On 23.03.2011, at 08:59, Stefan Hajnoczi wrote: > On Wed, Mar 23, 2011 at 7:58 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> On Wed, Mar 23, 2011 at 7:39 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >>> On Tue, Mar 22, 2011 at 11:52 PM, Andreas Färber <andreas.faer...@web.de> >>> 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 <stefa...@linux.vnet.ibm.com> >>>>> --- >>>>> 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
Yeah, either that or adding a configure check for the availability of atomic operations. If the glib folks did go through the work already, I agree that it'd be nice to reuse that work though. Alex
