dir-hold-afs-bufferlock-across-increment-of-buffer-lockers-to-prevent-newslot-from...

liberal debugging code and hammering produced a failure where newslot was
actually walking on another call to newslot because the bufferlock was
being dropped before newslot incremented lockers on the buffer it
was allocating, allowing someone else to come along and also think they
could allocate it.

by holding the bufferlock just a little longer we avoid this

diff --git a/src/dir/buffer.c b/src/dir/buffer.c
index 376ba81..018ddca 100644 (file)
--- a/src/dir/buffer.c
+++ b/src/dir/buffer.c
@@ -132,8 +132,8 @@ char *DRead(fid,page)
     if ( tb = phTable[pHash(fid)] ) {  /* ASSMT HERE */
        if (bufmatch(tb)) {
            ObtainWriteLock(&tb->lock);
-           ReleaseWriteLock(&afs_bufferLock);
            tb->lockers++;
+           ReleaseWriteLock(&afs_bufferLock);
            tb->accesstime = ++timecounter;
            ReleaseWriteLock(&tb->lock);
            return tb->data;
@@ -144,8 +144,8 @@ char *DRead(fid,page)
            if (bufmatch(tb2)) {
              buf_Front(bufhead,tb,tb2);
              ObtainWriteLock(&tb2->lock);
-             ReleaseWriteLock(&afs_bufferLock);
              tb2->lockers++;
+             ReleaseWriteLock(&afs_bufferLock);
              tb2->accesstime = ++timecounter;
              ReleaseWriteLock(&tb2->lock);
              return tb2->data;
@@ -154,8 +154,8 @@ char *DRead(fid,page)
              if (bufmatch(tb)) {
                buf_Front(bufhead,tb2,tb);
                ObtainWriteLock(&tb->lock);
-               ReleaseWriteLock(&afs_bufferLock);
                tb->lockers++;
+               ReleaseWriteLock(&afs_bufferLock);
                tb->accesstime = ++timecounter;
                ReleaseWriteLock(&tb->lock);
                return tb->data;
@@ -175,8 +175,8 @@ char *DRead(fid,page)
     tb = newslot(fid, page, (tb ? tb : tb2));
     ios++;
     ObtainWriteLock(&tb->lock);
-    ReleaseWriteLock(&afs_bufferLock);
     tb->lockers++;
+    ReleaseWriteLock(&afs_bufferLock);
     if (ReallyRead(tb->fid,tb->page,tb->data)) {
        tb->lockers--;
         FidZap(tb->fid);       /* disaster */
@@ -399,8 +399,8 @@ char *DNew (fid,page)
        return 0;
     }
     ObtainWriteLock(&tb->lock);
-    ReleaseWriteLock(&afs_bufferLock);
     tb->lockers++;
+    ReleaseWriteLock(&afs_bufferLock);
     ReleaseWriteLock(&tb->lock);
     return tb->data;
 }