[Xen-changelog] Better locking strategy in serial rx interrupt handler and getc. Release

[BitKeeper Bot <riel@xxxxxxxxxxx>]

ChangeSet 1.1595, 2005/05/30 17:04:00+01:00, kaf24@xxxxxxxxxxxxxxxxxxxx

        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@xxxxxxxxxxxxx>



 serial.c |   67 ++++++++++++++++++++++++++++-----------------------------------
 1 files changed, 30 insertions(+), 37 deletions(-)


diff -Nru a/xen/drivers/char/serial.c b/xen/drivers/char/serial.c
--- a/xen/drivers/char/serial.c 2005-05-30 13:03:06 -04:00
+++ b/xen/drivers/char/serial.c 2005-05-30 13:03:06 -04:00
@@ -28,12 +28,14 @@
     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 @@
         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 @@
         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 @@
     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)

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