2014607 - Snapshot the list of arenas before processing them

Status: NEW

(Core :: Memory Allocator, enhancement)

Description

[Paul Bone [:pbone]]

Bug 1364359 will add reference counting for arenas. While working on it this code could cause a problem.

MutexAutoLock lock(mLock);                                                   
for (RefPtr<arena_t> arena : iter()) {                                                 
  if (!arena->IsMainThreadOnly() || IsOnMainThreadWeak()) {                  
    RemoveFromOutstandingPurges(arena);                                      
    arena->PurgeLoop(aCond, aCaller);
  }
}

if the last reference to an arena is in the list of outstanding purges then the reference could be dropped by this loop. This is bad because arena_t destructor needs to remove a weak reference in the arena collection, but that's already locked by this code.

I will have Bug 1364359 work-around this but a future solution might be to build a temporary ref counted collection of arenas while holding the lock, then release the lock while processing them. This means the lock would be held for a MUCH shorter duration and may block other threads less, Currently the only performance-sensitive code using this lock is in jemalloc_stats_lite which is used by the profiler. Note that GetById doesn't use this lock in any of our builds since Bug 2004860.

Comment 1

[Paul Bone [:pbone]]

Actually this is simpler than the work-around I had in mind so I'll do it ahead of Bug 1364359.

Comment 2

[Paul Bone [:pbone]]

The changes to the iterator can go in Bug 1364359, they need the reference counting there as a dependency anyway.