linux-kernel - Re: [bisected] system hang after boot

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Date:   Mon, 27 Nov 2017 12:45:06 +0000
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [bisected] system hang after boot

Me again...

On Mon, Nov 27, 2017 at 11:49:47AM +0000, Will Deacon wrote:
> On Wed, Nov 22, 2017 at 09:22:17PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 22, 2017 at 06:26:59PM +0000, Will Deacon wrote:
> > 
> > > Now, I can't see what the break_lock is doing here other than causing
> > > problems. Is there a good reason for it, or can you just try removing it
> > > altogether? Patch below.
> > 
> > The main use is spin_is_contended(), which in turn ends up used in
> > __cond_resched_lock() through spin_needbreak().
> > 
> > This allows better lock wait times for PREEMPT kernels on platforms
> > where the lock implementation itself cannot provide 'contended' state.
> > 
> > In that capacity the write-write race shouldn't be a problem though.
> 
> I'm not sure why it isn't a problem: given that the break_lock variable
> can read as 1 for a lock that is no longer contended and 0 for a lock that
> is currently contended, then the __cond_resched_lock is likely to see a
> value of 0 (i.e. spin_needbreak always return false) more often than no
> since it's checked by the lock holder.
> 
> > That said, I'd not be horribly sad to see this go, I've always found it
> > to be quite the ugly hack and taking it out should provide some
> > incentive for better lock implementations for the archs relying on this.
> 
> Right, and they can always implement arch_spin_is_contended if they have
> a good way to do it.
> 
> I'll post this diff as a full patch, since it's clearly needed to get some
> s390 systems booting against with 4.15.

In writing up the patch, I realised that I might be being a bit too purist
about what we can actually rely on here, particularly as GENERIC_LOCKBREAK
isn't widely selected. I'm still suspicious, but perhaps on the architectures
that *do* select it, then the propagation of the lock_break variable between
CPUs works out because of the barriers in lock()/trylock(), and that's
sufficient for a "good enough" spin_is_contended() implementation.

Sebastian: could you try the diff below, please? If that fixes s390, then
we can debate the merits of GENERIC_LOCKBREAK independently of fixing this
issue.

Thanks,

Will

--->8

diff --git a/kernel/locking/spinlock.c b/kernel/locking/spinlock.c
index 1fd1a7543cdd..0ebb253e2199 100644
--- a/kernel/locking/spinlock.c
+++ b/kernel/locking/spinlock.c
@@ -68,8 +68,8 @@ void __lockfunc __raw_##op##_lock(locktype##_t *lock)			\
 									\
 		if (!(lock)->break_lock)				\
 			(lock)->break_lock = 1;				\
-		while ((lock)->break_lock)				\
-			arch_##op##_relax(&lock->raw_lock);		\
+									\
+		arch_##op##_relax(&lock->raw_lock);			\
 	}								\
 	(lock)->break_lock = 0;						\
 }									\
@@ -88,8 +88,8 @@ unsigned long __lockfunc __raw_##op##_lock_irqsave(locktype##_t *lock)	\
 									\
 		if (!(lock)->break_lock)				\
 			(lock)->break_lock = 1;				\
-		while ((lock)->break_lock)				\
-			arch_##op##_relax(&lock->raw_lock);		\
+									\
+		arch_##op##_relax(&lock->raw_lock);			\
 	}								\
 	(lock)->break_lock = 0;						\
 	return flags;							\