Re: Linux 3.16-rc6

From: Waiman Long
Date: Thu Jul 24 2014 - 16:38:37 EST

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@xxxxxxxxxxxxx> 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

+ /*
+ * 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,

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