Hello James, On Thu, Jun 4, 2015 at 1:27 PM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > 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. Thanks for the review. I'm not sure if I understood completely though. I see the check you mention in softirq_done and in the scsi_io_completion, however, I'm not sure if I see that in the situation I mentioned above (eh_timed_out() always returning returning BLK_EH_RESET_TIMER), how would the command ever end up in softirq_done or io_completion (instead of going on infinitely)? In my experiment, I actually instrumented the SCSI LLD to always ask for more time (BLK_EH_RESET_TIMER) for 1 of the disks, and I actually ended up with a system where I couldn't remove ANY of the disks in the system (for the reasons mentioned in the commit log sd_remove() waiting infinitely). I'm sure I'm missing something, but I'd appreciate if you could please help me understand? Thanks, Rajat -- 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
