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atomic: fix load and store for armv7 and higher

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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
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8
src/Makefile.am
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@ -293,7 +293,8 @@ TESTS_norun = \
sig2str-test \
stripnul \
echo-cancel-test \
lo-latency-test
lo-latency-test \
atomic-test
# These tests need a running pulseaudio daemon
TESTS_daemon = \
@ -412,6 +413,11 @@ srbchannel_test_CFLAGS = $(AM_CFLAGS) $(LIBCHECK_CFLAGS)
srbchannel_test_LDADD = $(AM_LDADD) libpulse.la libpulsecommon-@PA_MAJORMINOR@.la
srbchannel_test_LDFLAGS = $(AM_LDFLAGS) $(BINLDFLAGS) $(LIBCHECK_LIBS)
atomic_test_SOURCES = tests/atomic-test.c
atomic_test_CFLAGS = $(AM_CFLAGS) $(LIBCHECK_CFLAGS)
atomic_test_LDADD = $(AM_LDADD) libpulsecommon-@PA_MAJORMINOR@.la libpulse.la
atomic_test_LDFLAGS = $(AM_LDFLAGS) $(BINLDFLAGS) $(LIBCHECK_LIBS)
get_binary_name_test_SOURCES = tests/get-binary-name-test.c
get_binary_name_test_CFLAGS = $(AM_CFLAGS) $(LIBCHECK_CFLAGS)
get_binary_name_test_LDADD = $(AM_LDADD) libpulse.la libpulsecommon-@PA_MAJORMINOR@.la