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

[Stefan Hajnoczi]

On Wed, Mar 23, 2011 at 7:58 AM, Stefan Hajnoczi <address@hidden> wrote:
> 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

Corrected URI:
http://library.gnome.org/devel/glib/2.28/glib-Atomic-Operations.html#g-atomic-int-exchange-and-add

Stefan