linux-ext4 - Re: [PATCH 08/22] ext4: Provide function to handle transaction restarts

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Date:   Wed, 23 Oct 2019 18:25:13 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 08/22] ext4: Provide function to handle transaction
 restarts

On Mon 21-10-19 12:20:46, Theodore Y. Ts'o wrote:
> On Fri, Oct 04, 2019 at 12:05:54AM +0200, Jan Kara wrote:
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index fb0f99dc8c22..32f2c22c7ef2 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> 
> > +/*
> > + * Make sure 'handle' has at least 'check_cred' credits. If not, restart
> > + * transaction with 'restart_cred' credits. The function drops i_data_sem
> > + * when restarting transaction and gets it after transaction is restarted.
> > + *
> > + * The function returns 0 on success, 1 if transaction had to be restarted,
> > + * and < 0 in case of fatal error.
> > + */
> > +int ext4_datasem_ensure_credits(handle_t *handle, struct inode *inode,
> > +				int check_cred, int restart_cred)
> 
> This makes me super nervous.  This gets called by ext4_access_path(),
> which in turn is called by the insert_range, and collapse_range (among
> others) where we previously were not dropping i_data_sem.  This means
> we will be dropping i_data_sem while they are in the middle of doing
> surgery to the extent tree, which makes me super nervous.

But this patch changes nothing in that regard. Previously,
ext4_access_path() was using ext4_ext_truncate_extend_restart() which
called ext4_truncate_restart_trans() which was dropping i_data_sem as well.

> Granted, insert_range and collapse_range take a lot of locks,
> including the inode lock, but it's not obvious to me that this is
> safe, and at the very least the documentation for ext4_access_path
> should have a warning note in its comments that i_data_sem can get
> dropped, and its call sites audited if they haven't already.

Yeah, comment about that would be nice. I can add that when touching this
function anyway.

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