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 336E530BF7D; Wed, 31 Dec 2025 04:59:50 +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=1767157191; cv=none; b=RY/lpcTDPtFmqLHpRfeukUm7sQxU1Gk5rJ3VzaESJfXD9zIXYTtsttubR/WFnQrvf1dXwTnVIKXY9k3EbukPbX85JcQVD3h3Tbpm9/Q/4cOFLiQsT3Xtml6md3Y6H7JN8yFivbJFvQCKMcPtehrsThpyqtT4L++85tgoZzqkFwQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767157191; c=relaxed/simple; bh=1vkr9V/MD64Y0pYrKiO0vgFUHcY2dKFCgeu7wq+aQ8U=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=qCMyP/liR6/o10RxnhNBOD2H9W4w8pZRoAaNJoOZMdG6pyC83lUORdMfD/jKJU9kZmZ44AqyZiC3nLVzL1kfohVQbBmTnhyLxVxajd11WZ4H0yMBjGpmZf7RPpk1AeGFjyLU/0aXcDZLzvxdvvP/4oGGNpVqERgrwCusLcErnH0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=orhdW9k5; 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="orhdW9k5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6484DC116B1; Wed, 31 Dec 2025 04:59:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767157190; bh=1vkr9V/MD64Y0pYrKiO0vgFUHcY2dKFCgeu7wq+aQ8U=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=orhdW9k52n1qz2Vwl+eS/OF4qd4l+IvUqSRw7W5ioOn+70hj2LbGhwnszGW25Yp8U +0GpQm/8j9cMd/isE+mearVNJTkBerC1vDZIiPaTSsbc3+SLgAurRyLgjazuAR4tEU LyT62GQA4Ke/+q1xInM3Ka2SOPwqcEUpym7xfACWIG+6dIS4yl46J2N1KrYugU+OKI HtjKR8HnpxRzOYboVDxJnBDJywKJpyqfrN3vtvS3EweJsZ2qkhWJRfRupQLEUnv14K FyO/nx1uJqXB42TmrbuaHDoHKu6gDYCAY33sNgjTZsy5y3sDhK6hqlSEppImovdfHd UGvg6sQhSd9VA== From: SeongJae Park To: JaeJoon Jung Cc: SeongJae Park , Asier Gutierrez , akpm@linux-foundation.org, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, wangkefeng.wang@huawei.com, artem.kuzin@huawei.com, stepanov.anatoly@huawei.com Subject: Re: [RFC PATCH v1] mm: improve call_controls_lock Date: Tue, 30 Dec 2025 20:59:46 -0800 Message-ID: <20251231045948.77624-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Wed, 31 Dec 2025 11:15:00 +0900 JaeJoon Jung wrote: > On Tue, 30 Dec 2025 at 00:23, SeongJae Park wrote: > > > > Hello Asier, > > > > > > Thank you for sending this patch! > > > > On Mon, 29 Dec 2025 14:55:32 +0000 Asier Gutierrez wrote: > > > > > This is a minor patch set for a call_controls_lock synchronization improvement. > > > > Please break description lines to not exceed 75 characters per line. > > > > > > > > Spinlocks are faster than mutexes, even when the mutex takes the fast > > > path. Hence, this patch replaces the mutex call_controls_lock with a spinlock. > > > > But call_controls_lock is not being used on performance critical part. > > Actually, most of DAMON code is not performance critical. I really appreciate > > your patch, but I have to say I don't think this change is really needed now. > > Please let me know if I'm missing something. > > Paradoxically, when it comes to locking, spin_lock is better than > mutex_lock > because "most of DAMON code is not performance critical." > > DAMON code only accesses the ctx belonging to kdamond itself. For > example: > kdamond.0 --> ctx.0 > kdamond.1 --> ctx.1 > kdamond.2 --> ctx.2 > kdamond.# --> ctx.# > > There is no cross-approach as shown below: > kdamond.0 --> ctx.1 > kdamond.1 --> ctx.2 > kdamond.2 --> ctx.0 > > Only the data belonging to kdamond needs to be resolved for concurrent access. > most DAMON code needs to lock/unlock briefly when add/del linked > lists, > so spin_lock is effective. I don't disagree this. Both spinlock and mutex effectively work for DAMON's locking usages. > If you handle it with a mutex, it becomes > more > complicated because the rescheduling occurs as a context switch occurs > inside the kernel. Can you please elaborate what kind of complexities you are saying about? Adding some examples would be nice. > Moreover, since the call_controls_lock that is > currently > being raised as a problem only occurs in two places, the kdamon_call() > loop > and the damon_call() function, it is effective to handle it with a > spin_lock > as shown below. > > @@ -1502,14 +1501,15 @@ int damon_call(struct damon_ctx *ctx, struct > damon_call_control *control) > control->canceled = false; > INIT_LIST_HEAD(&control->list); > > - mutex_lock(&ctx->call_controls_lock); > + spin_lock(&ctx->call_controls_lock); > + /* damon_is_running */ > if (ctx->kdamond) { > list_add_tail(&control->list, &ctx->call_controls); > } else { > - mutex_unlock(&ctx->call_controls_lock); > + spin_unlock(&ctx->call_controls_lock); > return -EINVAL; > } > - mutex_unlock(&ctx->call_controls_lock); > + spin_unlock(&ctx->call_controls_lock); > > if (control->repeat) > return 0; Are you saying the above diff can fix the damon_call() use-after-free bug [1]? Can you please elaborate why you think so? [1] https://lore.kernel.org/20251231012315.75835-1-sj@kernel.org Thanks, SJ [...]