When the SCSI scanning code discovers a SCSI device it calls the driver core function device_add() to associate a SCSI ULD with the device. The driver core invokes the probing function for the matching SCSI ULP, e.g. sd_probe(). In order to minimize the time needed to scan SCSI targets that have a large number of LUNs, the SCSI disk driver scans LUNs asynchronously by starting the actual probing work asynchronously from inside sd_probe() An unfortunate aspect of how SCSI disk probing works today is that there is unnecessary serialization between probing and removal activity. For a possible approach of how to increase SCSI disk probing concurrency, see also [PATCH] sd: Increase SCSI disk probing concurrency, linux-scsi mailing list, December 2017 (https://www.spinics.net/lists/linux-scsi/msg115657.html). A second unfortunate aspect of SCSI disk probing is that certain race conditions in the block layer are hit if removal starts before asynchronous probing has finished. This is because the driver core is unaware that the SCSI disk code works asynchronously. Additionally, the SCSI disk asynchronous probing approach is incompatible with the power management code. The power management code calls wait_for_device_probe() in the driver core to wait for device probing activity to finish. wait_for_device_probe() however is unaware of the asynchronous probing in the SCSI sd driver and hence doesn't wait for the SCSI sd probing activity to finish. My proposal is to hold a session to discus potential solutions for increasing SCSI disk probing concurrency in a way that is compatible with the driver core and the power management subsystem.
