[Support][Parallel] Initialize threadIndex and add assertion checking its usage.

That patch adds a check for threadIndex being used with only threads
created by ThreadPoolExecutor. This helps catch two types of errors:

1. If a thread is created not by ThreadPoolExecutor its index may clash
   with the index of another thread. Using threadIndex, in that case, may
   lead to a data race.

2. Index of the main thread(threadIndex == 0) currently clashes with
   the index of thread0 in ThreadPoolExecutor threads. That may lead
   to a data race if main thread and thread0 are executed concurrently.

This patch allows execution tasks on the main thread only in case
parallel::strategy.ThreadsRequested == 1. In all other cases,
assertions check that threadIndex != UINT_MAX(i.e. that task
is executed on a thread created by ThreadPoolExecutor).

Differential Revision: https://reviews.llvm.org/D148916

1 files changed, 0 insertions, 3 deletions

diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index bda979c3066a..2f7fcd664c6a 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -1537,9 +1537,6 @@ template <class ELFT> void elf::scanRelocations() {
     tg.spawn(fn, serial);
   }
 
-  // Both the main thread and thread pool index 0 use getThreadIndex()==0. Be
-  // careful that they don't concurrently run scanSections. When serial is
-  // true, fn() has finished at this point, so running execute is safe.
   tg.spawn([] {
     RelocationScanner scanner;
     for (Partition &part : partitions) {