JFS: Releasing LOGGC_LOCK too early

In txLazyCommit, we are releasing log->gclock (LOGGC_LOCK) before
checking tblk->flag for tblkGC_LAZY.  For the case that tblkGC_LAZY
is not set, the user thread may release the tblk, and it may be
reused and the tblkGC_LAZY bit set again, between the time we release
the spinlock until we check the flag.  This is a lot to happen in an
SMP environment, but when CONFIG_PREEMPT is set, it is very easy to
see the problem.

The fix is to hold the spinlock until after we've checked the flag.

(Yes, I know the symbol names are ugly.)

fs/jfs/jfs_txnmgr.c

@@ -2746,13 +2746,16 @@ void txLazyCommit(struct tblock * tblk)
 	if (tblk->flag & tblkGC_READY)
 		wake_up(&tblk->gcwait);	// LOGGC_WAKEUP
 
-	spin_unlock_irq(&log->gclock);	// LOGGC_UNLOCK
-
+	/*
+	 * Can't release log->gclock until we've tested tblk->flag
+	 */
 	if (tblk->flag & tblkGC_LAZY) {
+		spin_unlock_irq(&log->gclock);	// LOGGC_UNLOCK
 		txUnlock(tblk);
 		tblk->flag &= ~tblkGC_LAZY;
 		txEnd(tblk - TxBlock);	/* Convert back to tid */
-	}
+	} else
+		spin_unlock_irq(&log->gclock);	// LOGGC_UNLOCK
 
 	jFYI(1, ("txLazyCommit: done: tblk = 0x%p\n", tblk));
 }