Re: [RFC] scsi: reduce protection of scan_mutex in scsi_remove_device

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Fri, 2017-04-21 at 22:31 +0000, Song Liu wrote:
> On Apr 21, 2017, at 2:20 PM, Bart Van Assche <Bart.VanAssche@sandisk.com> wrote:
> > On Fri, 2017-04-21 at 14:13 -0700, Song Liu wrote:
> > > On the other hand, some devices do long latency IO during deletion,
> > > for example, sd_shutdown() may do sync cache and/or start_stop.
> > > It is not necessary for these commands to run in series.
> > 
> > Have you noticed my patch series that makes sd_shutdown() submit the
> > SYNCHRONIZE CACHE command asynchronously? Have you tried whether that
> > patch series would be a good alternative?
> 
> The asynchronous SYNCHRONIZE CACHE will not help our use case, where the 
> latency comes from sd_start_stop_device(). Seems it is not easy to make 
> the START STOP UNIT command async. 

Hello Song,

It should be possible to make the START STOP UNIT command asynchronous too
by issuing it asynchronously from inside sd_sync_cache_done(). To avoid
dereferencing a stale struct scsi_disk pointer you will either have to hold
an additional reference as long as the SYNCHRONIZE CACHE command is in
progress or copy the data needed from struct scsi_disk into the SCSI request.

Bart.