[PATCH] Reimplementation of Metze's "lib/tdb: if we know pwrite and pread are thread/fork safe tdb_reopen_all() should be a noop".

Rusty Russell rusty at rustcorp.com.au
Wed Jul 29 20:22:08 MDT 2009


This version just wraps the reopen code, so we still re-grab the lock and do
the normal sanity checks.

The reason we do this at all is to avoid global fd limits, see:
http://forums.fedoraforum.org/showthread.php?t=210393

Note also that this whole reopen concept is fundamentally racy: if the parent
goes away before the child calls tdb_reopen_all, the database can be left
without an active lock and another TDB_CLEAR_IF_FIRST opener will clear it.
A fork_with_tdbs() wrapper could use a pipe to solve this, but it's hardly
elegant (what if there are other independent things which have similar needs?).

Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
---
 lib/tdb/common/open.c |   15 ++++++++++-----
 1 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/lib/tdb/common/open.c b/lib/tdb/common/open.c
index b19e4ce..eda4afd 100644
--- a/lib/tdb/common/open.c
+++ b/lib/tdb/common/open.c
@@ -425,6 +425,9 @@ int tdb_reopen(struct tdb_context *tdb)
 		goto fail;
 	}
 
+/* If we have real pread & pwrite, we can skip reopen. */
+#if !defined(LIBREPLACE_PREAD_NOT_REPLACED) || \
+	!defined(LIBREPLACE_PWRITE_NOT_REPLACED)
 	if (tdb_munmap(tdb) != 0) {
 		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_reopen: munmap failed (%s)\n", strerror(errno)));
 		goto fail;
@@ -436,11 +439,6 @@ int tdb_reopen(struct tdb_context *tdb)
 		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_reopen: open failed (%s)\n", strerror(errno)));
 		goto fail;
 	}
-	if ((tdb->flags & TDB_CLEAR_IF_FIRST) && 
-	    (tdb->methods->tdb_brlock(tdb, ACTIVE_LOCK, F_RDLCK, F_SETLKW, 0, 1) == -1)) {
-		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_reopen: failed to obtain active lock\n"));
-		goto fail;
-	}
 	if (fstat(tdb->fd, &st) != 0) {
 		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_reopen: fstat failed (%s)\n", strerror(errno)));
 		goto fail;
@@ -450,6 +448,13 @@ int tdb_reopen(struct tdb_context *tdb)
 		goto fail;
 	}
 	tdb_mmap(tdb);
+#endif /* fake pread or pwrite */
+
+	if ((tdb->flags & TDB_CLEAR_IF_FIRST) && 
+	    (tdb->methods->tdb_brlock(tdb, ACTIVE_LOCK, F_RDLCK, F_SETLKW, 0, 1) == -1)) {
+		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_reopen: failed to obtain active lock\n"));
+		goto fail;
+	}
 
 	return 0;
 
-- 
1.6.0.4





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