Backed out changeset f2359b80aaa2 (bug 1746447) for causing assertion failures in StartupCache. CLOSED TREE

startupcache/StartupCache.cpp

@@ -65,7 +65,6 @@ MOZ_DEFINE_MALLOC_SIZE_OF(StartupCacheMallocSizeOf)
 NS_IMETHODIMP
 StartupCache::CollectReports(nsIHandleReportCallback* aHandleReport,
                              nsISupports* aData, bool aAnonymize) {
-  MutexAutoLock lock(mTableLock);
   MOZ_COLLECT_REPORT(
       "explicit/startup-cache/mapping", KIND_NONHEAP, UNITS_BYTES,
       mCacheData.nonHeapSizeOfExcludingThis(),
@@ -227,11 +226,8 @@ nsresult StartupCache::Init() {
                                      false);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  {
-    MutexAutoLock lock(mTableLock);
   auto result = LoadArchive();
   rv = result.isErr() ? result.unwrapErr() : NS_OK;
-  }
 
   gFoundDiskCacheOnInit = rv != NS_ERROR_FILE_NOT_FOUND;
 
@@ -264,8 +260,6 @@ Result<Ok, nsresult> StartupCache::LoadArchive() {
   MOZ_ASSERT(NS_IsMainThread(), "Can only load startup cache on main thread");
   if (gIgnoreDiskCache) return Err(NS_ERROR_FAILURE);
 
-  mTableLock.AssertCurrentThreadOwns();
-
   MOZ_TRY(mCacheData.init(mFile));
   auto size = mCacheData.size();
   if (CanPrefetchMemory()) {
@@ -365,7 +359,6 @@ bool StartupCache::HasEntry(const char* id) {
 
   MOZ_ASSERT(NS_IsMainThread(), "Startup cache only available on main thread");
 
-  MutexAutoLock lock(mTableLock);
   return mTable.has(nsDependentCString(id));
 }
 
@@ -381,7 +374,6 @@ nsresult StartupCache::GetBuffer(const char* id, const char** outbuf,
   auto telemetry =
       MakeScopeExit([&label] { Telemetry::AccumulateCategorical(label); });
 
-  MutexAutoLock lock(mTableLock);
   decltype(mTable)::Ptr p = mTable.lookup(nsDependentCString(id));
   if (!p) {
     return NS_ERROR_NOT_AVAILABLE;
@@ -426,7 +418,6 @@ nsresult StartupCache::GetBuffer(const char* id, const char** outbuf,
           uncompressed.From(totalWritten), compressed.From(totalRead));
       if (NS_WARN_IF(result.isErr())) {
         value.mData = nullptr;
-        MutexAutoUnlock unlock(mTableLock);
         InvalidateCache();
         return NS_ERROR_FAILURE;
       }
@@ -466,19 +457,19 @@ nsresult StartupCache::PutBuffer(const char* id, UniquePtr<char[]>&& inbuf,
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  // Try to gain the table write lock. If the background task to write the
-  // cache is running, this will fail.
-  MutexAutoTryLock lock(mTableLock);
-  if (!lock) {
-    return NS_ERROR_NOT_AVAILABLE;
-  }
-  mTableLock.AssertCurrentThreadOwns();
   bool exists = mTable.has(nsDependentCString(id));
+
   if (exists) {
     NS_WARNING("Existing entry in StartupCache.");
     // Double-caching is undesirable but not an error.
     return NS_OK;
   }
+  // Try to gain the table write lock. If the background task to write the
+  // cache is running, this will fail.
+  if (!mTableLock.TryLock()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  auto lockGuard = MakeScopeExit([&] { mTableLock.Unlock(); });
 
   // putNew returns false on alloc failure - in the very unlikely event we hit
   // that and aren't going to crash elsewhere, there's no reason we need to
@@ -622,7 +613,7 @@ Result<Ok, nsresult> StartupCache::WriteToDisk() {
 void StartupCache::InvalidateCache(bool memoryOnly) {
   WaitOnPrefetchThread();
   // Ensure we're not writing using mTable...
-  MutexAutoLock lock(mTableLock);
+  MutexAutoLock unlock(mTableLock);
 
   mWrittenOnce = false;
   if (memoryOnly) {
@@ -672,16 +663,16 @@ void StartupCache::MaybeInitShutdownWrite() {
 void StartupCache::EnsureShutdownWriteComplete() {
   // If we've already written or there's nothing to write,
   // we don't need to do anything. This is the common case.
-  if (mWrittenOnce) {
-    return;
-  }
-  MutexAutoLock lock(mTableLock);
-  if (mCacheData.initialized() && !ShouldCompactCache()) {
+  if (mWrittenOnce || (mCacheData.initialized() && !ShouldCompactCache())) {
     return;
   }
   // Otherwise, ensure the write happens. The timer should have been cancelled
   // already in MaybeInitShutdownWrite.
-
+  if (!mTableLock.TryLock()) {
+    // Uh oh, we're writing away from the main thread. Wait to gain the lock,
+    // to ensure the write completes.
+    mTableLock.Lock();
+  } else {
     // We got the lock. Keep the following in sync with
     // MaybeWriteOffMainThread:
     WaitOnPrefetchThread();
@@ -695,6 +686,8 @@ void StartupCache::EnsureShutdownWriteComplete() {
     // We've had the lock, and `WriteToDisk()` sets mWrittenOnce and mDirty
     // when done, and checks for them when starting, so we don't need to do
     // anything else.
+  }
+  mTableLock.Unlock();
 }
 
 void StartupCache::IgnoreDiskCache() {
@@ -714,22 +707,14 @@ void StartupCache::ThreadedPrefetch(void* aClosure) {
   NS_SetCurrentThreadName("StartupCache");
   mozilla::IOInterposer::RegisterCurrentThread();
   StartupCache* startupCacheObj = static_cast<StartupCache*>(aClosure);
-  uint8_t* buf;
-  size_t size;
-  {
-    MutexAutoLock lock(startupCacheObj->mTableLock);
-    buf = startupCacheObj->mCacheData.get<uint8_t>().get();
-    size = startupCacheObj->mCacheData.size();
-  }
-  // PrefetchMemory does madvise/equivalent, but doesn't access the memory
-  // pointed to by buf
+  uint8_t* buf = startupCacheObj->mCacheData.get<uint8_t>().get();
+  size_t size = startupCacheObj->mCacheData.size();
   MMAP_FAULT_HANDLER_BEGIN_BUFFER(buf, size)
   PrefetchMemory(buf, size);
   MMAP_FAULT_HANDLER_CATCH()
   mozilla::IOInterposer::UnregisterCurrentThread();
 }
 
-// mTableLock must be held
 bool StartupCache::ShouldCompactCache() {
   // If we've requested less than 4/5 of the startup cache, then we should
   // probably compact it down. This can happen quite easily after the first run,
@@ -763,24 +748,19 @@ void StartupCache::MaybeWriteOffMainThread() {
     return;
   }
 
-  {
-    MutexAutoLock lock(mTableLock);
   if (mCacheData.initialized() && !ShouldCompactCache()) {
     return;
   }
-  }
+
   // Keep this code in sync with EnsureShutdownWriteComplete.
   WaitOnPrefetchThread();
-  {
-    MutexAutoLock lock(mTableLock);
   mDirty = true;
   mCacheData.reset();
-  }
 
   RefPtr<StartupCache> self = this;
   nsCOMPtr<nsIRunnable> runnable =
       NS_NewRunnableFunction("StartupCache::Write", [self]() mutable {
-        MutexAutoLock lock(self->mTableLock);
+        MutexAutoLock unlock(self->mTableLock);
         auto result = self->WriteToDisk();
         Unused << NS_WARN_IF(result.isErr());
       });