linux-kernel - Re: [PATCH v2 2/8] fix the broken lockdep logic in __sb_start

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Date:	Thu, 13 Aug 2015 15:22:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] fix the broken lockdep logic in
	__sb_start_write()

On 08/13, Jan Kara wrote:
>
> On Tue 11-08-15 19:04:01, Oleg Nesterov wrote:
> > 1. wait_event(frozen < level) without rwsem_acquire_read() is just
> >    wrong from lockdep perspective. If we are going to deadlock
> >    because the caller is buggy, lockdep detect this problem.
> >
> > 2. __sb_start_write() can race with thaw_super() + freeze_super(),
> >    and after "goto retry" the 2nd  acquire_freeze_lock() is wrong.
> >
> > 3. The "tell lockdep we are doing trylock" hack doesn't look nice.
> >
> >    I think this is correct, but this logic should be more explicit.
> >    Yes, the recursive read_lock() is fine if we hold the lock on a
> >    higher level. But we do not need to fool lockdep. If we can not
> >    deadlock in this case then try-lock must not fail and we can use
> >    use wait == F throughout this code.
> >
> > Note: as Dave Chinner explains, the "trylock" hack and the fat comment
> > can be probably removed. But this needs a separate change and it will
> > be trivial: just kill __sb_start_write() and rename do_sb_start_write()
> > back to __sb_start_write().
> >
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>
> Just a nit below...
>
> > +	if (wait)
> > +		rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
>
> If we provided also __sb_writers_acquire() helper (in addition to _nowait)
> variant, we could use these helpers in __sb_start_write() /
> __sb_end_write() as well which would look better to me when we already have
> them.

Why? This code goes away after 8/8.

Oleg.

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