On Thu, 2015-06-04 at 11:40 -0700, Rajat Jain wrote: > Each cmd timeout should result in scmd->retries++. Currently it happens > just only before a command is requeued back. However, if the LLD > eh_timed_out() handler asks to reset timer back again, then also it should > be incremented because effectively LLD will be given a full time period > (SD_TIMEOUT = 30 secs!) to attempt to complete the command. > > Why this is a problem: > > => Currently the SCSI low level transport drivers can provide > eh_timeout_handler() calls (for e.g. iscsi provides this) to deal > with command timeouts. > > => The eh_timeout_handler() can return BLK_EH_RESET_TIMER that causes the > SCSI / block layer to reset the timer, thus giving more time to the > LLD. > > => Currently a LLD can potentially loop infinitely on a command if it > always keeps on returning BLK_EH_RESET_TIMER. > > * => Other than choking its own devices, if the command that is stuck is a > command issued during sd_probe_async() (e.g. a partition table scan), > then it impacts all the disks because no other disks can be removed > from the system until sd_probe_async() returns. (sd_remove waits on > async_synchronize_full_domain(...)) > > => This problem actually resulted in the situation mentioned above, > whereby no disks in the system (on other scsi hosts) could be removed, > because of a stuck scsi command to read the partition tables of an > unrelated problematic disk during probe. The threads were stuck at: > > schedule+0x312/0x7a0 > async_synchronize_cookie_domain+0xb8/0x115 > ? __wake_up_bit+0x40/0x40 > async_synchronize_full_domain+0x15/0x17 > sd_remove+0x5f/0x135 > __device_release_driver+0x8a/0xe0 > device_release_driver+0x23/0x30 > bus_remove_device+0x10f/0x123 > device_del+0x132/0x18e > __scsi_remove_device+0x56/0xb6 > scsi_remove_device+0x26/0x33 > scsi_remove_target+0x12d/0x1a0 > ... > > What this patch does: > => Ensure that any quests to reset the timer are accounted for, so that > there is a finite upper bound on the time that a command is tried. > Once allowed number of retries is reached, we proceed to standard > error handling procedure (abort etc.) by scheduling the command > for EH. > > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx> This is actually wrong. Originally the code you're suggesting did exist and it used to cause us to time out far too early on some conditions. Now scmd->retries is for specific things that shouldn't be retried too often. Anything else appears to retry forever but in fact there's a specific check (in the softirq and io_completion) to check that a retryable failure hasn't taken longer than (cmd->allowed + 1) * req->timeout. This means effectively that nothing in SCSI is allowed to retry forever. James -- 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
