mirrors/pulseaudio
1
0
Fork 0
mirror of https://gitlab.freedesktop.org/pulseaudio/pulseaudio.git synced 2025-12-16 08:56:40 -05:00
Code Issues Projects Releases Packages Wiki Activity Actions

atomic: fix load and store for armv7 and higher

Browse source 
The original atomic implementation in pulseaudio based on
libatomic stated that the intent was to use full memory barriers.

According to [1], the load and store implementation based on
gcc builtins matches sequential consistent (i.e. full memory barrier)
load and store ordering only for x86.

I observed random crashes in client applications using memfd srbchannel
transport on an armv8-aarch64 platform (cortex-a57).
In all those crashes the first read on the pstream descriptor
(the size field) was wrong and looked like it contained old data.
I boiled the relevant parts of the srbchannel implementation down to
a simple test case and could observe random test failures.
So I figured that the atomic implementation was broken for armv8
with respect to cross-cpu memory access ordering consistency.

In order to come up with a minimal fix, I used the newer
__atomic_load_n/__atomic_store_n builtins from gcc.

With
aarch64-linux-gnu-gcc (Linaro GCC 7.3-2018.05) 7.3.1 20180425
they compile to
ldar and stlxr on arm64, which is correct according to [1] and [2].

The other atomic operations based on __sync builtins don't need
to be touched since they already are of the full memory barrier
variety.

[1] https://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html
[2] <https://community.arm.com/developer/ip-products/processors
    /b/processors-ip-blog/posts/armv8-a-architecture-2016-additions>
This commit is contained in:
Thomas Hutschenreuther 2019-05-29 19:20:37 +02:00 • committed by Tanu Kaskinen
parent 12bb46a768
commit d4ff4adce2
5 changed files with 198 additions and 1 deletions
Download patch file Download diff file Expand all files Collapse all files

12
configure.ac
View file

@ -246,6 +246,15 @@ fi
# If everything else fails use libatomic_ops
need_libatomic_ops=yes
AC_CACHE_CHECK([whether $CC knows __atomic_store_n()],
pulseaudio_cv_atomic_store_n, [
AC_LINK_IFELSE(
[AC_LANG_PROGRAM([], [[int c = 0; __atomic_store_n(&c, 4, __ATOMIC_SEQ_CST);]])],
[pulseaudio_cv_atomic_store_n=yes],
[pulseaudio_cv_atomic_store_n=no])
])
AC_CACHE_CHECK([whether $CC knows __sync_bool_compare_and_swap()],
pulseaudio_cv_sync_bool_compare_and_swap, [
AC_LINK_IFELSE(
@ -256,6 +265,9 @@ AC_CACHE_CHECK([whether $CC knows __sync_bool_compare_and_swap()],
if test "$pulseaudio_cv_sync_bool_compare_and_swap" = "yes" ; then
AC_DEFINE([HAVE_ATOMIC_BUILTINS], 1, [Have __sync_bool_compare_and_swap() and friends.])
if test "$pulseaudio_cv_atomic_store_n" = "yes" ; then
AC_DEFINE([HAVE_ATOMIC_BUILTINS_MEMORY_MODEL], 1, [Have __atomic_store_n() and friends.])
fi
need_libatomic_ops=no
else
# HW specific atomic ops stuff