From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from dggsgout11.his.huawei.com (dggsgout11.his.huawei.com [45.249.212.51]) (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 B74AA2472AE; Tue, 7 Jul 2026 08:24:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783412661; cv=none; b=Sg9uWDzlkyBRBszHBi45tbKvP/NgpOV1BO9xtJSMTZCX25qFTqex15N8fdVlOp7BYmdqMsH9+JSrNf2rK8Qf/Cl5YyT3jen2y8K51svNYKjG6yT2VJoC3z13NoJQIH38dg9eqxe6Wpfmrp+82/MhCbXnIfn29MUuMZqs1GvTaFw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783412661; c=relaxed/simple; bh=RdcY0+BSF7zN8poPmSC1SmtJ9BTekWE1dFvECPIYLT4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Du5HJqZ+cHt6YW6d4JlB3A972Um/cUMQHBsYcifosnet9erxc272irF7qGm4Gkm1W/S5Ac4RNi1fexbGuMtQntBXikKDKWwjlnLdReAvUf1ezYocpqqN5X6fPIxkLMswU7GA21DrEjlCeKhqjNADCxERZZINBnwmtOp3fuI5ZyI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com; spf=pass smtp.mailfrom=huaweicloud.com; arc=none smtp.client-ip=45.249.212.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huaweicloud.com Received: from mail.maildlp.com (unknown [172.19.163.170]) by dggsgout11.his.huawei.com (SkyGuard) with ESMTPS id 4gvZ5002wTzYQtkV; Tue, 7 Jul 2026 16:24:04 +0800 (CST) Received: from mail02.huawei.com (unknown [10.116.40.112]) by mail.maildlp.com (Postfix) with ESMTP id 06C464056E; Tue, 7 Jul 2026 16:24:07 +0800 (CST) Received: from [10.174.176.179] (unknown [10.174.176.179]) by APP1 (Coremail) with UTF8SMTPSA id cCh0CgC3xnKlt0xqV5pvAQ--.791S3; Tue, 07 Jul 2026 16:24:06 +0800 (CST) Message-ID: <13de8932-e9a4-4d29-8d98-4d3eb0bd91cd@huaweicloud.com> Date: Tue, 7 Jul 2026 16:24:04 +0800 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 5/6] null_blk: don't locklessly overwrite dev state after apply_fn To: Nilay Shroff , Zizhi Wo , axboe@kernel.dk, dlemoal@kernel.org, linux-block@vger.kernel.org, kch@nvidia.com, johannes.thumshirn@wdc.com, kbusch@kernel.org, bvanassche@acm.org Cc: linux-kernel@vger.kernel.org, yangerkun@huawei.com, chengzhihao1@huawei.com References: <20260707025542.1299859-1-wozizhi@huaweicloud.com> <20260707025542.1299859-6-wozizhi@huaweicloud.com> <94fcb5aa-0b1a-4d32-bc07-fd939367c6ee@linux.ibm.com> From: Zizhi Wo In-Reply-To: <94fcb5aa-0b1a-4d32-bc07-fd939367c6ee@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:cCh0CgC3xnKlt0xqV5pvAQ--.791S3 X-Coremail-Antispam: 1UD129KBjvJXoWxGr1kJr43ZF4fWFW3Wr15Jwb_yoWrKr1xpr 48Jr1UJryUJr1xJr4UJw1UXFyrJr1UJw1UGr1UJF1UA34UJr1jqr1UXF1jgw1UJw48Jr15 Xr1UXr1UZr1UJrUanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDU0xBIdaVrnRJUUUvE14x267AKxVW8JVW5JwAFc2x0x2IEx4CE42xK8VAvwI8IcIk0 rVWrJVCq3wAFIxvE14AKwVWUJVWUGwA2ocxC64kIII0Yj41l84x0c7CEw4AK67xGY2AK02 1l84ACjcxK6xIIjxv20xvE14v26r4j6ryUM28EF7xvwVC0I7IYx2IY6xkF7I0E14v26r4j 6F4UM28EF7xvwVC2z280aVAFwI0_Cr1j6rxdM28EF7xvwVC2z280aVCY1x0267AKxVW0oV Cq3wAS0I0E0xvYzxvE52x082IY62kv0487Mc02F40EFcxC0VAKzVAqx4xG6I80ewAv7VC0 I7IYx2IY67AKxVWUJVWUGwAv7VC2z280aVAFwI0_Jr0_Gr1lOx8S6xCaFVCjc4AY6r1j6r 4UM4x0Y48IcVAKI48JM4x0x7Aq67IIx4CEVc8vx2IErcIFxwACI402YVCY1x02628vn2kI c2xKxwCY1x0262kKe7AKxVWUtVW8ZwCF04k20xvY0x0EwIxGrwCFx2IqxVCFs4IE7xkEbV WUJVW8JwC20s026c02F40E14v26r1j6r18MI8I3I0E7480Y4vE14v26r106r1rMI8E67AF 67kF1VAFwI0_Jw0_GFylIxkGc2Ij64vIr41lIxAIcVC0I7IYx2IY67AKxVWUJVWUCwCI42 IY6xIIjxv20xvEc7CjxVAFwI0_Gr0_Cr1lIxAIcVCF04k26cxKx2IYs7xG6r1j6r1xMIIF 0xvEx4A2jsIE14v26r1j6r4UMIIF0xvEx4A2jsIEc7CjxVAFwI0_Gr1j6F4UJbIYCTnIWI evJa73UjIFyTuYvjfUonmRUUUUU X-CM-SenderInfo: pzr2x6tkl6x35dzhxuhorxvhhfrp/ 在 2026/7/7 15:33, Nilay Shroff 写道: > On 7/7/26 8:25 AM, Zizhi Wo wrote: >> From: Zizhi Wo >> >> The NULLB_DEVICE_ATTR macro unconditionally writes dev->NAME = new_value >> after apply_fn() returns. For attributes with an apply_fn (submit_queues, >> poll_queues), apply_fn already sets dev->NAME under &nullb_list_lock. >> >> configfs serializes writes via a per-open-file mutex (buffer->mutex), so >> two threads writing to the same attribute through separate open file >> descriptions run the store callback concurrently. The macro's write is >> redundant and lockless, so a concurrent store's losing thread can >> overwrite >> the winner's value after apply_fn set it, making dev->submit_queues >> mismatch the hardware state. null_map_queues() then hits a >> WARN_ON_ONCE and >> falls back to a single queue. >> >> Restructure the macro so that apply_fn attributes return directly after >> apply_fn, and only non-apply_fn attributes write dev->NAME -- those are >> only changeable while not CONFIGURED and have no live hardware state to >> mismatch. >> >> Fixes: 45919fbfe1c4 ("null_blk: Enable modifying 'submit_queues' after >> an instance has been configured") >> Signed-off-by: Zizhi Wo >> --- >>   drivers/block/null_blk/main.c | 14 ++++++++------ >>   1 file changed, 8 insertions(+), 6 deletions(-) >> >> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/ >> main.c >> index cab51301560e..e7555c47b671 100644 >> --- a/drivers/block/null_blk/main.c >> +++ b/drivers/block/null_blk/main.c >> @@ -360,13 +360,15 @@ nullb_device_##NAME##_store(struct config_item >> *item, const char *page,    \ >>       ret = nullb_device_##TYPE##_attr_store(&new_value, page, count);\ >>       if (ret < 0)                            \ >>           return ret;                        \ >> -    if (apply_fn)                            \ >> +    if (apply_fn) {                            \ >>           ret = apply_fn(dev, new_value);                \ >> -    else if (test_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags))     \ >> -        ret = -EBUSY;                        \ >> -    if (ret < 0)                            \ >> -        return ret;                        \ >> -    dev->NAME = new_value;                        \ >> +        if (ret < 0)                        \ >> +            return ret;                    \ >> +    } else {                            \ >> +        if (test_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags))    \ >> +            return -EBUSY;                    \ >> +        dev->NAME = new_value;                    \ >> +    }                                \ >>       return count;                            \ >>   }                                    \ >>   CONFIGFS_ATTR(nullb_device_, NAME); > > Your patch correctly addresses the synchronization issue regarding the > apply_fn attributes under the nullb_list_lock. However, even with this > restructuring, the 'else' block remains vulnerable to concurrent > unmarked accesses to dev->NAME. > > It seems when the device is not powered on, multiple threads can still > concurrently call the _show and _store callbacks on the same attribute > across separate open configfs file descriptions. > > While this may be considered a benign data race, it can trigger KCSAN > splats and may allow the compiler to perform potentially unsafe > optimization > heuristics (such as load/store tearing or merging). So I think we should > at-least mark those accesses using WRITE_ONCE() and READ_ONCE(). This also > help silence the KCSAN splat if it's configured. > > Thanks, > --Nilay > Thanks for catching this! The 'else' path can indeed be accessed concurrently; I'd left it alone earlier assuming there was no real impact, but from the angle you described, adding READ_ONCE()/ WRITE_ONCE() does make sense. I'll fix this in the next version. Thanks, Zizhi Wo