subject: FAILED: patch "[PATCH] mm/damon/core: fix damos_walk() vs kdamond_fn() exit race" failed to apply to 6.18-stable tree To: sj@kernel.org,akpm@linux-foundation.org,stable@vger.kernel.org Cc: From: Date: Fri, 01 May 2026 13:01:41 +0200 Message-ID: <2026050141-likewise-trapeze-f1cc@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.18-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y git checkout FETCH_HEAD git cherry-pick -x 33c3f6c2b48cd84b441dba1ee3e62290e53930f4 # git commit -s git send-email --to '' --in-reply-to '2026050141-likewise-trapeze-f1cc@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^.. Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 33c3f6c2b48cd84b441dba1ee3e62290e53930f4 Mon Sep 17 00:00:00 2001 From: SeongJae Park Date: Fri, 27 Mar 2026 16:33:15 -0700 Subject: [PATCH] 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: bf0eaba0ff9c ("mm/damon/core: implement damos_walk()") Signed-off-by: SeongJae Park Cc: # 6.14.x Signed-off-by: Andrew Morton 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);