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gefs: fix double-deadlist of block

Because we can have blocks in-flight with gen==memgen, which
both sides of the fork can free, we need to make sure that we
don't deadlist them in the new snapshot.

As a result, we need to use memgen, and not gen, in order to
prevent the potential for a double deadlisting and subsequent
free
.

--- a/sys/src/cmd/gefs/snap.c

+++ b/sys/src/cmd/gefs/snap.c

@@ -408,8 +408,16 @@

 		n->ht = t->ht;

 		n->bp = t->bp;

 		n->succ = -1;

+		/*

+		 * Because we can have blocks in-flight with gen==memgen,

+		 * which both sides of the fork can free, we need to make

+		 * sure that we don't deadlist them in the new snapshot.

+		 *

+		 * As a result, we need to use memgen, and not gen, in

+		 * order to prevent the potential for a double free.

+		 */

 		n->pred = t->gen;

-		n->base = t->gen;

+		n->base = t->memgen;

 		n->gen = fs->nextgen++;

 		n->memgen = fs->nextgen++;

-- 

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