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authorLinus Torvalds <torvalds@linux-foundation.org>2014-07-26 14:52:01 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-07-26 14:52:01 -0700
commit2062afb4f804afef61cbe62a30cac9a46e58e067 (patch)
tree58e6ca3b233436a8e1cd8c0ce473ff59bca96db0 /Makefile
parent8bdd638091605dc66d92c57c4b80eb87fffc15f7 (diff)
downloadlinux-linaro-stable-2062afb4f804afef61cbe62a30cac9a46e58e067.tar.gz
Fix gcc-4.9.0 miscompilation of load_balance() in scheduler
Michel Dänzer and a couple of other people reported inexplicable random oopses in the scheduler, and the cause turns out to be gcc mis-compiling the load_balance() function when debugging is enabled. The gcc bug apparently goes back to gcc-4.5, but slight optimization changes means that it now showed up as a problem in 4.9.0 and 4.9.1. The instruction scheduling problem causes gcc to schedule a spill operation to before the stack frame has been created, which in turn can corrupt the spilled value if an interrupt comes in. There may be other effects of this bug too, but that's the code generation problem seen in Michel's case. This is fixed in current gcc HEAD, but the workaround as suggested by Markus Trippelsdorf is pretty simple: use -fno-var-tracking-assignments when compiling the kernel, which disables the gcc code that causes the problem. This can result in slightly worse debug information for variable accesses, but that is infinitely preferable to actual code generation problems. Doing this unconditionally (not just for CONFIG_DEBUG_INFO) also allows non-debug builds to verify that the debug build would be identical: we can do export GCC_COMPARE_DEBUG=1 to make gcc internally verify that the result of the build is independent of the "-g" flag (it will make the compiler build everything twice, toggling the debug flag, and compare the results). Without the "-fno-var-tracking-assignments" option, the build would fail (even with 4.8.3 that didn't show the actual stack frame bug) with a gcc compare failure. See also gcc bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 Reported-by: Michel Dänzer <michel@daenzer.net> Suggested-by: Markus Trippelsdorf <markus@trippelsdorf.de> Cc: Jakub Jelinek <jakub@redhat.com> Cc: stable@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Makefile')
-rw-r--r--Makefile2
1 files changed, 2 insertions, 0 deletions
diff --git a/Makefile b/Makefile
index 6b2774145d66..5147f3f23504 100644
--- a/Makefile
+++ b/Makefile
@@ -688,6 +688,8 @@ KBUILD_CFLAGS += -fomit-frame-pointer
endif
endif
+KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments)
+
ifdef CONFIG_DEBUG_INFO
KBUILD_CFLAGS += -g
KBUILD_AFLAGS += -Wa,-gdwarf-2