On Wed, Mar 19, 2014 at 1:31 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 19 Mar 2014, Andreas Reis wrote: > >> I've uploaded a dmesg with the new debugging patch to bugzilla: >> https://bugzilla.kernel.org/attachment.cgi?id=130041 > > Thanks. I have now managed to reproduce many of the features of this > problem on my own computer. > > James, I will need your help (or help from somebody who understands the > SCSI error handler) to figure out how this problem should be fixed. > > Basically, usb-storage deadlocks when the SCSI error handler invokes > the eh_device_reset_handler callback while a command is running. The > command has timed out and will never complete normally, because the > device's firmware has crashed. But usb-storage's device-reset routine > waits for the current command to finish, which brings everything to a > standstill. > > Is this design wrong? That is, should the device-reset routine wait > for currently executing commands to finish, or should it abort them, or > what? > > Or should the SCSI error handler abort the running command before > invoking the eh_device_reset_handler callback? > > For the record, and in case anyone is curious, here's the detailed > sequence of events during my test: > > sd issues a READ(10) command. For whatever reason, the device > goes nuts and the command times out. > > scsi_times_out() calls scsi_abort_command(), which queues an > abort request. > > scmd_eh_abort_handler() calls scsi_try_to_abort_cmd(), which > succeeds in aborting the READ. > > The READ command is retried (I didn't trace through the details > of this). The retry fails with a Unit Attention (SK=6, > ASC=0x29, Reset or Bus Device Reset Occurred). > > The READ command is retried a second time, and it times out > again. > > This time around, scsi_times_out() calls scsi_abort_command() > unsuccessfully (because the SCSI_EH_ABORT_SCHEDULED flag is > still set). > > As a result, scsi_error_handler() calls scsi_unjam_host(), > which calls scsi_eh_get_sense(). > > That routine calls scsi_request_sense(), which goes into > scsi_send_eh_cmnd(). > > The calls to shost->hostt->queuecommand() all fail, because the > READ command is still running and usb-storage has a queue > depth of 1. The error messages produced by these failures are > disconcerting but not dangerous. > > Since the REQUEST SENSE command was never issued, > scsi_eh_get_sense() returns 0. > > scsi_unjam_host() goes on to call scsi_eh_abort_cmds(), which > does essentially nothing because the SCSI_EH_CANCEL_CMD flag > for the only command on work_q is clear. > scsi_eh_test_devices() returns 0 because check_list is empty > and work_q isn't. > > scsi_unjam_host() then calls scsi_eh_ready_devs(). This > routine ends up calling scsi_eh_bus_device_reset(), at which > point usb-storage deadlocks as described above. > > (On Andreas's system, the first READ retry times out as opposed to the > second retry as on my computer. I doubt this makes any difference.) > > I can't tell if this is all working as intended or if it went off the > tracks somewhere. > > Thanks for any guidance. > It seems to me we need an ->eh_strategy_handler() that understands the usb transport and will escalate to device reset around the time scsi_abort_command() fails. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
