__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
CC: Gautham R Shenoy
---
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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