Patchwork [1/4] trace: Fix simple trace dropped event record for big endian

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Submitter Stefan Hajnoczi
Date Feb. 6, 2013, 3:30 p.m.
Message ID <1360164617-12103-2-git-send-email-stefanha@redhat.com>
Download mbox | patch
Permalink /patch/218675/
State New
Headers show

Comments

Stefan Hajnoczi - Feb. 6, 2013, 3:30 p.m.
From: Markus Armbruster <armbru@redhat.com>

We use atomic operations to keep track of dropped events.

Inconveniently, GLib supports only int and void * atomics, but the
counter dropped_events is uint64_t.  Can't stop commit 62bab732: a
quick (gint *)&dropped_events bludgeons the compiler into submission.

That cast is okay only when int is exactly 64 bits wide, which it
commonly isn't.

If int is even wider, we clobber whatever follows dropped_events.  Not
worth worrying about, as none of the machines that interest us have
such morbidly obese ints.

That leaves the common case: int narrower than 64 bits.

Harmless on little endian hosts: we just don't access the most
significant bits of dropped_events.  They remain zero.

On big endian hosts, we use only the most significant bits of
dropped_events as counter.  The least significant bits remain zero.
However, we write out the full value, which is the correct counter
shifted left a bunch of places.

Fix by changing the variables involved to int.

There's another, equally suspicious-looking (gint *)&trace_idx
argument to g_atomic_int_compare_and_exchange(), but that one casts
unsigned *, so it's okay.  But it's also superfluous, because GLib's
atomic int operations work just fine for unsigned.  Drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 trace/simple.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

Patch

diff --git a/trace/simple.c b/trace/simple.c
index ce17d64..ccbdb6a 100644
--- a/trace/simple.c
+++ b/trace/simple.c
@@ -53,7 +53,7 @@  enum {
 uint8_t trace_buf[TRACE_BUF_LEN];
 static unsigned int trace_idx;
 static unsigned int writeout_idx;
-static uint64_t dropped_events;
+static int dropped_events;
 static FILE *trace_fp;
 static char *trace_file_name;
 
@@ -63,7 +63,7 @@  typedef struct {
     uint64_t timestamp_ns;
     uint32_t length;   /*    in bytes */
     uint32_t reserved; /*    unused */
-    uint8_t arguments[];
+    uint64_t arguments[];
 } TraceRecord;
 
 typedef struct {
@@ -160,7 +160,7 @@  static gpointer writeout_thread(gpointer opaque)
         uint8_t bytes[sizeof(TraceRecord) + sizeof(uint64_t)];
     } dropped;
     unsigned int idx = 0;
-    uint64_t dropped_count;
+    int dropped_count;
     size_t unused __attribute__ ((unused));
 
     for (;;) {
@@ -169,16 +169,16 @@  static gpointer writeout_thread(gpointer opaque)
         if (dropped_events) {
             dropped.rec.event = DROPPED_EVENT_ID,
             dropped.rec.timestamp_ns = get_clock();
-            dropped.rec.length = sizeof(TraceRecord) + sizeof(dropped_events),
+            dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t),
             dropped.rec.reserved = 0;
             while (1) {
                 dropped_count = dropped_events;
-                if (g_atomic_int_compare_and_exchange((gint *)&dropped_events,
+                if (g_atomic_int_compare_and_exchange(&dropped_events,
                                                       dropped_count, 0)) {
                     break;
                 }
             }
-            memcpy(dropped.rec.arguments, &dropped_count, sizeof(uint64_t));
+            dropped.rec.arguments[0] = dropped_count;
             unused = fwrite(&dropped.rec, dropped.rec.length, 1, trace_fp);
         }
 
@@ -220,11 +220,11 @@  int trace_record_start(TraceBufferRecord *rec, TraceEventID event, size_t datasi
 
         if (new_idx - writeout_idx > TRACE_BUF_LEN) {
             /* Trace Buffer Full, Event dropped ! */
-            g_atomic_int_inc((gint *)&dropped_events);
+            g_atomic_int_inc(&dropped_events);
             return -ENOSPC;
         }
 
-        if (g_atomic_int_compare_and_exchange((gint *)&trace_idx,
+        if (g_atomic_int_compare_and_exchange(&trace_idx,
                                               old_idx, new_idx)) {
             break;
         }