On Fri, 2017-04-21 at 14:13 -0700, Song Liu wrote: > When a device is deleted through sysfs handle "delete", the code > locks shost->scan_mutex. If multiple devices are deleted at the > same time, these deletes will be handled in series. > > 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. > > To reduce latency of parallel "delete" requests, this patch reduces > the protection of scan_mutex. The only function with Scsi_Host > called in __scsi_remove_device() is the optional slave_destroy(). > Therefore, the protection of scan_mutex is only necessary for this > function. Hello Song, What you wrote above is wrong. It is necessary to serialize SCSI scanning against SCSI device removal. That is why scan_mutex is held around the __scsi_remove_device() call. When adding a LUN the following (and several other) actions are performed: * Allocation of memory for struct scsi_device. * Allocation of a block layer request queue. * Initialization of the .sdev_gendev and .sdev_dev device structures. * Association of the transport layer driver with the SCSI device. * Association of a device handler with the SCSI device (e.g. ALUA). * Making the .sdev_gendev and .sdev_dev devices visible in sysfs. * Making the transport layer attributes visible in sysfs. * Creating a bsg device node in sysfs. * Association of an upper layer driver (e.g. sd or sr) with the SCSI LUN. Removal of a LUN means undoing all of the above. If adding and removing a LUN would not be not serialized then there would be a risk that removal and immediate reregistration of a LUN will fail due to the reregistration process trying to add sysfs attributes that were not yet removed by the removal step. Bart.
