From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7742410F287C for ; Fri, 27 Mar 2026 23:33:30 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id ED3EB6B0096; Fri, 27 Mar 2026 19:33:26 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E86346B0098; Fri, 27 Mar 2026 19:33:26 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D72DC6B0099; Fri, 27 Mar 2026 19:33:26 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id BD2AA6B0096 for ; Fri, 27 Mar 2026 19:33:26 -0400 (EDT) Received: from smtpin12.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 4E0A816053C for ; Fri, 27 Mar 2026 23:33:26 +0000 (UTC) X-FDA: 84593446812.12.6D077A1 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf02.hostedemail.com (Postfix) with ESMTP id 9513580008 for ; Fri, 27 Mar 2026 23:33:24 +0000 (UTC) Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=S+JFfCfT; spf=pass (imf02.hostedemail.com: domain of sj@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=sj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1774654404; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=bxVsiPS0xP1dceAU8EIPPNayC32A3kunb1RlapBhuj8=; b=SXKRnhXuJW7N4V5nIBpFfJxzz/eNETPwtlM26VtF24I4lzF33TF2Jyz+HS2+U3akLsOC7C WGrEj8n9O/CmWRvhEIbs+dl90vslycJSXJmVMvCto4riUjUrc4O8v64owZkb/OyRMSJaj2 FbpSqpWUcGiKQeIlO/R5G4xcqeVlkwg= ARC-Authentication-Results: i=1; imf02.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=S+JFfCfT; spf=pass (imf02.hostedemail.com: domain of sj@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=sj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1774654404; a=rsa-sha256; cv=none; b=IOlzhpN+uUHetLXHSA2Hl6MYsMYv3OCr4qNSFkIuHs9EkeACbeGnpE5siG7LxUtkfBbo9J fyHLS45SoYzWaUa/j1Ype/m5tL0YYqsIg5LNyBj5TTOaGJ4LH8hPDipi8hy1NYrjNlBzhx ZrxcVvQzgpzfQiN8FgxeNZyMyPNDoQg= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 9E2D04376A; Fri, 27 Mar 2026 23:33:23 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6012FC2BCB2; Fri, 27 Mar 2026 23:33:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774654403; bh=VSUrozLJroKMWrM7KxSMyE5+mSLHT7kOcYr4qKbQQJ4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=S+JFfCfT8CQ1EDMB6mu7gzb95mcb7qRbck02Q3vj3/Lh68f4nC1bboFOG+E9lhZIv qq6L/zMDCxZPvKxQazRGe37Fir+fOObkSDqQ0hD7LHG3jRfYkGQ3M+2t6l4tkLWlnw RruNY4xhMhTe8pu6GSBALQbqs468VASuNIbkKxs1AzLPLcBR5EEuf5ly6OmmDbApsu /s7V/LaQt+aCH/GGpB5p+1lzYp3Guq4Sz8kV25XgjcqYRlrRvfVKrJnmNuyzMr8HXk rcVQ6wvCvlfKXilvPLPW+oZNCrAjzY+qUPu7u1m9HwaA30L3dg8fXMXRdxBR7SvoOP 7YrcBI2upyjkg== From: SeongJae Park To: Andrew Morton Cc: SeongJae Park , "# 6 . 14 . x" , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH 2/2] mm/damon/core: fix damos_walk() vs kdamond_fn() exit race Date: Fri, 27 Mar 2026 16:33:15 -0700 Message-ID: <20260327233319.3528-3-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260327233319.3528-1-sj@kernel.org> References: <20260327233319.3528-1-sj@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: efqh6pn54ccuasxohddje93gu5x9hcr8 X-Rspamd-Queue-Id: 9513580008 X-Rspamd-Server: rspam09 X-HE-Tag: 1774654404-578824 X-HE-Meta: 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 kqp7jL9s dnkWpqoJXCF17xVogUmS9fHbnjNPmSJPc+4/vky12VN/NtupzEwvP0SLN+w/Wsgdau/nt20Lx9CuifGXvwS/+uIV43Ge+mK/IcN+VGCQ2SH2lpGwmtTRvDYl5iSvQskOOtNj9yst7I3PK09oNKOoM0imrA2IoMxIRRkLvLMWfIm00GQdqH47fQLuhUFgBTRnA/4y/OEpN1e3njgAi7NpD+YbuxyjtF6IZCl5VT7MGKdfUKuz+M29uUlAyFNXM+2BmSyeUpXZAMQQjbAIu4y+6feuqFw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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: # 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