Re: [Qemu-devel] [PATCH] simpletrace: Thread-safe tracing

[Andreas Färber]

Am 28.02.2011 um 10:38 schrieb Stefan Hajnoczi:

> Trace events outside the global mutex cannot be used with the simple
> trace backend since it is not thread-safe.  There is no check to  
> prevent
> them being enabled so people sometimes learn this the hard way.
>
> This patch restructures the simple trace backend with a ring buffer
> suitable for multiple concurrent writers.  A writeout thread empties  
> the
> trace buffer when threshold fill levels are reached.  Should the
> writeout thread be unable to keep up with trace generation, records  
> will
> simply be dropped.
>
> Each time events are dropped a special record is written to the trace
> file indicating how many events were dropped.  The event ID is
> 0xfffffffffffffffe and its signature is dropped(uint32_t count).
>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> v2:
> * Add 'dropped' event so we know when events were lost.
[...]
> +    __sync_synchronize(); /* read memory barrier before accessing  
> record */

Getting this at HEAD on Darwin/ppc64:

  CC    simpletrace.o
/Users/andreas/QEMU/qemu/simpletrace.c: In function ‘get_trace_record’:
/Users/andreas/QEMU/qemu/simpletrace.c:81: warning: implicit  
declaration of function ‘__sync_synchronize’
/Users/andreas/QEMU/qemu/simpletrace.c:81: warning: nested extern  
declaration of ‘__sync_synchronize’
/Users/andreas/QEMU/qemu/simpletrace.c: In function ‘trace’:
/Users/andreas/QEMU/qemu/simpletrace.c:161: warning: implicit  
declaration of function ‘__sync_fetch_and_add’
/Users/andreas/QEMU/qemu/simpletrace.c:161: warning: nested extern  
declaration of ‘__sync_fetch_and_add’
[...]
  LINK  qemu-nbd
Undefined symbols:
  "___sync_fetch_and_add", referenced from:
      _trace in simpletrace.o
  "___sync_synchronize", referenced from:
      _get_trace_record in simpletrace.o
      _trace in simpletrace.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [qemu-nbd] Error 1

Haven't investigated further yet.

Andreas