On Thu, Jun 21, 2018 at 08:07:23AM -0600, Jens Axboe wrote:

I'd be fine with that, if we knew that nobody uses it. But that's
really hard to figure out. I did see Jann's source code scan, which
even if non-exhaustive, still shows at least one user of it.

One is an example, and the other looks very close to an example,
as far as I can tell it was Nic doing a bsg read/write WIP for a
tgt module without anyone every picking up on it.  I did add the tgt
list to Cc and no one seemed to care about the bsg read/write support.
Adding the tgt list back, but I doubt anyone ever actually used it.

How about we just make the write interface sync? Then any copy can
happen while the we block the task, and the read side is just
copying the header info back, or dumping it if the task didn't
read it before it went away.

How is that going to work?  As far as I can tell each I/O using
bsg read/write needs a write and a read, so they need to pair
and thus can't be a purely sync interface.

It also doesn't help with the issue that bsg_write may possible
write to user memory, which is highly unusal and asking for security
issues itself.

Either way, we should probably at very least apply a respun version
of the patch from Jann to 4.18-rc and -stable while we keep discussing
this.

Jann, can you respin the bsg patch with the same changes as the now
included sg one?