[Xen-devel] [PATCH] [Mini-OS] Make semaphores callback-safe

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx>]

One may want to use semaphores in event handlers to wake threads waiting
for a resource, so semaphores then need to be callback-safe.

Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxx>

---
Note: there is no such use in the current code, but I have an overflow
fix for netfront which depends on this.

diff -r f2711b7eae95 -r 36bf1e737b87 extras/mini-os/include/semaphore.h
--- a/extras/mini-os/include/semaphore.h        Thu Nov 22 19:55:42 2007 +0000
+++ b/extras/mini-os/include/semaphore.h        Fri Nov 23 13:21:02 2007 +0000
@@ -41,14 +41,25 @@
 
 static void inline down(struct semaphore *sem)
 {
-    wait_event(sem->wait, sem->count > 0);
+    unsigned long flags;
+    while (1) {
+        wait_event(sem->wait, sem->count > 0);
+        local_irq_save(flags);
+        if (sem->count > 0)
+            break;
+        local_irq_restore(flags);
+    }
     sem->count--;
+    local_irq_restore(flags);
 }
 
 static void inline up(struct semaphore *sem)
 {
+    unsigned long flags;
+    local_irq_save(flags);
     sem->count++;
     wake_up(&sem->wait);
+    local_irq_restore(flags);
 }
 
 /* FIXME! Thre read/write semaphores are unimplemented! */

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