.

wcfs/notes.txt

@@ -44,8 +44,8 @@ somewhat acceptable (~ 0.01% of whole-file data size, i.e. ~ 128MB of index for
 ~ 1TB of data), it is not good from time overhead point of view - initial open
 of a file this way would be potentially slow.
 
--> we took the approach where we invalidate a block lazily only when it is
-actually accesses.
+-> we took the approach where we send invalidation to client about a block
+lazily only when the block is actually accessed.
 
 
 Changing mmapping while under pagefault is possible
@@ -142,6 +142,40 @@ waiting for read operation to finish for ptrace, and read will be first
 waiting on ptrace stopping to complete = deadlock)
 
 
+Kernel locks page on read/cache store/... - we have to be careful not to deadlock
+=================================================================================
+
+The kernel, when doing FUSE operations, locks corresponding pages. For example
+it locks a page, where it is going to read data, before issuing FUSE read
+request. Correspondingly, on e.g. cache store, the kernel also locks page where
+data has to be stored.
+
+It is easy to deadlock, if we don't take this locks into account. For example
+if we try to upload data to kernel pagecache from under serving read request,
+this can deadlock.
+
+Another case that needs to be cared about is interaction between uploadBlk and
+zwatcher: zconnMu being RWMutex, does not allow new RLocks to be taken once
+Lock request has been issued. Thus the following scenario is possible::
+
+   uploadBlk      os.Read        zwatcher
+
+                  page.Lock
+   zconnMu.Rlock
+                                 zconnMu.Lock
+   page.Lock
+                  zconnMu.Rlock
+
+
+- zwatcher is waiting for uploadBlk to release zconnMu;
+- uploadBlk is waiting for os.Read to release page;
+- os.Read is waiting for zwatcher to release zconnMu;
+- deadlock.
+
+To avoid such deadlocks zwatcher asks OS cache uploaders to pause while it is
+running, and retries taking zconnMu.Lock until all uploaders are indeed paused.
+
+
 δ(BTree) notes (XXX -> btreediff package)
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