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

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Message-ID: <20150813100200.GE26599@quack.suse.cz>
Date:	Thu, 13 Aug 2015 12:02:00 +0200
From:	Jan Kara <jack@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jan Kara <jack@...e.cz>,
	"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 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.

Once this is updated, feel free to add:

Reviewed-by: Jan Kara <jack@...e.com>

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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