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

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Message-ID: <20191021162046.GA27850@mit.edu>
Date:   Mon, 21 Oct 2019 12:20:46 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH 08/22] ext4: Provide function to handle transaction
 restarts

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.

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.

Thanks,

							- Ted