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 6A1D63F23C9; Fri, 27 Mar 2026 14:26:09 +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=1774621569; cv=none; b=gnc2yWGNkcZp5HjEa1esKdu4s5zl4B6M42cEJmHU6phwctdZtEX6Op+w0FeuM2uUwNa0M0wxirHuY82bbvq4lewEuXOWBR8LImxQcuBMIXMnau0hvpFCRvLWp1lq5GdVHYuQLeGGGvByVQKDj+hlbLhGIUVlaITH1j+eibauN78= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774621569; c=relaxed/simple; bh=SCd2raDJ8Tg1kT/fyAOHvNUkzbrdRCMrZxN6B+WI2i4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KwaVrM87k8YFCipyW+i1tv5q5bN4SY3ajyTSXfkTGifBw5rMdCPiabiuU8IHAWF/ZIwpuLcQ5M056BInEe1MUndzS5eV9KikjYySmiW7FYl82jNeZYKBPAeUS60BasarAdAOOT/r54WVFBMJeyXXwr6pe6n9qnNBFcvHyffY1j0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fcgKr2Jd; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fcgKr2Jd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 21CE0C2BCB1; Fri, 27 Mar 2026 14:26:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774621569; bh=SCd2raDJ8Tg1kT/fyAOHvNUkzbrdRCMrZxN6B+WI2i4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fcgKr2Jdfpp+obg1pcYwgv7gcq6LSa2j8dHOp/UGYYyBGWeL/4Oy98++Uy2DochQR fY5lzjhHGEXZL4rVfE/q1PEJDdc+BjJXU5pEcIEzmBxb2a5GxqQRF/J4WA4YE383XN zZVjw2dl3C/3kZSanuK7tgMyjONr55Qeu1HuKOqbXmtMOHXDXuR2RuJ8i7pxTQDPUf n2sQyScqDLgdDDz6E15tH46NWFy+In64svJMOfHX7N/968DjRqEqv3JGDdu2hpA6TF SdXXLia6mvcaFdyHlCQsqMeq7YAI1OJ75d31+h3rWm1ksKljGCLbOFSO0+NgqZCB84 SBlLbeI8scQsw== From: SeongJae Park To: Cc: SeongJae Park , "# 6 . 14 . x" , Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [RFC PATCH v3 2/2] mm/damon/core: fix damos_walk() vs kdamond_fn() exit race deadlock Date: Fri, 27 Mar 2026 07:26:04 -0700 Message-ID: <20260327142605.4834-3-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260327142605.4834-1-sj@kernel.org> References: <20260327142605.4834-1-sj@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When kdamond_fn() main loop is finished, the function cancels remaining damos_walk() request and unset the damon_ctx->kdamond so that API callers 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: # 6.14.x Signed-off-by: SeongJae Park --- 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