scsi: ufs: core: Revert "scsi: ufs: Synchronize SCSI and UFS error handling"

This reverts commit a113eaaf86373362b053279049907ff82b5df6c8.

There are a couple of issues with the commit:

 1. It causes deadlocks.

 2. It causes the shost->eh_cmd_q list of failed requests not to be
    processed, ever.

So revert it.

1. Deadlocks

The SCSI error handler runs with requests blocked beginning when
scsi_schedule_eh() sets SHOST_RECOVERY state, continuing through
scsi_error_handler() callback ->eh_strategy_handler() until
scsi_restart_operations() is called.  By setting eh_strategy_handler to
ufshcd_err_handler, the patch changed the UFS error handler to run with
requests blocked, including PM requests, for the entire run of the error
handler.

That conflicts with UFS error handler existing synchronization with UFS
device PM operations.  The UFS error handler synchronizes with runtime PM
by doing pm_runtime_get_sync() prior to blocking requests itself.  It
synchronizes with system PM by use of hba->host_sem, again before blocking
requests itself.  However, if requests are already blocked, then PM
operations will block.  So:

   the UFS error handler blocks waiting on PM
 + PM blocks waiting on SCSI PM requests to process or fail
 + PM requests are blocked waiting on error handling to finish
 =  deadlock

This happens both for runtime PM and system PM.

Prior to the patch, these deadlocks could not happen even if SCSI error
handling was running, because the presence of requests in shost->eh_cmd_q
would mean the queues could not be suspended, which would mean that, should
the UFS error handler run at the same time, it would not need to wait for
PM or vice versa.

Please note these scenarios are not just theoretical, they were found
during testing on a Samsung Galaxy Book S.

2. ->eh_strategy_handler() must process shost->eh_cmd_q list of failed
requests, as all other eh_strategy_handler's do except UFS error handler.
Refer for example: scsi_unjam_host(), ata_scsi_error() and
sas_scsi_recover_host().

Link: https://lore.kernel.org/r/20210917144349.14058-1-adrian.hunter@intel.com
Fixes: a113eaaf8637 ("scsi: ufs: Synchronize SCSI and UFS error handling")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/scsi/ufs/ufshcd.h b/drivers/scsi/ufs/ufshcd.h
index 4723f27a55d1..f0da5d3db1fa 100644
--- a/drivers/scsi/ufs/ufshcd.h
+++ b/drivers/scsi/ufs/ufshcd.h
@@ -741,6 +741,8 @@ struct ufs_hba_monitor {
  * @is_powered: flag to check if HBA is powered
  * @shutting_down: flag to check if shutdown has been invoked
  * @host_sem: semaphore used to serialize concurrent contexts
+ * @eh_wq: Workqueue that eh_work works on
+ * @eh_work: Worker to handle UFS errors that require s/w attention
  * @eeh_work: Worker to handle exception events
  * @errors: HBA errors
  * @uic_error: UFS interconnect layer error status
@@ -843,6 +845,8 @@ struct ufs_hba {
 	struct semaphore host_sem;
 
 	/* Work Queues */
+	struct workqueue_struct *eh_wq;
+	struct work_struct eh_work;
 	struct work_struct eeh_work;
 
 	/* HBA Errors */