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

[Stefan Hajnoczi]

On Wed, Mar 23, 2011 at 7:39 AM, Stefan Hajnoczi <address@hidden> wrote:
> 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.

Thinking about it more, the way I'd like to solve this (and make
simpletrace work on Windows too!) is to go ahead and use glib threads
and atomics.  I don't want to be in the business of writing
portability wrappers for different OSes and architectures, and glib
already does this:
file:///usr/share/doc/libglib2.0-doc/glib/glib-Atomic-Operations.html#g-atomic-int-exchange-and-add

Stefan