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>
---
drivers/scsi/scsi_error.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index c95a4e9..9671ec5 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -283,6 +283,17 @@ enum blk_eh_timer_return scsi_times_out(struct request *req)
else if (host->hostt->eh_timed_out)
rtn = host->hostt->eh_timed_out(scmd);
+ /*
+ * If a scmd times out because LLD failed to complete it, make sure that
+ * LLD can ask for more time only finite number of times. Also each such
+ * request must account towards the time the LLD has been spent on that
+ * cmd. Thus each timeout attempt by an LLD to complete a scmd must be
+ * treated as a retry since it involves waiting for another whole period
+ * of time before it times out again.
+ */
+ if (rtn == BLK_EH_RESET_TIMER && (++scmd->retries > scmd->allowed))
+ rtn = BLK_EH_NOT_HANDLED;
+
if (rtn == BLK_EH_NOT_HANDLED) {
if (!host->hostt->no_async_abort &&
scsi_abort_command(scmd) == SUCCESS)
--
2.2.0.rc0.207.ga3a616c
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