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[66.187.233.206]) by smtp.gmail.com with ESMTPSA id m15sm8809190qka.104.2019.10.08.12.06.13 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 08 Oct 2019 12:06:14 -0700 (PDT) Message-ID: <1570561573.5576.307.camel@lca.pw> Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() From: Qian Cai To: Michal Hocko Cc: Christian Borntraeger , Petr Mladek , Peter Oberparleiter , 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 Date: Tue, 08 Oct 2019 15:06:13 -0400 In-Reply-To: <20191008183525.GQ6681@dhcp22.suse.cz> 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> <1570550917.5576.303.camel@lca.pw> <20191008183525.GQ6681@dhcp22.suse.cz> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-10.el7) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2019-10-08 at 20:35 +0200, Michal Hocko wrote: > On Tue 08-10-19 12:08:37, Qian Cai wrote: > > On Tue, 2019-10-08 at 14:56 +0200, Christian Borntraeger wrote: > > > 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. > > > > > > > > The above fix is simply insufficient, > > Isn't this yet another init time lockdep false possitive? Again, this is not 100% false positive for sure yet. > > > 00: [    3.654337] -> #3 (console_owner){....}:                                  > > 00: [    3.654343]        lock_acquire+0x21a/0x468                               > > 00: [    3.654345]        console_unlock+0x3a6/0xa30                             > > 00: [    3.654346]        vprintk_emit+0x184/0x3c8                               > > 00: [    3.654348]        vprintk_default+0x44/0x50                              > > 00: [    3.654349]        printk+0xa8/0xc0                                       > > 00: [    3.654351]        get_random_u64+0x40/0x108                              > > 00: [    3.654360]        add_to_free_area_random+0x188/0x1c0                    > > 00: [    3.654364]        free_one_page+0x72/0x128                               > > 00: [    3.654366]        __free_pages_ok+0x51c/0xca0                            > > 00: [    3.654368]        memblock_free_all+0x30a/0x3b0                          > > 00: [    3.654370]        mem_init+0x84/0x200                                    > > 00: [    3.654371]        start_kernel+0x384/0x6a0                               > > 00: [    3.654373]        startup_continue+0x70/0xd0                             > > This one is actually a nice example why trying to get printk out of the > zone->lock is simply not viable. This one is likely a printk to warn > that the random pool is not fully intiailized. Just because the > allocator tries to randomize the initial free memory pool. You are not > going to remove that printk, right? Well, Sergey had a patch to convert that one to printk_deferred(), but even with his patch, it will still trigger the lockdep splat here because the lock dependency between zone->lock --> console_owner is still there from memory offline. > > I fully agree that this class of lockdep splats are annoying especially > when they make the lockdep unusable but please discuss this with lockdep > maintainers and try to find some solution rather than go and try to > workaround the problem all over the place. If there are places that > would result in a cleaner code then go for it but please do not make the > code worse just because of a non existent problem flagged by a false > positive. It makes me wonder what make you think it is a false positive for sure.