spi: Fix per-cpu stats access on 32 bit systems

On 32 bit systems, the following kernel BUG is hit:

BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
caller is debug_smp_processor_id+0x18/0x24
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.19.0-rc1-00001-g6ae0aec8a366 #181
Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
Backtrace:
 dump_backtrace from show_stack+0x20/0x24
 r7:81024ffd r6:00000000 r5:81024ffd r4:60000013
 show_stack from dump_stack_lvl+0x60/0x78
 dump_stack_lvl from dump_stack+0x14/0x1c
 r7:81024ffd r6:80f652de r5:80bec180 r4:819a2500
 dump_stack from check_preemption_disabled+0xc8/0xf0
 check_preemption_disabled from debug_smp_processor_id+0x18/0x24
 r8:8119b7e0 r7:81205534 r6:819f5c00 r5:819f4c00 r4:c083d724
 debug_smp_processor_id from __spi_sync+0x78/0x220
 __spi_sync from spi_sync+0x34/0x4c
 r10:bb7bf4e0 r9:c083d724 r8:00000007 r7:81a068c0 r6:822a83c0 r5:c083d724
 r4:819f4c00
 spi_sync from spi_mem_exec_op+0x338/0x370
 r5:000000b4 r4:c083d910
 spi_mem_exec_op from spi_nor_read_id+0x98/0xdc
 r10:bb7bf4e0 r9:00000000 r8:00000000 r7:00000000 r6:00000000 r5:82358040
 r4:819f7c40
 spi_nor_read_id from spi_nor_detect+0x38/0x114
 r7:82358040 r6:00000000 r5:819f7c40 r4:819f7c40
 spi_nor_detect from spi_nor_scan+0x11c/0xbec
 r10:bb7bf4e0 r9:00000000 r8:00000000 r7:c083da4c r6:00000000 r5:00010101
 r4:819f7c40
 spi_nor_scan from spi_nor_probe+0x10c/0x2d0
 r10:bb7bf4e0 r9:bb7bf4d0 r8:00000000 r7:819f4c00 r6:00000000 r5:00000000
 r4:819f7c40

per-cpu access needs to be guarded against preemption.

Fixes: 6598b91b ("spi: spi.c: Convert statistics to per-cpu u64_stats_t")
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David Jander <david@protonic.nl>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20220609121334.2984808-1-david@protonic.nlSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -314,11 +314,13 @@ static void spi_statistics_add_transfer_stats(struct spi_statistics *pcpu_stats,
 					      struct spi_controller *ctlr)
 {
 	int l2len = min(fls(xfer->len), SPI_STATISTICS_HISTO_SIZE) - 1;
-	struct spi_statistics *stats = this_cpu_ptr(pcpu_stats);
+	struct spi_statistics *stats;
 
 	if (l2len < 0)
 		l2len = 0;
 
+	get_cpu();
+	stats = this_cpu_ptr(pcpu_stats);
 	u64_stats_update_begin(&stats->syncp);
 
 	u64_stats_inc(&stats->transfers);
@@ -333,6 +335,7 @@ static void spi_statistics_add_transfer_stats(struct spi_statistics *pcpu_stats,
 		u64_stats_add(&stats->bytes_rx, xfer->len);
 
 	u64_stats_update_end(&stats->syncp);
+	put_cpu();
 }
 
 /*

include/linux/spi/spi.h

@@ -84,18 +84,24 @@ struct spi_statistics {
 
 #define SPI_STATISTICS_ADD_TO_FIELD(pcpu_stats, field, count)		\
 	do {								\
-		struct spi_statistics *__lstats = this_cpu_ptr(pcpu_stats); \
+		struct spi_statistics *__lstats;			\
+		get_cpu();						\
+		__lstats = this_cpu_ptr(pcpu_stats);			\
 		u64_stats_update_begin(&__lstats->syncp);		\
 		u64_stats_add(&__lstats->field, count);			\
 		u64_stats_update_end(&__lstats->syncp);			\
+		put_cpu();						\
 	} while (0)
 
 #define SPI_STATISTICS_INCREMENT_FIELD(pcpu_stats, field)		\
 	do {								\
-		struct spi_statistics *__lstats = this_cpu_ptr(pcpu_stats); \
+		struct spi_statistics *__lstats;			\
+		get_cpu();						\
+		__lstats = this_cpu_ptr(pcpu_stats);			\
 		u64_stats_update_begin(&__lstats->syncp);		\
 		u64_stats_inc(&__lstats->field);			\
 		u64_stats_update_end(&__lstats->syncp);			\
+		put_cpu();						\
 	} while (0)
 
 /**