linux-kernel - Re: [PATCH 2/4] fix the broken lockdep logic in __sb_start

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Message-ID: <20150722211353.GB19636@redhat.com>
Date:	Wed, 22 Jul 2015 23:13:53 +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>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] fix the broken lockdep logic in __sb_start_write()

On 07/21, Jan Kara wrote:
>
> On Mon 20-07-15 19:01:03, 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().
>
> The patch looks good. Did you test this BTW? You can add:

Yes, but "artificially". I just wrote the function which takes/drops
SB_FREEZE_FS twice with and then without SB_FREEZE_WRITE. It worked
as expected, lockdep complained when SB_FREEZE_WRITE wasn't held.

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

Thanks!

Oleg.

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