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

[Stefan Hajnoczi]

On Tue, Mar 22, 2011 at 11:52 PM, Andreas Färber <address@hidden> wrote:
> 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.

/me shakes his fist at Apple gcc!

These are gcc builtins, I believe the were added in gcc 4.1:
http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html#Atomic-Builtins

Which version of gcc are you running?

We can replace them with equivalent library functions or inline
assembly code.  Here's what we need:
Read memory barrier
Write memory barrier
Atomic load and increment

CCed Alex and Anthony who may have thoughts on adding these atomic ops to QEMU.

Stefan