linux-kernel - Re: [PATCH] bcachefs: no console_lock in bch2_print_string_as

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Message-ID: <ZpThlAGUsPXZArvk@phenom.ffwll.local>
Date: Mon, 15 Jul 2024 10:45:08 +0200
From: Daniel Vetter <daniel.vetter@...ll.ch>
To: John Ogness <john.ogness@...utronix.de>
Cc: Daniel Vetter <daniel.vetter@...ll.ch>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	syzbot+6cebc1af246fe020a2f0@...kaller.appspotmail.com,
	Daniel Vetter <daniel.vetter@...el.com>, stable@...r.kernel.org,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Brian Foster <bfoster@...hat.com>, linux-bcachefs@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] bcachefs: no console_lock in bch2_print_string_as_lines

On Wed, Jul 10, 2024 at 04:19:53PM +0206, John Ogness wrote:
> On 2024-07-10, Daniel Vetter <daniel.vetter@...ll.ch> wrote:
> > console_lock is the outermost subsystem lock for a lot of subsystems,
> > which means get/put_user must nest within. Which means it cannot be
> > acquired somewhere deeply nested in other locks, and most definitely
> > not while holding fs locks potentially needed to resolve faults.
> >
> > console_trylock is the best we can do here. But John pointed out on a
> > previous version that this is futile:
> >
> > "Using the console lock here at all is wrong. The console lock does not
> > prevent other CPUs from calling printk() and inserting lines in between.
> >
> > "There is no way to guarantee a contiguous ringbuffer block using
> > multiple printk() calls.
> >
> > "The console_lock usage should be removed."
> >
> > https://lore.kernel.org/lkml/87frsh33xp.fsf@jogness.linutronix.de/
> >
> > Do that.
> 
> Note that there is more of this incorrect usage of console lock in:
> 
> fs/bcachefs/debug.c:bch2_btree_verify_replica()
> 
> fs/bcachefs/bset.c:bch2_dump_btree_node()
> 
> from commit 1c6fdbd8f246("bcachefs: Initial commit")
> 
> ... and its parent bcache:
> 
> drivers/md/bcache/debug.c:bch_btree_verify()
> 
> drivers/md/bcache/bset.c:bch_dump_bucket()
> 
> from commit cafe56359144("bcache: A block layer cache")
> 
> These should also be removed. Although Kent should verify that the
> console lock is not providing some sort of necessary side-effect
> synchronization.

I'll take a look, at least some of them seem doable to audit without deep
bcachefs understanding. Thanks for pointing them out, I should have looked
a bit more at git grep ...
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch