Re: Linux 3.16-rc6

[Waiman Long <waiman.long@hp.com>]

On 07/24/2014 02:36 PM, Peter Zijlstra wrote:
> On Thu, Jul 24, 2014 at 11:18:16AM -0700, Linus Torvalds wrote:
>> On Thu, Jul 24, 2014 at 5:58 AM, Peter Zijlstra<peterz@infradead.org>  wrote:
>>> So going by the nifty picture rostedt made:
>>>
>>> [   61.454336]        CPU0                    CPU1
>>> [   61.454336]        ----                    ----
>>> [   61.454336]   lock(&(&p->alloc_lock)->rlock);
>>> [   61.454336]                                local_irq_disable();
>>> [   61.454336]                                lock(tasklist_lock);
>>> [   61.454336]                                lock(&(&p->alloc_lock)->rlock);
>>> [   61.454336]<Interrupt>
>>> [   61.454336]     lock(tasklist_lock);
>> So this *should* be fine. It always has been in the past, and it was
>> certainly the *intention* that it should continue to work with
>> qrwlock, even in the presense of pending writers on other cpu's.
>>
>> The qrwlock rules are that a read-lock in an interrupt is still going
>> to be unfair and succeed if there are other readers.
> Ah, indeed. Should have checked :/
>
>> So it sounds to me like the new lockdep rules in tip/master are too
>> strict and are throwing a false positive.
> Right. Waiman can you have a look?

Yes, I think I may have a solution for that.

Borislav, can you apply the following patch on top of the lockdep patch 
to see if it can fix the problem?

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index d24e433..507a8ce 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3595,6 +3595,12 @@ void lock_acquire(struct lockdep_map *lock, 
unsigned int
         raw_local_irq_save(flags);
         check_flags(flags);

+       /*
+        * An interrupt recursive read in interrupt context can be 
considered
+        * to be the same as a recursive read from checking perspective.
+        */
+       if ((read == 3) && in_interrupt())
+               read = 2;
         current->lockdep_recursion = 1;
         trace_lock_acquire(lock, subclass, trylock, read, check, 
nest_lock, ip);
         __lock_acquire(lock, subclass, trylock, read, check,