[PATCH v2 04/20] scsi: core: Fix a race between scsi_done() and scsi_times_out()

[Bart Van Assche <bvanassche@acm.org>]

This patch restores the behavior of the following algorithm from the legacy
block layer:
- Before completing a request, test-and-set REQ_ATOM_COMPLETE atomically.
  Only call the block driver completion function if that flag was not yet
  set.
- Before calling the block driver timeout function, test-and-set
  REQ_ATOM_COMPLETE atomically. Only call the timeout handler if that flag
  was not yet set. If that flag was already set, do not restart the timer.

Cc: Keith Busch <kbusch@kernel.org>
Reported-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 065990bd198e ("scsi: set timed out out mq requests to complete")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/scsi_error.c | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 9cb0f9df621a..cd05f2db3339 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -331,6 +331,14 @@ enum blk_eh_timer_return scsi_times_out(struct request *req)
 	enum blk_eh_timer_return rtn = BLK_EH_DONE;
 	struct Scsi_Host *host = scmd->device->host;
 
+	/*
+	 * scsi_done() may be called concurrently with scsi_times_out(). Only
+	 * one of these two functions should proceed. Hence return early if
+	 * scsi_done() won the race.
+	 */
+	if (test_and_set_bit(SCMD_STATE_COMPLETE, &scmd->state))
+		return BLK_EH_DONE;
+
 	trace_scsi_dispatch_cmd_timeout(scmd);
 	scsi_log_completion(scmd, TIMEOUT_ERROR);
 
@@ -341,20 +349,6 @@ enum blk_eh_timer_return scsi_times_out(struct request *req)
 		rtn = host->hostt->eh_timed_out(scmd);
 
 	if (rtn == BLK_EH_DONE) {
-		/*
-		 * Set the command to complete first in order to prevent a real
-		 * completion from releasing the command while error handling
-		 * is using it. If the command was already completed, then the
-		 * lower level driver beat the timeout handler, and it is safe
-		 * to return without escalating error recovery.
-		 *
-		 * If timeout handling lost the race to a real completion, the
-		 * block layer may ignore that due to a fake timeout injection,
-		 * so return RESET_TIMER to allow error handling another shot
-		 * at this command.
-		 */
-		if (test_and_set_bit(SCMD_STATE_COMPLETE, &scmd->state))
-			return BLK_EH_RESET_TIMER;
 		if (scsi_abort_command(scmd) != SUCCESS) {
 			set_host_byte(scmd, DID_TIME_OUT);
 			scsi_eh_scmd_add(scmd);