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 AE088C433F5 for ; Tue, 14 Dec 2021 18:41:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237074AbhLNSlR (ORCPT ); Tue, 14 Dec 2021 13:41:17 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36130 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231756AbhLNSlR (ORCPT ); Tue, 14 Dec 2021 13:41:17 -0500 Received: from mail-qv1-xf2e.google.com (mail-qv1-xf2e.google.com [IPv6:2607:f8b0:4864:20::f2e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 17A6EC061574 for ; Tue, 14 Dec 2021 10:41:17 -0800 (PST) Received: by mail-qv1-xf2e.google.com with SMTP id u16so18118768qvk.4 for ; Tue, 14 Dec 2021 10:41:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=broadcom.com; s=google; h=from:references:in-reply-to:mime-version:thread-index:date :message-id:subject:to:cc; bh=Hbn/AxB91HmkwwtH/dhLGpQ/ltRDkxXYipVduIB6Tk0=; b=MHRJwugqRhEw6AMoDiMWBCDzzOMDqdx3a65vyraIzdHAO1xJQOj27aTb9H/zoUhN/M J2y3dIIteD/m3ADrHTERgZrLVDBUfCOumsrw9FhiFQNIj3AUjtvhAE9v2XIU1xVRHdHJ AD6tPn8xgP/VC/Zp0GMYu2YtZ8cu0pY83tnkk= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:references:in-reply-to:mime-version :thread-index:date:message-id:subject:to:cc; bh=Hbn/AxB91HmkwwtH/dhLGpQ/ltRDkxXYipVduIB6Tk0=; b=EUmAEvZaIbP1SxohSheLbPeoJ5TPNBLWlnBcbpBP3gLLJXeFvM02wKtfMc0xT3D7Nb 69zUt+bRwqtGdIzj2sKd1RA/nF72DeQkJnV6wzuP+UGTj17WRquJfisioh9XGe4xtZr/ QURDcf/Un16qbKft3ltIjoj02b4hqcXmkV0o0ZRGjCtbDbLUuxl0uGWHiYxP5FWHKi2n zZh4+jJJ0ixxM15qwVFAj2OqGlZqoPNwVQkJe9jBmOfdIF+iePx5N+/5CK2jOOFKjzrq U+2S6Ly2DrMjUVB559LC0SNB/hmDS93kU56wVZl73HOKBWBfxvez0kPSGdV9X0mJ2H72 SidA== X-Gm-Message-State: AOAM530sgTuC2nDHxywzC7zyDs2LCd02eNIroOXdWZpKnhmdVkLAmB5o eGutkZpLPx+SyflOjoWl4uZ4mc0t654Lkr7Wdh2Wlw== X-Google-Smtp-Source: ABdhPJzdWdT6fkUlsCkUoy0Cm/mlOpmp5mPLdWalI2L8LEaklpwBLhB0XgwKmsngd1yv+VSkDnA3FFUd0DiFzXTHCys= X-Received: by 2002:a05:6214:21e3:: with SMTP id p3mr7244873qvj.93.1639507275925; Tue, 14 Dec 2021 10:41:15 -0800 (PST) From: Kashyap Desai References: <20210906065003.439019-1-ming.lei@redhat.com> In-Reply-To: <20210906065003.439019-1-ming.lei@redhat.com> MIME-Version: 1.0 X-Mailer: Microsoft Outlook 15.0 Thread-Index: AQCw9RdeG8B11ACtJOqN2MhOUMiBIa6AQIaA Date: Wed, 15 Dec 2021 00:11:13 +0530 Message-ID: <0d8666c9983158a4954f30f6b429e797@mail.gmail.com> Subject: RE: [PATCH] blk-mq: avoid to iterate over stale request To: Ming Lei , luojiaxing Cc: Jens Axboe , linux-block@vger.kernel.org, linux-scsi@vger.kernel.org, "Martin K. Petersen" , John Garry Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-256; boundary="000000000000cff5c005d31f88cd" Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org --000000000000cff5c005d31f88cd Content-Type: text/plain; charset="UTF-8" + John Garry > blk-mq can't run allocating driver tag and updating ->rqs[tag] atomically, > meantime blk-mq doesn't clear ->rqs[tag] after the driver tag is released. > > So there is chance to iterating over one stale request just after the tag is > allocated and before updating ->rqs[tag]. > > scsi_host_busy_iter() calls scsi_host_check_in_flight() to count scsi in-flight > requests after scsi host is blocked, so no new scsi command can be marked as > SCMD_STATE_INFLIGHT. However, driver tag allocation still can be run by blk- > mq core. One request is marked as SCMD_STATE_INFLIGHT, but this request > may have been kept in another slot of ->rqs[], meantime the slot can be > allocated out but ->rqs[] isn't updated yet. Then this in-flight request is > counted twice as SCMD_STATE_INFLIGHT. This way causes trouble in handling > scsi error. Hi Ming, We found similar issue on RHEL8.5 (kernel does not have this patch in discussion.). Issue reproduced on 5.15 kernel as well. I understood this commit will fix specific race condition and avoid reading incorrect host_busy value. As per commit message - That incorrect host_busy will be just transient. If we read after some delay, correct host_busy count will be available. Right ? In my case (I am using shared host tag enabled driver), it is not race condition issue but stale rqs[] entries create permanent incorrect count of host_busy. Example - There are two pending IOs. This IOs are timed out. Bitmap of pending IO is tag#5 (actually belongs to hctx0), tag#10 (actually belongs to hctx1). Note - This is a shared bit map. If hctx0 has same address of the request at 5th and 10th index, we will count total 2 inflight commands instead of 1 from hctx0 context + From hctx1 context, we will count 1 inflight command = Total is 3. Even though we read after some delay, host_busy will be incorrect. We expect host_busy = 2 but it will return 3. This patch fix my issue explained above for shared host-tag case. I am confused reading the commit message. You may not have intentionally fix the issue as I explained but indirectly it fixes my issue. Am I correct ? What was an issue reported by Luojiaxiang ? I am interested to know if issue reported by Luojiaxiang had shared host tagset enabled ? Kashyap > > Fixes the issue by not iterating over stale request. > > Cc: linux-scsi@vger.kernel.org > Cc: "Martin K. Petersen" > Reported-by: luojiaxing > Signed-off-by: Ming Lei > --- > block/blk-mq-tag.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index > 86f87346232a..ff5caeb82542 100644 > --- a/block/blk-mq-tag.c > +++ b/block/blk-mq-tag.c > @@ -208,7 +208,7 @@ static struct request > *blk_mq_find_and_get_req(struct blk_mq_tags *tags, > > spin_lock_irqsave(&tags->lock, flags); > rq = tags->rqs[bitnr]; > - if (!rq || !refcount_inc_not_zero(&rq->ref)) > + if (!rq || rq->tag != bitnr || !refcount_inc_not_zero(&rq->ref)) > rq = NULL; > spin_unlock_irqrestore(&tags->lock, flags); > return rq; > -- > 2.31.1 --000000000000cff5c005d31f88cd Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIIQcAYJKoZIhvcNAQcCoIIQYTCCEF0CAQExDzANBglghkgBZQMEAgEFADALBgkqhkiG9w0BBwGg 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