On Thu, 19 Oct 2017, Bart Van Assche wrote:

On Thu, 2017-10-19 at 14:55 +0900, Byungchul Park wrote:

quoted

Now the performance regression was fixed, re-enable
CONFIG_LOCKDEP_CROSSRELEASE and CONFIG_LOCKDEP_COMPLETIONS.

Signed-off-by: Byungchul Park <redacted>
---
 lib/Kconfig.debug | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 90ea784..fe8fceb 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug

@@ -1138,8 +1138,8 @@ config PROVE_LOCKING
 	select DEBUG_MUTEXES
 	select DEBUG_RT_MUTEXES if RT_MUTEXES
 	select DEBUG_LOCK_ALLOC
-	select LOCKDEP_CROSSRELEASE if BROKEN
-	select LOCKDEP_COMPLETIONS if BROKEN
+	select LOCKDEP_CROSSRELEASE
+	select LOCKDEP_COMPLETIONS
 	select TRACE_IRQFLAGS
 	default n
 	help

I do not agree with this patch. Although the traditional lock validation
code can be proven not to produce false positives, that is not the case for
the cross-release checks. These checks are prone to produce false positives.
Many kernel developers, including myself, are not interested in spending
time on analyzing false positive deadlock reports. So I think that it is
wrong to enable cross-release checking unconditionally if PROVE_LOCKING has
been enabled. What I think that should happen is that either the cross-
release checking code is removed from the kernel or that
LOCKDEP_CROSSRELEASE becomes a new kernel configuration option. That will
give kernel developers who choose to enable PROVE_LOCKING the freedom to
decide whether or not to enable LOCKDEP_CROSSRELEASE.

I really disagree with your reasoning completely

1) When lockdep was introduced more than ten years ago it was far from
   perfect and we spent a reasonable amount of time to improve it, analyze
   false positives and add the missing annotations all over the tree. That
   was a process which took years.

2) Surely nobody is interested in wasting time on analyzing false
   positives, but your (and other peoples) attidute of 'none of my
   business' is what makes kernel development extremly frustrating.

   It should be in the interest of everybody involved in kernel development
   to help with improving such features and not to lean back and wait for
   others to bring it into a shape which allows you to use it as you see
   fit.

That's not how community works and lockdep would not be in the shape it is
today, if only a handful of people would have used and improved it. Such
things only work when used widely and when we get enough information so we
can address the weak spots.

Thanks,

	tglx


   

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