[PATCH 10/14] spi: Split spi_write_then_read()

[Joseph CT Chan <jctchan@gmail.com>]

From: Bart Van Assche <bvanassche@acm.org>

Extract spi_write_then_read_buf() from spi_write_then_read(), move the
buffer mutex to file scope as spi_buf_lock, and annotate buf with
__guarded_by(&spi_buf_lock). Eliminate the correlated conditional unlock
in spi_write_then_read() to enable Clang's static context analysis to
verify lock balance and buffer access safety at compile time.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Joseph CT Chan <josephctchan@google.com>
---
 drivers/spi/spi.c | 95 ++++++++++++++++++++++++++---------------------
 1 file changed, 53 insertions(+), 42 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 0d36460e2c87..85089b0ebf4b 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -5135,7 +5135,39 @@ EXPORT_SYMBOL_GPL(spi_bus_unlock);
 /* Portable code must never pass more than 32 bytes */
 #define	SPI_BUFSIZ	max(32, SMP_CACHE_BYTES)
 
-static u8	*buf;
+static DEFINE_MUTEX(spi_buf_lock);
+static u8	*buf __guarded_by(&spi_buf_lock);
+
+static int spi_write_then_read_buf(struct spi_device *spi,
+		const void *txbuf, unsigned int n_tx,
+		void *rxbuf, unsigned int n_rx, u8 *local_buf)
+{
+	struct spi_message	message;
+	struct spi_transfer	x[2];
+	int			status;
+
+	spi_message_init(&message);
+	memset(x, 0, sizeof(x));
+	if (n_tx) {
+		x[0].len = n_tx;
+		spi_message_add_tail(&x[0], &message);
+	}
+	if (n_rx) {
+		x[1].len = n_rx;
+		spi_message_add_tail(&x[1], &message);
+	}
+
+	memcpy(local_buf, txbuf, n_tx);
+	x[0].tx_buf = local_buf;
+	x[1].rx_buf = local_buf + n_tx;
+
+	/* Do the I/O */
+	status = spi_sync(spi, &message);
+	if (status == 0)
+		memcpy(rxbuf, x[1].rx_buf, n_rx);
+
+	return status;
+}
 
 /**
  * spi_write_then_read - SPI synchronous write followed by read
@@ -5161,11 +5193,7 @@ int spi_write_then_read(struct spi_device *spi,
 		const void *txbuf, unsigned n_tx,
 		void *rxbuf, unsigned n_rx)
 {
-	static DEFINE_MUTEX(lock);
-
 	int			status;
-	struct spi_message	message;
-	struct spi_transfer	x[2];
 	u8			*local_buf;
 
 	/*
@@ -5174,39 +5202,21 @@ int spi_write_then_read(struct spi_device *spi,
 	 * keep heap costs out of the hot path unless someone else is
 	 * using the pre-allocated buffer or the transfer is too large.
 	 */
-	if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
-		local_buf = kmalloc(max((unsigned)SPI_BUFSIZ, n_tx + n_rx),
-				    GFP_KERNEL | GFP_DMA);
-		if (!local_buf)
-			return -ENOMEM;
-	} else {
-		local_buf = buf;
-	}
-
-	spi_message_init(&message);
-	memset(x, 0, sizeof(x));
-	if (n_tx) {
-		x[0].len = n_tx;
-		spi_message_add_tail(&x[0], &message);
-	}
-	if (n_rx) {
-		x[1].len = n_rx;
-		spi_message_add_tail(&x[1], &message);
+	if ((n_tx + n_rx) <= SPI_BUFSIZ && mutex_trylock(&spi_buf_lock)) {
+		status = spi_write_then_read_buf(spi, txbuf, n_tx, rxbuf, n_rx,
+						 buf);
+		mutex_unlock(&spi_buf_lock);
+		return status;
 	}
 
-	memcpy(local_buf, txbuf, n_tx);
-	x[0].tx_buf = local_buf;
-	x[1].rx_buf = local_buf + n_tx;
-
-	/* Do the I/O */
-	status = spi_sync(spi, &message);
-	if (status == 0)
-		memcpy(rxbuf, x[1].rx_buf, n_rx);
+	local_buf = kmalloc(max((unsigned int)SPI_BUFSIZ, n_tx + n_rx),
+			    GFP_KERNEL | GFP_DMA);
+	if (!local_buf)
+		return -ENOMEM;
 
-	if (x[0].tx_buf == buf)
-		mutex_unlock(&lock);
-	else
-		kfree(local_buf);
+	status = spi_write_then_read_buf(spi, txbuf, n_tx, rxbuf, n_rx,
+					 local_buf);
+	kfree(local_buf);
 
 	return status;
 }
@@ -5371,10 +5381,10 @@ static int __init spi_init(void)
 {
 	int	status;
 
-	buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
-	if (!buf) {
-		status = -ENOMEM;
-		goto err0;
+	scoped_guard(mutex, &spi_buf_lock) {
+		buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
 	}
 
 	status = bus_register(&spi_bus_type);
@@ -5403,9 +5413,10 @@ static int __init spi_init(void)
 err2:
 	bus_unregister(&spi_bus_type);
 err1:
-	kfree(buf);
-	buf = NULL;
-err0:
+	scoped_guard(mutex, &spi_buf_lock) {
+		kfree(buf);
+		buf = NULL;
+	}
 	return status;
 }
 
-- 
2.56.0.317.g454f8302e1-goog