USB: usbfs: Use a spinlock instead of atomic accesses to tally used memory.

While the existing code code imposes a limit on the used memory, it might be
over pessimistic (even if this is unlikely).

Example scenario:
8 threads running in parallel, all entering
"usbfs_increase_memory_usage()" at the same time.
The atomic accesses in "usbfs_increase_memory_usage()" could be
serialized like this:
  8 x "atomic64_add"
  8 x "atomic64_read"
If the 8 x "atomic64_add" raise "usbfs_memory_usage" above the limit,
then all 8 calls of "usbfs_increase_memory_usage()" will return with
-ENOMEM.  If you instead serialize over the whole access to
"usbfs_memory_usage" by using a spinlock, some of these calls will
succeed.
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ingo Rohloff <ingo.rohloff@lauterbach.com>
Link: https://lore.kernel.org/r/20220209123303.103340-2-ingo.rohloff@lauterbach.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/devio.c

@@ -139,30 +139,42 @@ MODULE_PARM_DESC(usbfs_memory_mb,
 /* Hard limit, necessary to avoid arithmetic overflow */
 #define USBFS_XFER_MAX         (UINT_MAX / 2 - 1000000)
 
-static atomic64_t usbfs_memory_usage;	/* Total memory currently allocated */
+static DEFINE_SPINLOCK(usbfs_memory_usage_lock);
+static u64 usbfs_memory_usage;	/* Total memory currently allocated */
 
 /* Check whether it's okay to allocate more memory for a transfer */
 static int usbfs_increase_memory_usage(u64 amount)
 {
-	u64 lim;
+	u64 lim, total_mem;
+	unsigned long flags;
+	int ret;
 
 	lim = READ_ONCE(usbfs_memory_mb);
 	lim <<= 20;
 
-	atomic64_add(amount, &usbfs_memory_usage);
-
-	if (lim > 0 && atomic64_read(&usbfs_memory_usage) > lim) {
-		atomic64_sub(amount, &usbfs_memory_usage);
-		return -ENOMEM;
-	}
+	ret = 0;
+	spin_lock_irqsave(&usbfs_memory_usage_lock, flags);
+	total_mem = usbfs_memory_usage + amount;
+	if (lim > 0 && total_mem > lim)
+		ret = -ENOMEM;
+	else
+		usbfs_memory_usage = total_mem;
+	spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags);
 
-	return 0;
+	return ret;
 }
 
 /* Memory for a transfer is being deallocated */
 static void usbfs_decrease_memory_usage(u64 amount)
 {
-	atomic64_sub(amount, &usbfs_memory_usage);
+	unsigned long flags;
+
+	spin_lock_irqsave(&usbfs_memory_usage_lock, flags);
+	if (amount > usbfs_memory_usage)
+		usbfs_memory_usage = 0;
+	else
+		usbfs_memory_usage -= amount;
+	spin_unlock_irqrestore(&usbfs_memory_usage_lock, flags);
 }
 
 static int connected(struct usb_dev_state *ps)