Re: [PATCH v3 2/3] block: introduce new field bd_flags in block_device

[Ming Lei <ming.lei@redhat.com>]

On Wed, Nov 22, 2023 at 04:47:51AM -0800, Christoph Hellwig wrote:
> 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

Yeah, C/C++ doesn't provide such guarantee, but many modern ARCHs [1]
guarantees that RW on naturally aligned type is atomic.

I verified the point on x86/arm64/ppc64le by the following code, and
all three STOREs are done in single instruction.

	struct data {
		int b;
		char a;
		char a2;
		char a3;
		char a4;
	} __attribute__((aligned(8)));
	
	void atomic_test()
	{
		struct data d;
	
		d.b = 1;
		d.a = 2;
		d.a3 = 3;
	
		printf("%d %d %d\n", d.b, d.a, d.a3);
	}

[1] https://preshing.com/20130618/atomic-vs-non-atomic-operations/

> and even worse bd_partno.  Granted that is only alpha these
> days IIRC, but it's still buggy.

bd_has_submit_bio and bd_partno can be thought as read only, and the
two can be corrupted?

bd_dev may have similar trouble with bd_partno for ARCHs which don't
provide atomic RW on naturally aligned int.


Thanks,
Ming