From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 28D23155C8A for ; Thu, 6 Jun 2024 09:02:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717664566; cv=none; b=QCgoUVnPvE6wRjHFGsyphQYWXUF5Rse9mQg2W5jkQvi777tFnWzkwXWwBP6FX+1zvnJ9cqaBdn8p/wnvvB7BNqjq/66ryZ1tISk7tYzkoCXqiJ+hy8mvhd3ocULqxIEkg0Ce7kqmxF48QegrrXhlgFq9DDXIZ1gKxuhvKiz+8BA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717664566; c=relaxed/simple; bh=9icnmEfpu4qV4+inQ0UmDn7/E9tga/lNpi/8MPeEJbs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fMQ2ihOwDyilAB8LMGv9J4V+gXjJb5hjCCDGK1L1g8BuaIK1nnUIwzFEReP/azgnmpPDzHKUO75z4d626i6XVe/ifZm+YdXiqZje/Bgds4o6kIF2yMSNrp7jlJNH54xX4ZLeClJQrQwcOW39q2XZYSL1ohozmMrx7UEUwKdCESQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=IA9PizxD; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="IA9PizxD" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1717664564; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=LeIpi3AlPo5Ron82kvJE1KgranvjfzkDt2CgEkhJ+AQ=; b=IA9PizxDsAzku09WqXUOik0zehNLT0g4739OvjSnlV10ycXF+xHSmLaswt+8PbWpYq3G9r 7AUdXAkiUbK6dKkEQ8BV45BMYz6qsJCERmXlxhqMBj960dvASsLGH3pYWabztC3+w16Ckq s5hIq/NLVeXDBv5XoZQ6KoiTByib3A8= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-219-KtvURmmHOEOHZXqGUnOUMA-1; Thu, 06 Jun 2024 05:02:40 -0400 X-MC-Unique: KtvURmmHOEOHZXqGUnOUMA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id DB4F529AA387; Thu, 6 Jun 2024 09:02:39 +0000 (UTC) Received: from fedora (unknown [10.72.113.78]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 16B6B20229B8; Thu, 6 Jun 2024 09:02:34 +0000 (UTC) Date: Thu, 6 Jun 2024 17:02:30 +0800 From: Ming Lei To: YangYang Cc: Jens Axboe , Andrew Morton , Pavel Begunkov , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, "yukuai (C)" , Yu Kuai Subject: Re: [PATCH v2] sbitmap: fix io hung due to race on sbitmap_word::cleared Message-ID: References: <20240604031124.2261-1-yang.yang@vivo.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 On Thu, Jun 06, 2024 at 03:21:38PM +0800, YangYang wrote: > On 2024/6/6 11:12, Ming Lei wrote: > > On Tue, Jun 04, 2024 at 02:12:22PM +0800, Yu Kuai wrote: > > > Hi, > > > > > > 在 2024/06/04 11:25, Ming Lei 写道: > > > > On Tue, Jun 4, 2024 at 11:12 AM Yang Yang wrote: > > > > > > > > > > Configuration for sbq: > > > > > depth=64, wake_batch=6, shift=6, map_nr=1 > > > > > > > > > > 1. There are 64 requests in progress: > > > > > map->word = 0xFFFFFFFFFFFFFFFF > > > > > 2. After all the 64 requests complete, and no more requests come: > > > > > map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF > > > > > 3. Now two tasks try to allocate requests: > > > > > T1: T2: > > > > > __blk_mq_get_tag . > > > > > __sbitmap_queue_get . > > > > > sbitmap_get . > > > > > sbitmap_find_bit . > > > > > sbitmap_find_bit_in_word . > > > > > __sbitmap_get_word -> nr=-1 __blk_mq_get_tag > > > > > sbitmap_deferred_clear __sbitmap_queue_get > > > > > /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit > > > > > if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word > > > > > return false; __sbitmap_get_word -> nr=-1 > > > > > mask = xchg(&map->cleared, 0) sbitmap_deferred_clear > > > > > atomic_long_andnot() /* map->cleared=0 */ > > > > > if (!(map->cleared)) > > > > > return false; > > > > > /* > > > > > * map->cleared is cleared by T1 > > > > > * T2 fail to acquire the tag > > > > > */ > > > > > > > > > > 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken > > > > > up due to the wake_batch being set at 6. If no more requests come, T1 > > > > > will wait here indefinitely. > > > > > > > > > > To fix this issue, simply revert commit 661d4f55a794 ("sbitmap: > > > > > remove swap_lock"), which causes this issue. > > > > > > > > I'd suggest to add the following words in commit log: > > > > > > > > Check on ->cleared and update on both ->cleared and ->word need to be > > > > done atomically, and using spinlock could be the simplest solution. > > > > > > > > Otherwise, the patch looks fine for me. > > > > > > Maybe I'm noob, but I'm confused how can this fix the problem, looks > > > like the race condition doesn't change. > > > > > > In sbitmap_find_bit_in_word: > > > > > > 1) __sbitmap_get_word read word; > > > 2) sbitmap_deferred_clear clear cleared; > > > 3) sbitmap_deferred_clear update word; > > > > > > 2) and 3) are done atomically while 1) can still concurrent with 3): > > > > After 1) fails, sbitmap_deferred_clear() is called with spinlock, > > then it is pretty easy to solve the race, such as, the following patch > > against the revert patch. > > > > > > diff --git a/lib/sbitmap.c b/lib/sbitmap.c > > index dee02a0266a6..c015ecd8e10e 100644 > > --- a/lib/sbitmap.c > > +++ b/lib/sbitmap.c > > @@ -63,13 +63,15 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb, > > static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) > > { > > unsigned long mask; > > - bool ret = false; > > unsigned long flags; > > + bool ret; > > spin_lock_irqsave(&map->swap_lock, flags); > > - if (!map->cleared) > > + if (!map->cleared) { > > + ret = !!map->word; > > After atomic_long_andnot(mask, (atomic_long_t *)&map->word), map->word > may be 0 if all requests have completed, or not 0 if some requests are > still in flight. setting ->word is lockless, but zeroing ->word is serialized with ->swap_lock. > Therefore, using !!map->word to determine the > availability of free tags is inaccurate. The check should be changed to decide if any free bit is available in map->word instead of !!map->word, and 'shift' need to be passed in sbitmap_deferred_clear(). Just be curious, do you have reproducer for this issue? Thanks, Ming