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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8A8B9C4321E for ; Mon, 27 Sep 2021 16:38:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 75EA96113A for ; Mon, 27 Sep 2021 16:38:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235680AbhI0QkH (ORCPT ); Mon, 27 Sep 2021 12:40:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59764 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235555AbhI0Qju (ORCPT ); Mon, 27 Sep 2021 12:39:50 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:e::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 73A60C061771; Mon, 27 Sep 2021 09:38:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Sender:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description; bh=b92mAeHk3N99JPkWqvyo9aOeWc+3cnxJoDw3jDX8Mm4=; b=4iFsa2NhMsYXEgtvOhS6QX/tkz tyYKgFv1fS0sfrcdKD3juEPK8CF/Ol1yZHGmzIRua97s7LFVp5KSx/EhG/Rs5uPqn0jrEAYu1SLD1 mctlFau4QBg0iNjKr3CDZtUGpkNOY1kiPQEtwLoHZ58BNFZVz8PL1cxZHtdFj3+r9++NLGzMG3xiV KJUPX+jcfgPQidVAkAXeUPEl/dQplFwMDdkQ8UnAubdVFxZmZ9pOGXj2sNmH2vgsF9PHJbHCc8knW E4xFkJ9YhZ2zxWdzSkbcm0jADND9t1MZAC0sAXr6u3iOqqvrgCHiqVh/G0utH/Ech2Ug46pvNBfFQ 01F5fLUw==; Received: from mcgrof by bombadil.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mUtdm-003OSE-Rl; Mon, 27 Sep 2021 16:38:06 +0000 From: Luis Chamberlain To: tj@kernel.org, gregkh@linuxfoundation.org, akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org, shuah@kernel.org Cc: bvanassche@acm.org, dan.j.williams@intel.com, joe@perches.com, tglx@linutronix.de, mcgrof@kernel.org, keescook@chromium.org, rostedt@goodmis.org, linux-spdx@vger.kernel.org, linux-doc@vger.kernel.org, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v8 06/12] kernel/module: add documentation for try_module_get() Date: Mon, 27 Sep 2021 09:37:59 -0700 Message-Id: <20210927163805.808907-7-mcgrof@kernel.org> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210927163805.808907-1-mcgrof@kernel.org> References: <20210927163805.808907-1-mcgrof@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: Luis Chamberlain Precedence: bulk List-ID: X-Mailing-List: linux-spdx@vger.kernel.org There is quite a bit of tribal knowledge around proper use of try_module_get() and that it must be used only in a context which can ensure the module won't be gone during the operation. Document this little bit of tribal knowledge. I'm extending this tribal knowledge with new developments which it seems some folks do not yet believe to be true: we can be sure a module will exist during the lifetime of a sysfs file operation. For proof, refer to test_sysfs test #32: ./tools/testing/selftests/sysfs/sysfs.sh -t 0032 Without this being true, the write would fail or worse, a crash would happen, in this test. It does not. Signed-off-by: Luis Chamberlain --- include/linux/module.h | 34 ++++++++++++++++++++++++++++++++-- 1 file changed, 32 insertions(+), 2 deletions(-) diff --git a/include/linux/module.h b/include/linux/module.h index c9f1200b2312..22eacd5e1e85 100644 --- a/include/linux/module.h +++ b/include/linux/module.h @@ -609,10 +609,40 @@ void symbol_put_addr(void *addr); to handle the error case (which only happens with rmmod --wait). */ extern void __module_get(struct module *module); -/* This is the Right Way to get a module: if it fails, it's being removed, - * so pretend it's not there. */ +/** + * try_module_get() - yields to module removal and bumps refcnt otherwise + * @module: the module we should check for + * + * This can be used to try to bump the reference count of a module, so to + * prevent module removal. The reference count of a module is not allowed + * to be incremented if the module is already being removed. + * + * Care must be taken to ensure the module cannot be removed during the call to + * try_module_get(). This can be done by having another entity other than the + * module itself increment the module reference count, or through some other + * means which guarantees the module could not be removed during an operation. + * An example of this later case is using try_module_get() in a sysfs file + * which the module created. The sysfs store / read file operations are + * gauranteed to exist through the use of kernfs's active reference (see + * kernfs_active()). If a sysfs file operation is being run, the module which + * created it must still exist as the module is in charge of removing the same + * sysfs file being read. Also, a sysfs / kernfs file removal cannot happen + * unless the same file is not active. + * + * One of the real values to try_module_get() is the module_is_live() check + * which ensures this the caller of try_module_get() can yield to userspace + * module removal requests and fail whatever it was about to process. + */ extern bool try_module_get(struct module *module); +/** + * module_put() - release a reference count to a module + * @module: the module we should release a reference count for + * + * If you successfully bump a reference count to a module with try_module_get(), + * when you are finished you must call module_put() to release that reference + * count. + */ extern void module_put(struct module *module); #else /*!CONFIG_MODULE_UNLOAD*/ -- 2.30.2