linux-kernel - Re: [PATCH v3 2/3] block: introduce new field bd_flags in block

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Message-ID: <ZV34d/hI12pKFUzj@infradead.org>
Date:   Wed, 22 Nov 2023 04:47:51 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 2/3] block: introduce new field bd_flags in
 block_device

On Wed, Nov 22, 2023 at 04:19:40PM +0800, Ming Lei wrote:
> On Tue, Nov 21, 2023 at 11:53:17PM -0800, Christoph Hellwig wrote:
> > On Wed, Nov 22, 2023 at 03:45:24PM +0800, Ming Lei wrote:
> > > All the existed 'bool' flags are not atomic RW, so I think it isn't
> > > necessary to define 'bd_flags' as 'unsigned long' for replacing them.
> > 
> > So because the old code wasn't correct we'll never bother?  The new
> > flag and the new placement certainly make this more critical as well.
> 
> Can you explain why the old code was wrong?
> 
> 1) ->bd_read_only and ->bd_make_it_fail
> 
> - set from userspace interface(ioctl or sysfs)
> - check in IO code path
> 
> so changing it into atomic bit doesn't make difference from user
> viewpoint.

> 
> 2) ->bd_write_holder
> 
> disk->open_mutex is held for read & write this flag
> 
> 3) ->bd_has_submit_bio
> 
> This flag is setup as oneshot before adding disk, and check in FS io code
> path.

On architectures that can't do byte-level atomics all three can corrupt
each other, and even worse bd_partno.  Granted that is only alpha these
days IIRC, but it's still buggy.