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chain_key collision check triggers

lockdep Peter Zijlstra 2 participants 3 messages Jun 3, 2016 1 message not in the archive

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s390 Locking primitives
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Peter Zijlstra

author maintainer
#1 lore raw
In reply to a message not in the archive
Quoted 5 lines · Heiko Carstens

Hi all,

the new lockdep chain_key collision detection code triggers quite reliably on s390, so it looks like we need a different iterate_chain_key() implementation?

That too, I have one queued up; but if you can realiably trigger this that's 'good' news. Most reports so far have been one offs that people could not reproduce.

Let me wake up a bit more, but in the meantime, the below is what I had queued.

---
Subject: locking,lockdep: Use __jhash_mix for iterate_chain_key
From: Peter Zijlstra <[email protected]>
Date: Mon May 30 18:31:33 CEST 2016

Use __jhash_mix() to mix the class_idx into the class_key. This function provides better mixing than the previously used, home grown mix function.

Leave hashing to the professionals :-)

Cc [email protected]
Suggested-by George Spelvin <[email protected]>
Signed-off-by Peter Zijlstra (Intel) <[email protected]>
---
 kernel/locking/lockdep.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)
kernel/locking/lockdep.c +9 -4
@@ -46,6 +46,7 @@
 #include <linux/gfp.h>
 #include <linux/kmemcheck.h>
 #include <linux/random.h>
+#include <linux/jhash.h>
 
 #include <asm/sections.h>
 

@@ -309,10 +310,14 @@ static struct hlist_head chainhash_table
  * It's a 64-bit hash, because it's important for the keys to be
  * unique.
  */
-#define iterate_chain_key(key1, key2) \
-	(((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
-	((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
-	(key2))
+static inline u64 iterate_chain_key(u64 key, u32 idx)
+{
+	u32 k0 = key, k1 = key >> 32;
+
+	__jhash_mix(idx, k0, k1); /* Macro that modifies arguments! */
+
+	return k0 | (u64)k1 << 32;
+}
 
 void lockdep_off(void)
 {
#2 lore raw
Quoted 14 lines · Peter Zijlstra
Quoted 5 lines · Heiko Carstens

Hi all,

the new lockdep chain_key collision detection code triggers quite reliably on s390, so it looks like we need a different iterate_chain_key() implementation?

That too, I have one queued up; but if you can realiably trigger this that's 'good' news. Most reports so far have been one offs that people could not reproduce.

Let me wake up a bit more, but in the meantime, the below is what I had queued.

Quoted 8 lines

---
Subject: locking,lockdep: Use __jhash_mix for iterate_chain_key
From: Peter Zijlstra <[email protected]>
Date: Mon May 30 18:31:33 CEST 2016

Use __jhash_mix() to mix the class_idx into the class_key. This function provides better mixing than the previously used, home grown mix function.

Ok, thanks! I'll use that then. Unfortunately the new hash function generates 28 instead of 2 instructions on s390.

author maintainer
#3 lore raw
Quoted 20 lines · Heiko Carstens
Quoted 19 lines · Peter Zijlstra
Quoted 5 lines · Heiko Carstens

Hi all,

the new lockdep chain_key collision detection code triggers quite reliably on s390, so it looks like we need a different iterate_chain_key() implementation?

That too, I have one queued up; but if you can realiably trigger this that's 'good' news. Most reports so far have been one offs that people could not reproduce.

Let me wake up a bit more, but in the meantime, the below is what I had queued.

---
Subject: locking,lockdep: Use __jhash_mix for iterate_chain_key
From: Peter Zijlstra <[email protected]>
Date: Mon May 30 18:31:33 CEST 2016

Quoted 6 lines
Quoted 3 lines

Use __jhash_mix() to mix the class_idx into the class_key. This function provides better mixing than the previously used, home grown mix function.

Ok, thanks! I'll use that then. Unfortunately the new hash function generates 28 instead of 2 instructions on s390.

Yeah, but I doubt you can actually measure the performance difference, lockdep is a pig anyway.