[PATCH 4.6 189/203] scsi: fix race between simultaneous decrements of ->host_failed

From: Greg Kroah-Hartman
Date: Mon Jul 25 2016 - 17:49:52 EST

4.6-stable review patch. If anyone has any objections, please let me know.


From: Wei Fang <fangwei1@xxxxxxxxxx>

commit 72d8c36ec364c82bf1bf0c64dfa1041cfaf139f7 upstream.

sas_ata_strategy_handler() adds the works of the ata error handler to
system_unbound_wq. This workqueue asynchronously runs work items, so the
ata error handler will be performed concurrently on different CPUs. In
this case, ->host_failed will be decreased simultaneously in
scsi_eh_finish_cmd() on different CPUs, and become abnormal.

It will lead to permanently inequality between ->host_failed and
->host_busy, and scsi error handler thread won't start running. IO
errors after that won't be handled.

Since all scmds must have been handled in the strategy handler, just
remove the decrement in scsi_eh_finish_cmd() and zero ->host_busy after
the strategy handler to fix this race.

Fixes: 50824d6c5657 ("[SCSI] libsas: async ata-eh")
Signed-off-by: Wei Fang <fangwei1@xxxxxxxxxx>
Reviewed-by: James Bottomley <jejb@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Documentation/scsi/scsi_eh.txt | 8 ++++++--
drivers/ata/libata-eh.c | 2 +-
drivers/scsi/scsi_error.c | 4 +++-
3 files changed, 10 insertions(+), 4 deletions(-)

--- a/Documentation/scsi/scsi_eh.txt
+++ b/Documentation/scsi/scsi_eh.txt
@@ -263,19 +263,23 @@ scmd->allowed.

3. scmd recovered
ACTION: scsi_eh_finish_cmd() is invoked to EH-finish scmd
- - shost->host_failed--
- clear scmd->eh_eflags
- scsi_setup_cmd_retry()
- move from local eh_work_q to local eh_done_q
+ CONCURRENCY: at most one thread per separate eh_work_q to
+ keep queue manipulation lockless

4. EH completes
ACTION: scsi_eh_flush_done_q() retries scmds or notifies upper
- layer of failure.
+ layer of failure. May be called concurrently but must have
+ a no more than one thread per separate eh_work_q to
+ manipulate the queue locklessly
- scmd is removed from eh_done_q and scmd->eh_entry is cleared
- if retry is necessary, scmd is requeued using
- otherwise, scsi_finish_command() is invoked for scmd
+ - zero shost->host_failed
LOCKING: queue or finish function performs appropriate locking

--- a/drivers/ata/libata-eh.c
+++ b/drivers/ata/libata-eh.c
@@ -606,7 +606,7 @@ void ata_scsi_error(struct Scsi_Host *ho
ata_scsi_port_error_handler(host, ap);

/* finish or retry handled scmd's and clean up */
- WARN_ON(host->host_failed || !list_empty(&eh_work_q));
+ WARN_ON(!list_empty(&eh_work_q));

--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -1127,7 +1127,6 @@ static int scsi_eh_action(struct scsi_cm
void scsi_eh_finish_cmd(struct scsi_cmnd *scmd, struct list_head *done_q)
- scmd->device->host->host_failed--;
scmd->eh_eflags = 0;
list_move_tail(&scmd->eh_entry, done_q);
@@ -2226,6 +2225,9 @@ int scsi_error_handler(void *data)

+ /* All scmds have been handled */
+ shost->host_failed = 0;
* Note - if the above fails completely, the action is to take
* individual devices offline and flush the queue of any