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Thu, 10 Oct 2019 07:57:57 +0000 (UTC) Date: Thu, 10 Oct 2019 09:57:56 +0200 From: Petr Mladek To: Qian Cai Cc: Christian Borntraeger , Heiko Carstens , sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, Michal Hocko , linux-mm@kvack.org, john.ogness@linutronix.de, akpm@linux-foundation.org, Vasily Gorbik , PeterOberparleiter , david@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() Message-ID: <20191010075756.nyix7l32ai6fylzn@pathway.suse.cz> References:<20191008183525.GQ6681@dhcp22.suse.cz> <1570561573.5576.307.camel@lca.pw> <20191008191728.GS6681@dhcp22.suse.cz> <1570563324.5576.309.camel@lca.pw> <20191009114903.aa6j6sa56z2cssom@pathway.suse.cz> <1570626402.5937.1.camel@lca.pw> <20191009132746.GA6681@dhcp22.suse.cz> <1570628593.5937.3.camel@lca.pw> <20191009142438.yx74ukfqwy2hr4fz@pathway.suse.cz> <1570632374.5937.8.camel@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To:<1570632374.5937.8.camel@lca.pw> User-Agent: NeoMutt/20170912 (1.9.0) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000085, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed 2019-10-09 10:46:14, Qian Cai wrote: > On Wed, 2019-10-09 at 16:24 +0200, Petr Mladek wrote: > > On Wed 2019-10-09 09:43:13, Qian Cai wrote: > > > On Wed, 2019-10-09 at 15:27 +0200, Michal Hocko wrote: > > > > On Wed 09-10-19 09:06:42, Qian Cai wrote: > > > > [...] > > > > > https://lore.kernel.org/linux-mm/1570460350.5576.290.camel@lca.= pw/ > > > > >=20 > > > > > [=A0=A0297.425964] -> #1 (&port_lock_key){-.-.}: > > > > > [=A0=A0297.425967]=A0=A0=A0=A0=A0=A0=A0=A0__lock_acquire+0x5b3/= 0xb40 > > > > > [=A0=A0297.425967]=A0=A0=A0=A0=A0=A0=A0=A0lock_acquire+0x126/0x= 280 > > > > > [=A0=A0297.425968]=A0=A0=A0=A0=A0=A0=A0=A0_raw_spin_lock_irqsav= e+0x3a/0x50 > > > > > [=A0=A0297.425969]=A0=A0=A0=A0=A0=A0=A0=A0serial8250_console_wr= ite+0x3e4/0x450 > > > > > [=A0=A0297.425970]=A0=A0=A0=A0=A0=A0=A0=A0univ8250_console_writ= e+0x4b/0x60 > > > > > [=A0=A0297.425970]=A0=A0=A0=A0=A0=A0=A0=A0console_unlock+0x501/= 0x750 > > > > > [=A0=A0297.425971]=A0=A0=A0=A0=A0=A0=A0=A0vprintk_emit+0x10d/0x= 340 > > > > > [=A0=A0297.425972]=A0=A0=A0=A0=A0=A0=A0=A0vprintk_default+0x1f/= 0x30 > > > > > [=A0=A0297.425972]=A0=A0=A0=A0=A0=A0=A0=A0vprintk_func+0x44/0xd= 4 > > > > > [=A0=A0297.425973]=A0=A0=A0=A0=A0=A0=A0=A0printk+0x9f/0xc5 > > > > > [=A0=A0297.425974]=A0=A0=A0=A0=A0=A0=A0=A0register_console+0x39= c/0x520 > > > > > [=A0=A0297.425975]=A0=A0=A0=A0=A0=A0=A0=A0univ8250_console_init= +0x23/0x2d > > > > > [=A0=A0297.425975]=A0=A0=A0=A0=A0=A0=A0=A0console_init+0x338/0x= 4cd > > > > > [=A0=A0297.425976]=A0=A0=A0=A0=A0=A0=A0=A0start_kernel+0x534/0x= 724 > > > > > [=A0=A0297.425977]=A0=A0=A0=A0=A0=A0=A0=A0x86_64_start_reservat= ions+0x24/0x26 > > > > > [=A0=A0297.425977]=A0=A0=A0=A0=A0=A0=A0=A0x86_64_start_kernel+0= xf4/0xfb > > > > > [=A0=A0297.425978]=A0=A0=A0=A0=A0=A0=A0=A0secondary_startup_64+= 0xb6/0xc0 > > > > >=20 > > > > > where=A0the report again show the early boot call trace for the= locking > > > > > dependency, > > > > >=20 > > > > > console_owner --> port_lock_key > > > > >=20 > > > > > but that dependency clearly not only happen in the early boot. > > > >=20 > > > > Can you provide an example of the runtime dependency without any = early > > > > boot artifacts? Because this discussion really doens't make much = sense > > > > without a clear example of a _real_ lockdep report that is not a = false > > > > possitive. All of them so far have been concluded to be false pos= sitive > > > > AFAIU. > > >=20 > > > An obvious one is in the above link. Just replace the trace in #1 a= bove with > > > printk() from anywhere, i.e., just ignore the early boot calls ther= e as they are > > > not important. > > >=20 > > > printk() > > > console_unlock() > > > console_lock_spinning_enable() --> console_owner_lock > > > call_console_drivers() > > > serial8250_console_write() --> port->lock > >=20 > > Please, find the location where this really happens and then suggests > > how the real deadlock could get fixed. So far, we have seen only > > false positives and theoretical scenarios. >=20 > Now the bar is higher again. You are now asking me to actually trigger = this > potential deadlock live. I am probably better off buying some lottery t= ickets > then if I could be that lucky. No, we just do not want to comlicate the code too much just to hide false positives from lockdep. I do not ask you to reproduce the deadlock. I ask you to find a code path where the deadlock might really happen. It seems that you actually found one in the tty code in the other mail. Best Regards, Petr