[PATCH 1/2] lockdep: fix recursive read lock validation

[Peter Zijlstra]

__lock_acquire( .read = 2 )
  hlock->read = read; /* [1] */
  validate_chain()
    ret = check_deadlock(); /* returns 2 when recursive */

    if (ret == 2)
      hlock->read = 2; /* but it was already 2 from [1] */

    check_prevs_add()
      if (hlock->read != 2)
        /* add to dependency chain */

So it will never add a recursive read lock to the dependency chain. Fix this
by setting hlock->read to 1 when its the first recursive lock instance.

This means that the following sequence is now invalid, whereas previously
it was considered valid:

  rlock(a); rlock(b); runlock(b); runlock(a)
  rlock(b); rlock(a);

It really is invalid when considered against write locks.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: Gautham R Shenoy <ego@xxxxxxxxxx>
---
 kernel/lockdep.c       |    9 ++++-----
 lib/locking-selftest.c |   12 ++++++------
 2 files changed, 10 insertions(+), 11 deletions(-)

Index: linux-2.6-2/kernel/lockdep.c
===================================================================
--- linux-2.6-2.orig/kernel/lockdep.c
+++ linux-2.6-2/kernel/lockdep.c
@@ -1557,12 +1557,11 @@ static int validate_chain(struct task_st
 		if (!ret)
 			return 0;
 		/*
-		 * Mark recursive read, as we jump over it when
-		 * building dependencies (just like we jump over
-		 * trylock entries):
+		 * If we are the first recursive read, don't jump over our
+		 * dependency.
 		 */
-		if (ret == 2)
-			hlock->read = 2;
+		if (hlock->read == 2 && ret != 2)
+			hlock->read = 1;
 		/*
 		 * Add dependency only if this lock is not the head
 		 * of the chain, and if it's not a secondary read-lock:
Index: linux-2.6-2/lib/locking-selftest.c
===================================================================
--- linux-2.6-2.orig/lib/locking-selftest.c
+++ linux-2.6-2/lib/locking-selftest.c
@@ -1135,12 +1135,12 @@ void locking_selftest(void)
 	debug_locks_silent = !debug_locks_verbose;
 
 	DO_TESTCASE_6R("A-A deadlock", AA);
-	DO_TESTCASE_6R("A-B-B-A deadlock", ABBA);
-	DO_TESTCASE_6R("A-B-B-C-C-A deadlock", ABBCCA);
-	DO_TESTCASE_6R("A-B-C-A-B-C deadlock", ABCABC);
-	DO_TESTCASE_6R("A-B-B-C-C-D-D-A deadlock", ABBCCDDA);
-	DO_TESTCASE_6R("A-B-C-D-B-D-D-A deadlock", ABCDBDDA);
-	DO_TESTCASE_6R("A-B-C-D-B-C-D-A deadlock", ABCDBCDA);
+	DO_TESTCASE_6("A-B-B-A deadlock", ABBA);
+	DO_TESTCASE_6("A-B-B-C-C-A deadlock", ABBCCA);
+	DO_TESTCASE_6("A-B-C-A-B-C deadlock", ABCABC);
+	DO_TESTCASE_6("A-B-B-C-C-D-D-A deadlock", ABBCCDDA);
+	DO_TESTCASE_6("A-B-C-D-B-D-D-A deadlock", ABCDBDDA);
+	DO_TESTCASE_6("A-B-C-D-B-C-D-A deadlock", ABCDBCDA);
 	DO_TESTCASE_6("double unlock", double_unlock);
 	DO_TESTCASE_6("initialize held", init_held);
 	DO_TESTCASE_6_SUCCESS("bad unlock order", bad_unlock_order);

--

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