scsi: fcoe: Use per-CPU API to update per-CPU statistics

The per-CPU statistics (struct fc_stats) is updated by getting a stable
per-CPU pointer via get_cpu() + per_cpu_ptr() and then performing the
increment. This can be optimized by using this_cpu_*() which will do
whatever is needed on the architecture to perform the update safe and
efficient.  The read out of the individual value (fc_get_host_stats())
should be done by using READ_ONCE() instead of a plain-C access. The
difference is that READ_ONCE() will always perform a single access while
the plain-C access can be split by the compiler into two loads if it
appears beneficial.  The usage of u64 has the side-effect that it is also
64bit wide on 32bit architectures and the read is always split into two
loads. The can lead to strange values if the read happens during an update
which alters both 32bit parts of the 64bit value. This can be circumvented
by either using a 32bit variables on 32bit architecures or extending the
statistics with a sequence counter.

Use this_cpu_*() API to update the statistics and READ_ONCE() to read it.

Link: https://lore.kernel.org/r/20220506105758.283887-3-bigeasy@linutronix.de
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

2 files changed, 25 insertions, 34 deletions

diff --git a/drivers/scsi/libfc/fc_fcp.c b/drivers/scsi/libfc/fc_fcp.c
index bce90eb56c9c..945adca5e72f 100644
--- a/drivers/scsi/libfc/fc_fcp.c
+++ b/drivers/scsi/libfc/fc_fcp.c
@@ -143,8 +143,7 @@ static struct fc_fcp_pkt *fc_fcp_pkt_alloc(struct fc_lport *lport, gfp_t gfp)
 		INIT_LIST_HEAD(&fsp->list);
 		spin_lock_init(&fsp->scsi_pkt_lock);
 	} else {
-		per_cpu_ptr(lport->stats, get_cpu())->FcpPktAllocFails++;
-		put_cpu();
+		this_cpu_inc(lport->stats->FcpPktAllocFails);
 	}
 	return fsp;
 }
@@ -266,8 +265,7 @@ static int fc_fcp_send_abort(struct fc_fcp_pkt *fsp)
 	if (!fsp->seq_ptr)
 		return -EINVAL;
 
-	per_cpu_ptr(fsp->lp->stats, get_cpu())->FcpPktAborts++;
-	put_cpu();
+	this_cpu_inc(fsp->lp->stats->FcpPktAborts);
 
 	fsp->state |= FC_SRB_ABORT_PENDING;
 	rc = fc_seq_exch_abort(fsp->seq_ptr, 0);
@@ -436,8 +434,7 @@ static inline struct fc_frame *fc_fcp_frame_alloc(struct fc_lport *lport,
 	if (likely(fp))
 		return fp;
 
-	per_cpu_ptr(lport->stats, get_cpu())->FcpFrameAllocFails++;
-	put_cpu();
+	this_cpu_inc(lport->stats->FcpFrameAllocFails);
 	/* error case */
 	fc_fcp_can_queue_ramp_down(lport);
 	shost_printk(KERN_ERR, lport->host,
@@ -471,7 +468,6 @@ static void fc_fcp_recv_data(struct fc_fcp_pkt *fsp, struct fc_frame *fp)
 {
 	struct scsi_cmnd *sc = fsp->cmd;
 	struct fc_lport *lport = fsp->lp;
-	struct fc_stats *stats;
 	struct fc_frame_header *fh;
 	size_t start_offset;
 	size_t offset;
@@ -533,14 +529,12 @@ static void fc_fcp_recv_data(struct fc_fcp_pkt *fsp, struct fc_frame *fp)
 
 		if (~crc != le32_to_cpu(fr_crc(fp))) {
 crc_err:
-			stats = per_cpu_ptr(lport->stats, get_cpu());
-			stats->ErrorFrames++;
+			this_cpu_inc(lport->stats->ErrorFrames);
 			/* per cpu count, not total count, but OK for limit */
-			if (stats->InvalidCRCCount++ < FC_MAX_ERROR_CNT)
+			if (this_cpu_inc_return(lport->stats->InvalidCRCCount) < FC_MAX_ERROR_CNT)
 				printk(KERN_WARNING "libfc: CRC error on data "
 				       "frame for port (%6.6x)\n",
 				       lport->port_id);
-			put_cpu();
 			/*
 			 * Assume the frame is total garbage.
 			 * We may have copied it over the good part
@@ -1861,7 +1855,6 @@ int fc_queuecommand(struct Scsi_Host *shost, struct scsi_cmnd *sc_cmd)
 	struct fc_fcp_pkt *fsp;
 	int rval;
 	int rc = 0;
-	struct fc_stats *stats;
 
 	rval = fc_remote_port_chkready(rport);
 	if (rval) {
@@ -1913,20 +1906,18 @@ int fc_queuecommand(struct Scsi_Host *shost, struct scsi_cmnd *sc_cmd)
 	/*
 	 * setup the data direction
 	 */
-	stats = per_cpu_ptr(lport->stats, get_cpu());
 	if (sc_cmd->sc_data_direction == DMA_FROM_DEVICE) {
 		fsp->req_flags = FC_SRB_READ;
-		stats->InputRequests++;
-		stats->InputBytes += fsp->data_len;
+		this_cpu_inc(lport->stats->InputRequests);
+		this_cpu_add(lport->stats->InputBytes, fsp->data_len);
 	} else if (sc_cmd->sc_data_direction == DMA_TO_DEVICE) {
 		fsp->req_flags = FC_SRB_WRITE;
-		stats->OutputRequests++;
-		stats->OutputBytes += fsp->data_len;
+		this_cpu_inc(lport->stats->OutputRequests);
+		this_cpu_add(lport->stats->OutputBytes, fsp->data_len);
 	} else {
 		fsp->req_flags = 0;
-		stats->ControlRequests++;
+		this_cpu_inc(lport->stats->ControlRequests);
 	}
-	put_cpu();
 
 	/*
 	 * send it to the lower layer
diff --git a/drivers/scsi/libfc/fc_lport.c b/drivers/scsi/libfc/fc_lport.c
index 19cd4a95d354..9c02c9523c4d 100644
--- a/drivers/scsi/libfc/fc_lport.c
+++ b/drivers/scsi/libfc/fc_lport.c
@@ -308,21 +308,21 @@ struct fc_host_statistics *fc_get_host_stats(struct Scsi_Host *shost)
 
 		stats = per_cpu_ptr(lport->stats, cpu);
 
-		fc_stats->tx_frames += stats->TxFrames;
-		fc_stats->tx_words += stats->TxWords;
-		fc_stats->rx_frames += stats->RxFrames;
-		fc_stats->rx_words += stats->RxWords;
-		fc_stats->error_frames += stats->ErrorFrames;
-		fc_stats->invalid_crc_count += stats->InvalidCRCCount;
-		fc_stats->fcp_input_requests += stats->InputRequests;
-		fc_stats->fcp_output_requests += stats->OutputRequests;
-		fc_stats->fcp_control_requests += stats->ControlRequests;
-		fcp_in_bytes += stats->InputBytes;
-		fcp_out_bytes += stats->OutputBytes;
-		fc_stats->fcp_packet_alloc_failures += stats->FcpPktAllocFails;
-		fc_stats->fcp_packet_aborts += stats->FcpPktAborts;
-		fc_stats->fcp_frame_alloc_failures += stats->FcpFrameAllocFails;
-		fc_stats->link_failure_count += stats->LinkFailureCount;
+		fc_stats->tx_frames += READ_ONCE(stats->TxFrames);
+		fc_stats->tx_words += READ_ONCE(stats->TxWords);
+		fc_stats->rx_frames += READ_ONCE(stats->RxFrames);
+		fc_stats->rx_words += READ_ONCE(stats->RxWords);
+		fc_stats->error_frames += READ_ONCE(stats->ErrorFrames);
+		fc_stats->invalid_crc_count += READ_ONCE(stats->InvalidCRCCount);
+		fc_stats->fcp_input_requests += READ_ONCE(stats->InputRequests);
+		fc_stats->fcp_output_requests += READ_ONCE(stats->OutputRequests);
+		fc_stats->fcp_control_requests += READ_ONCE(stats->ControlRequests);
+		fcp_in_bytes += READ_ONCE(stats->InputBytes);
+		fcp_out_bytes += READ_ONCE(stats->OutputBytes);
+		fc_stats->fcp_packet_alloc_failures += READ_ONCE(stats->FcpPktAllocFails);
+		fc_stats->fcp_packet_aborts += READ_ONCE(stats->FcpPktAborts);
+		fc_stats->fcp_frame_alloc_failures += READ_ONCE(stats->FcpFrameAllocFails);
+		fc_stats->link_failure_count += READ_ONCE(stats->LinkFailureCount);
 	}
 	fc_stats->fcp_input_megabytes = div_u64(fcp_in_bytes, 1000000);
 	fc_stats->fcp_output_megabytes = div_u64(fcp_out_bytes, 1000000);