On Mon, Feb 29, 2016 at 11:47:14AM -0800, Wan-Teh Chang wrote: > On Fri, Feb 26, 2016 at 5:04 PM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > > > > To be a little bit more explicit, it should be relatively easy to > > accomplish this by changing the position of qscale in the relevant struct, > > and only copy that half of the struct (where entries are obviously grouped) > > where we actually care about the copied value. > > > > See e.g. the copy_fields macro [1] on how we do partial-struct copies. > > > > Ronald > > > > [1] > > http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/h264_slice.c;h=9a5bc3f63fb5c6aae3747a03657b8dc821c7d750;hb=HEAD#l427 > > Hi Ronald, > > Thanks a lot for the hints. As soon as I had access to a computer last > Saturday, I implemented your suggestion. Although the data race on > |qscale| is gone, ThreadSantizer warned about the next data race of > this kind. I fixed that using the same approach, and then > ThreadSantizer warned about the next data race of this kind. > > That made me realize the fix for this class of data race is going to > big, and I should do this work in the ffmpeg HEAD rather than the old > version I'm using. I followed the instructions James Almer gave > earlier in this email thread: > > ./configure --samples=fate-suite/ --toolchain=clang-tsan --disable-stripping > make fate-h264 THREADS=4 > > (Note: James suggested --toolchain=gcc-tsan. That didn't work for me > for some reason. I tried --toolchain=clang-tsan instead and it > worked.) > > ThreadSanitizer reported the same class of data races in the h264 > decoder with multiple threads. To understand the code and propose a > proper fix, I am afraid that I will need to live and breathe this code > for several days because the structs involved (H264Context, > H264SliceContext, etc.) have a lot of members. > > So, I am wondering if you could run fate-h264 under tsan and take a > look at the data races in the h264 decoding code. I think you will be > able to fix the data races much faster than I can. If you don't have a > lot of time, I'll be happy to implement a solution if you can outline > it after your investigation. Thanks. >
BTW, thanks a lot for working on this. You can also play with valgrind (tool helgrind or drd) if it's supported on your system. You won't need a special build to use it (although for fate, you'll need something like --target-exec="valgrind --tool=helgrind" or get the command with make fate-foobar V=1 and prepend valgrind). Note: according to Ronald it's actually very close to TSAN. -- Clément B.
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