[PATCH] xentrace: Clear lost records when disabling tracing

[George Dunlap <George.Dunlap@eu.citrix.com>]

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This patch clears the "lost records" flag on each cpu when tracing is disabled.

Without this patch, the next time tracing starts, cpus with lost
records will generate lost record traces, even though buffers are
empty and no tracing has recently happened.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

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diff -r 2c5b7da4b914 xen/common/trace.c
--- a/xen/common/trace.c	Wed Jan 06 16:22:18 2010 +0000
+++ b/xen/common/trace.c	Thu Jan 07 15:24:15 2010 +0000
@@ -230,11 +230,26 @@
             tb_init_done = 1;
         break;
     case XEN_SYSCTL_TBUFOP_disable:
+    {
         /*
          * Disable trace buffers. Just stops new records from being written,
          * does not deallocate any memory.
          */
+        int i;
+
         tb_init_done = 0;
+        wmb();
+        /* Clear any lost-record info so we don't get phantom lost records next time we
+         * start tracing.  Grab the lock to make sure we're not racing anyone.  After this
+         * hypercall returns, no more records should be placed into the buffers. */
+        for_each_online_cpu(i)
+        {
+            int flags;
+            spin_lock_irqsave(&per_cpu(t_lock, i), flags);
+            per_cpu(lost_records, i)=0;
+            spin_unlock_irqrestore(&per_cpu(t_lock, i), flags);
+        }
+    }
         break;
     default:
         rc = -EINVAL;

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