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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 8 Oct 2019 13:56:47 +0100 Received: from d06av25.portsmouth.uk.ibm.com (d06av25.portsmouth.uk.ibm.com [9.149.105.61]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x98CujvH52625654 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 8 Oct 2019 12:56:45 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9B61511C04C; Tue, 8 Oct 2019 12:56:45 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2342111C050; Tue, 8 Oct 2019 12:56:45 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.152.224.119]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 8 Oct 2019 12:56:45 +0000 (GMT) Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() To: Michal Hocko , Petr Mladek , Peter Oberparleiter Cc: Qian Cai , akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, david@redhat.com, linux-kernel@vger.kernel.org, Heiko Carstens , Vasily Gorbik References: <1570228005-24979-1-git-send-email-cai@lca.pw> <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> <20191007144937.GO2381@dhcp22.suse.cz> <20191008074357.f33f6pbs4cw5majk@pathway.suse.cz> <20191008082752.GB6681@dhcp22.suse.cz> From: Christian Borntraeger Openpgp: preference=signencrypt Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20191008082752.GB6681@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 19100812-0012-0000-0000-00000356174C X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19100812-0013-0000-0000-00002191189D Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-10-08_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=65 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1908290000 definitions=main-1910080124 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Adding Peter Oberparleiter. Peter, can you have a look? On 08.10.19 10:27, Michal Hocko wrote: > On Tue 08-10-19 09:43:57, Petr Mladek wrote: >> On Mon 2019-10-07 16:49:37, Michal Hocko wrote: >>> [Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@lca.pw >>> Petr has explained it is a false positive >>> http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz] >>> On Mon 07-10-19 16:30:02, Petr Mladek wrote: >>> [...] >>>> I believe that it cannot really happen because: >>>> >>>> static int __init >>>> sclp_console_init(void) >>>> { >>>> [...] >>>> rc = sclp_rw_init(); >>>> [...] >>>> register_console(&sclp_console); >>>> return 0; >>>> } >>>> >>>> sclp_rw_init() is called before register_console(). And >>>> console_unlock() will never call sclp_console_write() before >>>> the console is registered. >>>> >>>> AFAIK, lockdep only compares existing chain of locks. It does >>>> not know about console registration that would make some >>>> code paths mutually exclusive. >>>> >>>> I believe that it is a false positive. I do not know how to >>>> avoid this lockdep report. I hope that it will disappear >>>> by deferring all printk() calls rather soon. >>> >>> Thanks a lot for looking into this Petr. I have also checked the code >>> and I really fail to see why the allocation has to be done under the >>> lock in the first place. sclp_read_sccb and sclp_init_sccb are global >>> variables but I strongly suspect that they need a synchronization during >>> early init, callbacks are registered only later IIUC: >> >> Good idea. It would work when the init function is called only once. >> But see below. >> >>> diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c >>> index d2ab3f07c008..4b1c033e3255 100644 >>> --- a/drivers/s390/char/sclp.c >>> +++ b/drivers/s390/char/sclp.c >>> @@ -1169,13 +1169,13 @@ sclp_init(void) >>> unsigned long flags; >>> int rc = 0; >>> >>> + sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); >>> + sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); >>> spin_lock_irqsave(&sclp_lock, flags); >>> /* Check for previous or running initialization */ >>> if (sclp_init_state != sclp_init_state_uninitialized) >>> goto fail_unlock; >> >> It seems that sclp_init() could be called several times in parallel. >> I see it called from sclp_register() and sclp_initcall(). > > Interesting. Something for s390 people to answer I guess. > Anyway, this should be quite trivial to workaround by a cmpxch or alike. >