Fixed two buffer manager performance problems (Markos, Vinayak, thanks for pointing these out!):

- No longer hold global mutex during page writeback
 
 - Don't reserve 90% of the buffer pool for the free list.  Now the freelist reserves 1% of the buffer pool + 6 pages, and expands if it runs out of space.

src/stasis/bufferManager/bufferHash.c

@@ -196,7 +196,10 @@ static Page * bhLoadPageImpl_helper(int xid, const int pageid, int uninitialized
   // Is the page in cache?
   Page * ret = LH_ENTRY(find)(cachedPages, &pageid,sizeof(int));
 
-  // Is the page already being read from disk?  (If ret == 0, then no...)
+  do {
+
+    // Is the page already in memory or being read from disk?
+    // (If ret == 0, then no...)
     while(ret) {
       checkPageState(ret);
       if(*pagePendingPtr(ret)) {
@@ -224,8 +227,32 @@ static Page * bhLoadPageImpl_helper(int xid, const int pageid, int uninitialized
     // The page is not in cache, and is not (no longer is) pending.
     assert(!ret);
 
-  // Remove a page from the freelist.
+    // Remove a page from the freelist.  This may cause writeback, and release our latch.
-  ret = getFreePage();
+    Page * ret2 = getFreePage();
+
+    // Did some other thread put the page in cache for us?
+    ret = LH_ENTRY(find)(cachedPages, &pageid,sizeof(int));
+
+    if(!ret) {
+      // No, so we're ready to add it.
+      ret = ret2;
+      // Esacpe from this loop.
+      break;
+    } else {
+      // Put the page back on the free list
+
+      // It's possible that we wrote this page back even though the
+      // freeList didn't have any free space; extend free list if necessary.
+      if(freeListLength == freeCount) {
+        freeList = realloc(freeList, freeListLength+1);
+        freeListLength++;
+      }
+
+      freeList[freeCount] = ret2;
+      freeCount++;
+    }
+    // try again.
+  } while(1);
 
   // Add a pending entry to cachedPages to block like-minded threads and writeback
   (*pagePendingPtr(ret)) = (void*)1;
@@ -371,7 +398,7 @@ void bhBufInit() {
 
   cachedPages = LH_ENTRY(create)(MAX_BUFFER_SIZE);
 
-  freeListLength = 9 * MAX_BUFFER_SIZE / 10;
+  freeListLength = 6 + MAX_BUFFER_SIZE / 100;
   freeLowWater  = freeListLength - 5;
   freeCount = 0;
   pageCount = 0;