forked from OSchip/llvm-project
7ec308715c
Prevent the following pathological behavior: Since memory access handling is not synchronized with DoReset, a thread running concurrently with DoReset can leave a bogus shadow value that will be later falsely detected as a race. For such false races RestoreStack will return false and we will not report it. However, consider that a thread leaves a whole lot of such bogus values and these values are later read by a whole lot of threads. This will cause massive amounts of ReportRace calls and lots of serialization. In very pathological cases the resulting slowdown can be >100x. This is very unlikely, but it was presumably observed in practice: https://github.com/google/sanitizers/issues/1552 If this happens, previous access sid+epoch will be the same for all of these false races b/c if the thread will try to increment epoch, it will notice that DoReset has happened and will stop producing bogus shadow values. So, last_spurious_race is used to remember the last sid+epoch for which RestoreStack returned false. Then it is used to filter out races with the same sid+epoch very early and quickly. It is of course possible that multiple threads left multiple bogus shadow values and all of them are read by lots of threads at the same time. In such case last_spurious_race will only be able to deduplicate a few races from one thread, then few from another and so on. An alternative would be to hold an array of such sid+epoch, but we consider such scenario as even less likely. Note: this can lead to some rare false negatives as well: 1. When a legit access with the same sid+epoch participates in a race as the "previous" memory access, it will be wrongly filtered out. 2. When RestoreStack returns false for a legit memory access because it was already evicted from the thread trace, we will still remember it in last_spurious_race. Then if there is another racing memory access from the same thread that happened in the same epoch, but was stored in the next thread trace part (which is still preserved in the thread trace), we will also wrongly filter it out while RestoreStack would actually succeed for that second memory access. Reviewed By: melver Differential Revision: https://reviews.llvm.org/D130269 |
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| tools | ||
| unittests | ||
| utils | ||
| www | ||
| .clang-tidy | ||
| .gitignore | ||
| CMakeLists.txt | ||
| CODE_OWNERS.TXT | ||
| CREDITS.TXT | ||
| LICENSE.TXT | ||
| README.txt | ||
README.txt
Compiler-RT ================================ This directory and its subdirectories contain source code for the compiler support routines. Compiler-RT is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. ================================