bitkeeper revision 1.1595 (429b3970iSiBE6JNLmeO-KDY_v3Hdg)

Better locking strategy in serial rx interrupt handler and getc. Release
the spinlock after each loop iteration to prevent starvation of other
cpus and of interrupt handling. Also neatly folds byte_matches() into
serial_getc().
Signed-off-by: Keir Fraser <keir@xensource.com>

diff --git a/xen/drivers/char/serial.c b/xen/drivers/char/serial.c
index c578de152c043ff0af9ada1dc424bef3d2876c84..f3828716bce89737e8f8e2622b52f31653d47ced 100644 (file)
--- a/xen/drivers/char/serial.c
+++ b/xen/drivers/char/serial.c
@@ -28,12 +28,14 @@ void serial_rx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
     BUG_ON(!port->driver);
     BUG_ON(!port->driver->getc);
 
-    spin_lock_irqsave(&port->lock, flags);
-
-    while ( port->driver->getc(port, &c) )
+    for ( ; ; )
     {
-        fn = NULL;
+        spin_lock_irqsave(&port->lock, flags);
+
+        if ( !port->driver->getc(port, &c) )
+            break;
 
+        fn = NULL;
         if ( port->rx != NULL )
             fn = port->rx;
         else if ( (c & 0x80) && (port->rx_hi != NULL) )
@@ -43,12 +45,12 @@ void serial_rx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
         else if ( (port->rxbufp - port->rxbufc) != RXBUFSZ )
             port->rxbuf[MASK_RXBUF_IDX(port->rxbufp++)] = c;            
 
+        spin_unlock_irqrestore(&port->lock, flags);
+
         if ( fn != NULL )
-        {
-            spin_unlock_irqrestore(&port->lock, flags);
             (*fn)(c & 0x7f, regs);
-            spin_lock_irqsave(&port->lock, flags);
-        }
+
+        cpu_relax();
     }
 
     spin_unlock_irqrestore(&port->lock, flags);
@@ -83,22 +85,6 @@ void serial_puts(int handle, const char *s)
         serial_putc(handle, *s++);
 }
 
-/* Returns TRUE if given character (*pc) matches the serial handle. */
-static int byte_matches(int handle, unsigned char *pc)
-{
-    if ( !(handle & SERHND_HI) )
-    {
-        if ( !(handle & SERHND_LO) || !(*pc & 0x80) )
-            return 1;
-    }
-    else if ( *pc & 0x80 )
-    {
-        *pc &= 0x7f;
-        return 1;
-    }
-    return 0;
-}
-
 char serial_getc(int handle)
 {
     struct serial_port *port = &com[handle & SERHND_IDX];
@@ -108,21 +94,28 @@ char serial_getc(int handle)
     if ( (handle == -1) || !port->driver || !port->driver->getc )
         return '\0';
 
-    spin_lock_irqsave(&port->lock, flags);
+    do {        
+        for ( ; ; )
+        {
+            spin_lock_irqsave(&port->lock, flags);
+            
+            if ( port->rxbufp != port->rxbufc )
+            {
+                c = port->rxbuf[MASK_RXBUF_IDX(port->rxbufc++)];
+                break;
+            }
+            
+            if ( port->driver->getc(port, &c) )
+                break;
 
-    while ( port->rxbufp != port->rxbufc )
-    {
-        c = port->rxbuf[MASK_RXBUF_IDX(port->rxbufc++)];
-        if ( byte_matches(handle, &c) )
-            goto out;
-    }
-    
-    while ( !port->driver->getc(port, &c) && !byte_matches(handle, &c) )
-        continue;
+            spin_unlock_irqrestore(&port->lock, flags);
 
- out:
-    spin_unlock_irqrestore(&port->lock, flags);
-    return c;
+            cpu_relax();
+        }
+    } while ( ((handle & SERHND_LO) &&  (c & 0x80)) ||
+              ((handle & SERHND_HI) && !(c & 0x80)) );
+    
+    return c & 0x7f;
 }
 
 int serial_parse_handle(char *conf)