linux-ext4 - [PATCH RFC v5 21/21] dept: Don't create dependencies between different depths in any case

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Date:   Wed, 16 Mar 2022 11:26:33 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     torvalds@...ux-foundation.org
Cc:     damien.lemoal@...nsource.wdc.com, linux-ide@...r.kernel.org,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
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Subject: [PATCH RFC v5 21/21] dept: Don't create dependencies between different depths in any case

Dept already prevents creating dependencies between different depths of
the class indicated by *_lock_nested() when the lock acquisitions happen
consecutively. For example:

   lock A0 with depth
   lock_nested A1 with depth + 1

   ...

   unlock A1
   unlock A0

Dept does not create A0 -> A1 dependency in this case. However, once
another class cut in, the code becomes problematic. When Dept tries to
create real dependencies, it does not only create real ones but also
wrong ones between different depths of the class. For example:

   lock A0 with depth
   lock B
   lock_nested A1 with depth + 1

   ...

   unlock A1
   unlock B
   unlock A0

Even in this case, Dept should not create A0 -> A1 dependency but it
does. Let Dept not create wrong dependencies between different depths
of the class in any case.

Reported-by: 42.hyeyoo@...il.com
Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/dependency/dept.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/kernel/dependency/dept.c b/kernel/dependency/dept.c
index 10801783..a2088685 100644
--- a/kernel/dependency/dept.c
+++ b/kernel/dependency/dept.c
@@ -1458,14 +1458,7 @@ static void add_wait(struct dept_class *c, unsigned long ip,
 
 		eh = dt->ecxt_held + i;
 		if (eh->ecxt->class != c || eh->nest == ne)
-			break;
-	}
-
-	for (; i >= 0; i--) {
-		struct dept_ecxt_held *eh;
-
-		eh = dt->ecxt_held + i;
-		add_dep(eh->ecxt, w);
+			add_dep(eh->ecxt, w);
 	}
 
 	if (!wait_consumed(w) && !rich_stack) {
-- 
1.9.1