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Diffstat (limited to 'fs_mgr/libsnapshot/snapuserd.cpp')
-rw-r--r--fs_mgr/libsnapshot/snapuserd.cpp611
1 files changed, 30 insertions, 581 deletions
diff --git a/fs_mgr/libsnapshot/snapuserd.cpp b/fs_mgr/libsnapshot/snapuserd.cpp
index 4c4a34225..f1fcb70a4 100644
--- a/fs_mgr/libsnapshot/snapuserd.cpp
+++ b/fs_mgr/libsnapshot/snapuserd.cpp
@@ -32,41 +32,6 @@ using android::base::unique_fd;
#define SNAP_LOG(level) LOG(level) << misc_name_ << ": "
#define SNAP_PLOG(level) PLOG(level) << misc_name_ << ": "
-static constexpr size_t PAYLOAD_SIZE = (1UL << 20);
-
-static_assert(PAYLOAD_SIZE >= BLOCK_SZ);
-
-void BufferSink::Initialize(size_t size) {
- buffer_size_ = size;
- buffer_offset_ = 0;
- buffer_ = std::make_unique<uint8_t[]>(size);
-}
-
-void* BufferSink::GetPayloadBuffer(size_t size) {
- if ((buffer_size_ - buffer_offset_) < size) return nullptr;
-
- char* buffer = reinterpret_cast<char*>(GetBufPtr());
- struct dm_user_message* msg = (struct dm_user_message*)(&(buffer[0]));
- return (char*)msg->payload.buf + buffer_offset_;
-}
-
-void* BufferSink::GetBuffer(size_t requested, size_t* actual) {
- void* buf = GetPayloadBuffer(requested);
- if (!buf) {
- *actual = 0;
- return nullptr;
- }
- *actual = requested;
- return buf;
-}
-
-struct dm_user_header* BufferSink::GetHeaderPtr() {
- CHECK(sizeof(struct dm_user_header) <= buffer_size_);
- char* buf = reinterpret_cast<char*>(GetBufPtr());
- struct dm_user_header* header = (struct dm_user_header*)(&(buf[0]));
- return header;
-}
-
Snapuserd::Snapuserd(const std::string& misc_name, const std::string& cow_device,
const std::string& backing_device) {
misc_name_ = misc_name;
@@ -75,356 +40,32 @@ Snapuserd::Snapuserd(const std::string& misc_name, const std::string& cow_device
control_device_ = "/dev/dm-user/" + misc_name;
}
-// Construct kernel COW header in memory
-// This header will be in sector 0. The IO
-// request will always be 4k. After constructing
-// the header, zero out the remaining block.
-void Snapuserd::ConstructKernelCowHeader() {
- void* buffer = bufsink_.GetPayloadBuffer(BLOCK_SZ);
- CHECK(buffer != nullptr);
-
- memset(buffer, 0, BLOCK_SZ);
-
- struct disk_header* dh = reinterpret_cast<struct disk_header*>(buffer);
-
- dh->magic = SNAP_MAGIC;
- dh->valid = SNAPSHOT_VALID;
- dh->version = SNAPSHOT_DISK_VERSION;
- dh->chunk_size = CHUNK_SIZE;
-}
-
-// Start the replace operation. This will read the
-// internal COW format and if the block is compressed,
-// it will be de-compressed.
-bool Snapuserd::ProcessReplaceOp(const CowOperation* cow_op) {
- if (!reader_->ReadData(*cow_op, &bufsink_)) {
- SNAP_LOG(ERROR) << "ProcessReplaceOp failed for block " << cow_op->new_block;
- return false;
- }
+bool Snapuserd::InitializeWorkers() {
+ for (int i = 0; i < NUM_THREADS_PER_PARTITION; i++) {
+ std::unique_ptr<WorkerThread> wt = std::make_unique<WorkerThread>(
+ cow_device_, backing_store_device_, control_device_, misc_name_, GetSharedPtr());
- return true;
-}
-
-// Start the copy operation. This will read the backing
-// block device which is represented by cow_op->source.
-bool Snapuserd::ProcessCopyOp(const CowOperation* cow_op) {
- void* buffer = bufsink_.GetPayloadBuffer(BLOCK_SZ);
- CHECK(buffer != nullptr);
-
- // Issue a single 4K IO. However, this can be optimized
- // if the successive blocks are contiguous.
- if (!android::base::ReadFullyAtOffset(backing_store_fd_, buffer, BLOCK_SZ,
- cow_op->source * BLOCK_SZ)) {
- SNAP_PLOG(ERROR) << "Copy-op failed. Read from backing store: " << backing_store_device_
- << "at block :" << cow_op->source;
- return false;
+ worker_threads_.push_back(std::move(wt));
}
-
return true;
}
-bool Snapuserd::ProcessZeroOp() {
- // Zero out the entire block
- void* buffer = bufsink_.GetPayloadBuffer(BLOCK_SZ);
- CHECK(buffer != nullptr);
-
- memset(buffer, 0, BLOCK_SZ);
- return true;
-}
-
-bool Snapuserd::ProcessCowOp(const CowOperation* cow_op) {
- CHECK(cow_op != nullptr);
-
- switch (cow_op->type) {
- case kCowReplaceOp: {
- return ProcessReplaceOp(cow_op);
- }
-
- case kCowZeroOp: {
- return ProcessZeroOp();
- }
-
- case kCowCopyOp: {
- return ProcessCopyOp(cow_op);
- }
-
- default: {
- SNAP_LOG(ERROR) << "Unknown operation-type found: " << cow_op->type;
- }
- }
- return false;
-}
-
-int Snapuserd::ReadUnalignedSector(sector_t sector, size_t size,
- std::map<sector_t, const CowOperation*>::iterator& it) {
- size_t skip_sector_size = 0;
-
- SNAP_LOG(DEBUG) << "ReadUnalignedSector: sector " << sector << " size: " << size
- << " Aligned sector: " << it->second;
-
- if (!ProcessCowOp(it->second)) {
- SNAP_LOG(ERROR) << "ReadUnalignedSector: " << sector << " failed of size: " << size;
- return -1;
- }
-
- int num_sectors_skip = sector - it->first;
-
- if (num_sectors_skip > 0) {
- skip_sector_size = num_sectors_skip << SECTOR_SHIFT;
- char* buffer = reinterpret_cast<char*>(bufsink_.GetBufPtr());
- struct dm_user_message* msg = (struct dm_user_message*)(&(buffer[0]));
-
- memmove(msg->payload.buf, (char*)msg->payload.buf + skip_sector_size,
- (BLOCK_SZ - skip_sector_size));
- }
-
- bufsink_.ResetBufferOffset();
- return std::min(size, (BLOCK_SZ - skip_sector_size));
-}
-
-/*
- * Read the data for a given COW Operation.
- *
- * Kernel can issue IO at a sector granularity.
- * Hence, an IO may end up with reading partial
- * data from a COW operation or we may also
- * end up with interspersed request between
- * two COW operations.
- *
- */
-int Snapuserd::ReadData(sector_t sector, size_t size) {
- /*
- * chunk_map stores COW operation at 4k granularity.
- * If the requested IO with the sector falls on the 4k
- * boundary, then we can read the COW op directly without
- * any issue.
- *
- * However, if the requested sector is not 4K aligned,
- * then we will have the find the nearest COW operation
- * and chop the 4K block to fetch the requested sector.
- */
- std::map<sector_t, const CowOperation*>::iterator it = chunk_map_.find(sector);
- if (it == chunk_map_.end()) {
- it = chunk_map_.lower_bound(sector);
- if (it != chunk_map_.begin()) {
- --it;
- }
-
- /*
- * If the IO is spanned between two COW operations,
- * split the IO into two parts:
- *
- * 1: Read the first part from the single COW op
- * 2: Read the second part from the next COW op.
- *
- * Ex: Let's say we have a 1024 Bytes IO request.
- *
- * 0 COW OP-1 4096 COW OP-2 8192
- * |******************|*******************|
- * |*****|*****|
- * 3584 4608
- * <- 1024B - >
- *
- * We have two COW operations which are 4k blocks.
- * The IO is requested for 1024 Bytes which are spanned
- * between two COW operations. We will split this IO
- * into two parts:
- *
- * 1: IO of size 512B from offset 3584 bytes (COW OP-1)
- * 2: IO of size 512B from offset 4096 bytes (COW OP-2)
- */
- return ReadUnalignedSector(sector, size, it);
- }
-
- int num_ops = DIV_ROUND_UP(size, BLOCK_SZ);
- while (num_ops) {
- if (!ProcessCowOp(it->second)) {
- return -1;
- }
- num_ops -= 1;
- it++;
- // Update the buffer offset
- bufsink_.UpdateBufferOffset(BLOCK_SZ);
-
- SNAP_LOG(DEBUG) << "ReadData at sector: " << sector << " size: " << size;
- }
-
- // Reset the buffer offset
- bufsink_.ResetBufferOffset();
- return size;
-}
-
-/*
- * dm-snap does prefetch reads while reading disk-exceptions.
- * By default, prefetch value is set to 12; this means that
- * dm-snap will issue 12 areas wherein each area is a 4k page
- * of disk-exceptions.
- *
- * If during prefetch, if the chunk-id seen is beyond the
- * actual number of metadata page, fill the buffer with zero.
- * When dm-snap starts parsing the buffer, it will stop
- * reading metadata page once the buffer content is zero.
- */
-bool Snapuserd::ZerofillDiskExceptions(size_t read_size) {
- size_t size = exceptions_per_area_ * sizeof(struct disk_exception);
-
- if (read_size > size) {
- return false;
- }
-
- void* buffer = bufsink_.GetPayloadBuffer(size);
- CHECK(buffer != nullptr);
-
- memset(buffer, 0, size);
- return true;
-}
-
-/*
- * A disk exception is a simple mapping of old_chunk to new_chunk.
- * When dm-snapshot device is created, kernel requests these mapping.
- *
- * Each disk exception is of size 16 bytes. Thus a single 4k page can
- * have:
- *
- * exceptions_per_area_ = 4096/16 = 256. This entire 4k page
- * is considered a metadata page and it is represented by chunk ID.
- *
- * Convert the chunk ID to index into the vector which gives us
- * the metadata page.
- */
-bool Snapuserd::ReadDiskExceptions(chunk_t chunk, size_t read_size) {
- uint32_t stride = exceptions_per_area_ + 1;
- size_t size;
-
- // ChunkID to vector index
- lldiv_t divresult = lldiv(chunk, stride);
-
- if (divresult.quot < vec_.size()) {
- size = exceptions_per_area_ * sizeof(struct disk_exception);
-
- CHECK(read_size == size);
-
- void* buffer = bufsink_.GetPayloadBuffer(size);
- CHECK(buffer != nullptr);
-
- memcpy(buffer, vec_[divresult.quot].get(), size);
- } else {
- return ZerofillDiskExceptions(read_size);
- }
-
- return true;
-}
-
-loff_t Snapuserd::GetMergeStartOffset(void* merged_buffer, void* unmerged_buffer,
- int* unmerged_exceptions) {
- loff_t offset = 0;
- *unmerged_exceptions = 0;
-
- while (*unmerged_exceptions <= exceptions_per_area_) {
- struct disk_exception* merged_de =
- reinterpret_cast<struct disk_exception*>((char*)merged_buffer + offset);
- struct disk_exception* cow_de =
- reinterpret_cast<struct disk_exception*>((char*)unmerged_buffer + offset);
-
- // Unmerged op by the kernel
- if (merged_de->old_chunk != 0 || merged_de->new_chunk != 0) {
- CHECK(merged_de->old_chunk == cow_de->old_chunk);
- CHECK(merged_de->new_chunk == cow_de->new_chunk);
-
- offset += sizeof(struct disk_exception);
- *unmerged_exceptions += 1;
- continue;
- }
-
- break;
- }
-
- CHECK(!(*unmerged_exceptions == exceptions_per_area_));
-
- SNAP_LOG(DEBUG) << "Unmerged_Exceptions: " << *unmerged_exceptions << " Offset: " << offset;
- return offset;
-}
-
-int Snapuserd::GetNumberOfMergedOps(void* merged_buffer, void* unmerged_buffer, loff_t offset,
- int unmerged_exceptions) {
- int merged_ops_cur_iter = 0;
-
- // Find the operations which are merged in this cycle.
- while ((unmerged_exceptions + merged_ops_cur_iter) < exceptions_per_area_) {
- struct disk_exception* merged_de =
- reinterpret_cast<struct disk_exception*>((char*)merged_buffer + offset);
- struct disk_exception* cow_de =
- reinterpret_cast<struct disk_exception*>((char*)unmerged_buffer + offset);
-
- CHECK(merged_de->new_chunk == 0);
- CHECK(merged_de->old_chunk == 0);
-
- if (cow_de->new_chunk != 0) {
- merged_ops_cur_iter += 1;
- offset += sizeof(struct disk_exception);
- const CowOperation* cow_op = chunk_map_[ChunkToSector(cow_de->new_chunk)];
- CHECK(cow_op != nullptr);
-
- CHECK(cow_op->new_block == cow_de->old_chunk);
- // zero out to indicate that operation is merged.
- cow_de->old_chunk = 0;
- cow_de->new_chunk = 0;
- } else if (cow_de->old_chunk == 0) {
- // Already merged op in previous iteration or
- // This could also represent a partially filled area.
- //
- // If the op was merged in previous cycle, we don't have
- // to count them.
- CHECK(cow_de->new_chunk == 0);
- break;
- } else {
- SNAP_LOG(ERROR) << "Error in merge operation. Found invalid metadata: "
- << " merged_de-old-chunk: " << merged_de->old_chunk
- << " merged_de-new-chunk: " << merged_de->new_chunk
- << " cow_de-old-chunk: " << cow_de->old_chunk
- << " cow_de-new-chunk: " << cow_de->new_chunk
- << " unmerged_exceptions: " << unmerged_exceptions
- << " merged_ops_cur_iter: " << merged_ops_cur_iter
- << " offset: " << offset;
- return -1;
+bool Snapuserd::CommitMerge(int num_merge_ops) {
+ {
+ std::lock_guard<std::mutex> lock(lock_);
+ CowHeader header;
+
+ reader_->GetHeader(&header);
+ header.num_merge_ops += num_merge_ops;
+ reader_->UpdateMergeProgress(num_merge_ops);
+ if (!writer_->CommitMerge(num_merge_ops)) {
+ SNAP_LOG(ERROR) << "CommitMerge failed... merged_ops_cur_iter: " << num_merge_ops
+ << " Total-merged-ops: " << header.num_merge_ops;
+ return false;
}
- }
- return merged_ops_cur_iter;
-}
-
-bool Snapuserd::ProcessMergeComplete(chunk_t chunk, void* buffer) {
- uint32_t stride = exceptions_per_area_ + 1;
- CowHeader header;
-
- if (!reader_->GetHeader(&header)) {
- SNAP_LOG(ERROR) << "Failed to get header";
- return false;
- }
-
- // ChunkID to vector index
- lldiv_t divresult = lldiv(chunk, stride);
- CHECK(divresult.quot < vec_.size());
- SNAP_LOG(DEBUG) << "ProcessMergeComplete: chunk: " << chunk
- << " Metadata-Index: " << divresult.quot;
-
- int unmerged_exceptions = 0;
- loff_t offset = GetMergeStartOffset(buffer, vec_[divresult.quot].get(), &unmerged_exceptions);
-
- int merged_ops_cur_iter =
- GetNumberOfMergedOps(buffer, vec_[divresult.quot].get(), offset, unmerged_exceptions);
-
- // There should be at least one operation merged in this cycle
- CHECK(merged_ops_cur_iter > 0);
-
- header.num_merge_ops += merged_ops_cur_iter;
- reader_->UpdateMergeProgress(merged_ops_cur_iter);
- if (!writer_->CommitMerge(merged_ops_cur_iter)) {
- SNAP_LOG(ERROR) << "CommitMerge failed... merged_ops_cur_iter: " << merged_ops_cur_iter;
- return false;
+ merge_initiated_ = true;
}
- SNAP_LOG(DEBUG) << "Merge success: " << merged_ops_cur_iter << "chunk: " << chunk;
- merge_initiated_ = true;
return true;
}
@@ -836,7 +477,6 @@ bool Snapuserd::ReadMetadata() {
// Total number of sectors required for creating dm-user device
num_sectors_ = ChunkToSector(data_chunk_id);
- metadata_read_done_ = true;
merge_initiated_ = false;
return true;
}
@@ -850,37 +490,6 @@ void MyLogger(android::base::LogId, android::base::LogSeverity severity, const c
}
}
-// Read Header from dm-user misc device. This gives
-// us the sector number for which IO is issued by dm-snapshot device
-bool Snapuserd::ReadDmUserHeader() {
- if (!android::base::ReadFully(ctrl_fd_, bufsink_.GetBufPtr(), sizeof(struct dm_user_header))) {
- SNAP_PLOG(ERROR) << "Control-read failed";
- return false;
- }
-
- return true;
-}
-
-// Send the payload/data back to dm-user misc device.
-bool Snapuserd::WriteDmUserPayload(size_t size) {
- if (!android::base::WriteFully(ctrl_fd_, bufsink_.GetBufPtr(),
- sizeof(struct dm_user_header) + size)) {
- SNAP_PLOG(ERROR) << "Write to dm-user failed size: " << size;
- return false;
- }
-
- return true;
-}
-
-bool Snapuserd::ReadDmUserPayload(void* buffer, size_t size) {
- if (!android::base::ReadFully(ctrl_fd_, buffer, size)) {
- SNAP_PLOG(ERROR) << "ReadDmUserPayload failed size: " << size;
- return false;
- }
-
- return true;
-}
-
bool Snapuserd::InitCowDevice() {
cow_fd_.reset(open(cow_device_.c_str(), O_RDWR));
if (cow_fd_ < 0) {
@@ -888,186 +497,26 @@ bool Snapuserd::InitCowDevice() {
return false;
}
- // Allocate the buffer which is used to communicate between
- // daemon and dm-user. The buffer comprises of header and a fixed payload.
- // If the dm-user requests a big IO, the IO will be broken into chunks
- // of PAYLOAD_SIZE.
- size_t buf_size = sizeof(struct dm_user_header) + PAYLOAD_SIZE;
- bufsink_.Initialize(buf_size);
-
return ReadMetadata();
}
-bool Snapuserd::InitBackingAndControlDevice() {
- backing_store_fd_.reset(open(backing_store_device_.c_str(), O_RDONLY));
- if (backing_store_fd_ < 0) {
- SNAP_PLOG(ERROR) << "Open Failed: " << backing_store_device_;
- return false;
- }
-
- ctrl_fd_.reset(open(control_device_.c_str(), O_RDWR));
- if (ctrl_fd_ < 0) {
- SNAP_PLOG(ERROR) << "Unable to open " << control_device_;
- return false;
- }
-
- return true;
-}
-
-bool Snapuserd::DmuserWriteRequest() {
- struct dm_user_header* header = bufsink_.GetHeaderPtr();
-
- // device mapper has the capability to allow
- // targets to flush the cache when writes are completed. This
- // is controlled by each target by a flag "flush_supported".
- // This flag is set by dm-user. When flush is supported,
- // a number of zero-length bio's will be submitted to
- // the target for the purpose of flushing cache. It is the
- // responsibility of the target driver - which is dm-user in this
- // case, to remap these bio's to the underlying device. Since,
- // there is no underlying device for dm-user, this zero length
- // bio's gets routed to daemon.
- //
- // Flush operations are generated post merge by dm-snap by having
- // REQ_PREFLUSH flag set. Snapuser daemon doesn't have anything
- // to flush per se; hence, just respond back with a success message.
- if (header->sector == 0) {
- CHECK(header->len == 0);
- header->type = DM_USER_RESP_SUCCESS;
- if (!WriteDmUserPayload(0)) {
- return false;
- }
- return true;
- }
-
- size_t remaining_size = header->len;
- size_t read_size = std::min(PAYLOAD_SIZE, remaining_size);
- CHECK(read_size == BLOCK_SZ);
-
- CHECK(header->sector > 0);
- chunk_t chunk = SectorToChunk(header->sector);
- CHECK(chunk_map_.find(header->sector) == chunk_map_.end());
-
- void* buffer = bufsink_.GetPayloadBuffer(read_size);
- CHECK(buffer != nullptr);
- header->type = DM_USER_RESP_SUCCESS;
-
- if (!ReadDmUserPayload(buffer, read_size)) {
- SNAP_LOG(ERROR) << "ReadDmUserPayload failed for chunk id: " << chunk
- << "Sector: " << header->sector;
- header->type = DM_USER_RESP_ERROR;
- }
-
- if (header->type == DM_USER_RESP_SUCCESS && !ProcessMergeComplete(chunk, buffer)) {
- SNAP_LOG(ERROR) << "ProcessMergeComplete failed for chunk id: " << chunk
- << "Sector: " << header->sector;
- header->type = DM_USER_RESP_ERROR;
- } else {
- SNAP_LOG(DEBUG) << "ProcessMergeComplete success for chunk id: " << chunk
- << "Sector: " << header->sector;
- }
-
- if (!WriteDmUserPayload(0)) {
- return false;
- }
-
- return true;
-}
-
-bool Snapuserd::DmuserReadRequest() {
- struct dm_user_header* header = bufsink_.GetHeaderPtr();
- size_t remaining_size = header->len;
- loff_t offset = 0;
- sector_t sector = header->sector;
- do {
- size_t read_size = std::min(PAYLOAD_SIZE, remaining_size);
-
- int ret = read_size;
- header->type = DM_USER_RESP_SUCCESS;
- chunk_t chunk = SectorToChunk(header->sector);
-
- // Request to sector 0 is always for kernel
- // representation of COW header. This IO should be only
- // once during dm-snapshot device creation. We should
- // never see multiple IO requests. Additionally this IO
- // will always be a single 4k.
- if (header->sector == 0) {
- CHECK(metadata_read_done_ == true);
- CHECK(read_size == BLOCK_SZ);
- ConstructKernelCowHeader();
- SNAP_LOG(DEBUG) << "Kernel header constructed";
- } else {
- if (!offset && (read_size == BLOCK_SZ) &&
- chunk_map_.find(header->sector) == chunk_map_.end()) {
- if (!ReadDiskExceptions(chunk, read_size)) {
- SNAP_LOG(ERROR) << "ReadDiskExceptions failed for chunk id: " << chunk
- << "Sector: " << header->sector;
- header->type = DM_USER_RESP_ERROR;
- } else {
- SNAP_LOG(DEBUG) << "ReadDiskExceptions success for chunk id: " << chunk
- << "Sector: " << header->sector;
- }
- } else {
- chunk_t num_sectors_read = (offset >> SECTOR_SHIFT);
- ret = ReadData(sector + num_sectors_read, read_size);
- if (ret < 0) {
- SNAP_LOG(ERROR) << "ReadData failed for chunk id: " << chunk
- << " Sector: " << (sector + num_sectors_read)
- << " size: " << read_size << " header-len: " << header->len;
- header->type = DM_USER_RESP_ERROR;
- } else {
- SNAP_LOG(DEBUG) << "ReadData success for chunk id: " << chunk
- << "Sector: " << header->sector;
- }
- }
- }
-
- // Daemon will not be terminated if there is any error. We will
- // just send the error back to dm-user.
- if (!WriteDmUserPayload(ret)) {
- return false;
- }
-
- remaining_size -= ret;
- offset += ret;
- } while (remaining_size > 0);
-
- return true;
-}
-
-bool Snapuserd::Run() {
- struct dm_user_header* header = bufsink_.GetHeaderPtr();
-
- bufsink_.Clear();
+/*
+ * Entry point to launch worker threads
+ */
+bool Snapuserd::Start() {
+ std::vector<std::future<bool>> threads;
- if (!ReadDmUserHeader()) {
- SNAP_LOG(ERROR) << "ReadDmUserHeader failed";
- return false;
+ for (int i = 0; i < worker_threads_.size(); i++) {
+ threads.emplace_back(
+ std::async(std::launch::async, &WorkerThread::RunThread, worker_threads_[i].get()));
}
- SNAP_LOG(DEBUG) << "msg->seq: " << std::hex << header->seq;
- SNAP_LOG(DEBUG) << "msg->type: " << std::hex << header->type;
- SNAP_LOG(DEBUG) << "msg->flags: " << std::hex << header->flags;
- SNAP_LOG(DEBUG) << "msg->sector: " << std::hex << header->sector;
- SNAP_LOG(DEBUG) << "msg->len: " << std::hex << header->len;
-
- switch (header->type) {
- case DM_USER_REQ_MAP_READ: {
- if (!DmuserReadRequest()) {
- return false;
- }
- break;
- }
-
- case DM_USER_REQ_MAP_WRITE: {
- if (!DmuserWriteRequest()) {
- return false;
- }
- break;
- }
+ bool ret = true;
+ for (auto& t : threads) {
+ ret = t.get() && ret;
}
- return true;
+ return ret;
}
} // namespace snapshot