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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 118E2C433FE for ; Tue, 1 Mar 2022 01:49:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232007AbiCABti (ORCPT ); Mon, 28 Feb 2022 20:49:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39928 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232069AbiCABtb (ORCPT ); Mon, 28 Feb 2022 20:49:31 -0500 Received: from mp-relay-01.fibernetics.ca (mp-relay-01.fibernetics.ca [208.85.217.136]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 67E325DE42 for ; Mon, 28 Feb 2022 17:48:49 -0800 (PST) Received: from mailpool-fe-02.fibernetics.ca (mailpool-fe-02.fibernetics.ca [208.85.217.145]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mp-relay-01.fibernetics.ca (Postfix) with ESMTPS id 1A247E0F68; Tue, 1 Mar 2022 01:48:48 +0000 (UTC) Received: from localhost (mailpool-mx-02.fibernetics.ca [208.85.217.141]) by mailpool-fe-02.fibernetics.ca (Postfix) with ESMTP id 11EF263086; Tue, 1 Mar 2022 01:48:48 +0000 (UTC) X-Virus-Scanned: Debian amavisd-new at Received: from mailpool-fe-02.fibernetics.ca ([208.85.217.145]) by localhost (mail-mx-02.fibernetics.ca [208.85.217.141]) (amavisd-new, port 10024) with ESMTP id E9tgzuaWfdRv; Tue, 1 Mar 2022 01:48:47 +0000 (UTC) Received: from [192.168.48.23] (host-45-78-195-155.dyn.295.ca [45.78.195.155]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: dgilbert@interlog.com) by mail.ca.inter.net (Postfix) with ESMTPSA id 37BAD62664; Tue, 1 Mar 2022 01:48:47 +0000 (UTC) Message-ID: Date: Mon, 28 Feb 2022 20:48:46 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Reply-To: dgilbert@interlog.com Subject: Re: [PATCH 2/2] scsi: scsi_debug: fix sparse lock warnings in sdebug_blk_mq_poll() Content-Language: en-CA To: Damien Le Moal , linux-scsi@vger.kernel.org, "Martin K . Petersen" References: <20220225084527.523038-1-damien.lemoal@opensource.wdc.com> <20220225084527.523038-3-damien.lemoal@opensource.wdc.com> <2dda2a2a-dc54-e335-e0eb-574868397277@opensource.wdc.com> From: Douglas Gilbert In-Reply-To: <2dda2a2a-dc54-e335-e0eb-574868397277@opensource.wdc.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-scsi@vger.kernel.org See below: On 2022-02-28 08:46, Damien Le Moal wrote: > On 2022/02/28 4:05, Douglas Gilbert wrote: >> On 2022-02-25 03:45, Damien Le Moal wrote: >>> The use of the locked boolean variable to control locking and unlocking >>> of the qc_lock of struct sdebug_queue confuses sparse, leading to a >>> warning about an unexpected unlock. Simplify the qc_lock lock/unlock >>> handling code of this function to avoid this warning by removing the >>> locked boolean variable. >> >> See below. >> >>> >>> Signed-off-by: Damien Le Moal >>> --- >>> drivers/scsi/scsi_debug.c | 19 +++++++++---------- >>> 1 file changed, 9 insertions(+), 10 deletions(-) >>> >>> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c >>> index f4e97f2224b2..acb32f3e38eb 100644 >>> --- a/drivers/scsi/scsi_debug.c >>> +++ b/drivers/scsi/scsi_debug.c >>> @@ -7509,7 +7509,6 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num) >>> { >>> bool first; >>> bool retiring = false; >>> - bool locked = false; >>> int num_entries = 0; >>> unsigned int qc_idx = 0; >>> unsigned long iflags; >>> @@ -7525,18 +7524,17 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num) >>> if (qc_idx >= sdebug_max_queue) >>> return 0; >>> >>> + spin_lock_irqsave(&sqp->qc_lock, iflags); >>> + >>> for (first = true; first || qc_idx + 1 < sdebug_max_queue; ) { >>> - if (!locked) { >>> - spin_lock_irqsave(&sqp->qc_lock, iflags); >>> - locked = true; >>> - } >>> if (first) { >>> first = false; >>> if (!test_bit(qc_idx, sqp->in_use_bm)) >>> continue; >>> - } else { >>> - qc_idx = find_next_bit(sqp->in_use_bm, sdebug_max_queue, qc_idx + 1); >>> } >>> + >>> + qc_idx = find_next_bit(sqp->in_use_bm, sdebug_max_queue, >>> + qc_idx + 1); >> >> The original logic is wrong or the above line is wrong. find_next_bit() is not >> called on the first iteration in the original, but it is with this patch. >> >>> if (qc_idx >= sdebug_max_queue) >>> break; >>> >>> @@ -7586,14 +7584,15 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num) >>> } >>> WRITE_ONCE(sd_dp->defer_t, SDEB_DEFER_NONE); >>> spin_unlock_irqrestore(&sqp->qc_lock, iflags); >>> - locked = false; >>> scsi_done(scp); /* callback to mid level */ >>> num_entries++; >>> + spin_lock_irqsave(&sqp->qc_lock, iflags); >>> if (find_first_bit(sqp->in_use_bm, sdebug_max_queue) >= sdebug_max_queue) >>> break; /* if no more then exit without retaking spinlock */ >> >> See that comment on the line above? That is the reason for the guard variable. >> Defying that comment, the modified code does a superfluous spinlock irqsave >> and irqrestore. > > Rechecking this, there is one point that is bothering me: is it OK to have the > find_first_bit() outside of the sqp lock ? If not, then this is a bug and the > extra lock/unlock that my patch add is a fix... I think you are correct, please fix it. You will notice that when the spinlock_irq is dropped to call scsi_done(), that the iteration is restarted. >> >> Sparse could be taken as a comment on the amount of grey matter that tool has. >> >> >>> } >>> - if (locked) >>> - spin_unlock_irqrestore(&sqp->qc_lock, iflags); >>> + >>> + spin_unlock_irqrestore(&sqp->qc_lock, iflags); >>> + >>> if (num_entries > 0) >>> atomic_add(num_entries, &sdeb_mq_poll_count); >>> return num_entries; >> >> Locking issues are extremely difficult to analyze via a unified diff of >> the function. A copy of the original function is required to make any >> sense of it. I was trying to say: it is difficult to understand what diff style output of a change as shown in a [PATCH] post like this, especially to a function's locking, will do, without see the __whole__ function. It is not a criticism of this patchset, but the process in general which loses important context of the function being patched.