gc: fix assertion / ASAN violation in gc_big_object_link #56944
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vtjnash
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We somehow just got (un)lucky that `DFS!` at Compiler/src/ssair/domtree.jl:184 just happened to store exactly the same value as this pointer in this particular memory location previously. What are the odds? Seen on a CI run (with rr)
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KristofferC
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We somehow just got (un)lucky that `DFS!` at Compiler/src/ssair/domtree.jl:184 just happened to store exactly the same value as this pointer in this particular memory location previously, so that this branch on `undef` hit exactly the right value to fail. What are the odds? Seen on a CI run (with rr) The odds of this happening seem somewhere around 2^60 against, to 1 for each time. So that seems impressive we hit this even this once. But we did, and the proof is here, caught in rr: https://buildkite.com/julialang/julia-master/builds/43366#019425d7-67fd-4f33-a025-6d7cd6181649 ``` From worker 6: julia: /cache/build/tester-amdci5-10/julialang/julia-master/src/gc-stock.h:492: gc_big_object_link: Assertion `node->header != gc_bigval_sentinel_tag' failed. 2025-01-02 07:47:22 UTC From worker 6: 2025-01-02 07:47:22 UTC From worker 6: [3877] signal 6 (-6): Aborted 2025-01-02 07:47:22 UTC From worker 6: in expression starting at none:1 2025-01-02 07:47:22 UTC From worker 6: gsignal at /lib/x86_64-linux-gnu/libc.so.6 (unknown line) 2025-01-02 07:47:22 UTC From worker 6: abort at /lib/x86_64-linux-gnu/libc.so.6 (unknown line) 2025-01-02 07:47:22 UTC From worker 6: unknown function (ip: 0x7fb9a4b5040e) at /lib/x86_64-linux-gnu/libc.so.6 2025-01-02 07:47:22 UTC From worker 6: __assert_fail at /lib/x86_64-linux-gnu/libc.so.6 (unknown line) 2025-01-02 07:47:22 UTC From worker 6: gc_big_object_link at /cache/build/tester-amdci5-10/julialang/julia-master/src/gc-stock.h:492 [inlined] 2025-01-02 07:47:22 UTC From worker 6: gc_setmark_big at /cache/build/tester-amdci5-10/julialang/julia-master/src/gc-stock.c:276 2025-01-02 07:47:22 UTC From worker 6: jl_gc_big_alloc_inner at /cache/build/tester-amdci5-10/julialang/julia-master/src/gc-stock.h:491 ``` (cherry picked from commit 36472a7)
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We somehow just got (un)lucky that
DFS!at Compiler/src/ssair/domtree.jl:184 just happened to store exactly the same value as this pointer in this particular memory location previously, so that this branch onundefhit exactly the right value to fail. What are the odds?Seen on a CI run (with rr)
The odds of this happening seem somewhere around 2^60 against, to 1 for each time. So that seems impressive we hit this even this once.
But we did, and the proof is here, caught in rr:
https://buildkite.com/julialang/julia-master/builds/43366#019425d7-67fd-4f33-a025-6d7cd6181649