netdev - Re: [PATCH v5 bpf-next 1/9] bpf: introduce bpf_spin

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Message-ID: <20190131085122.GE3103@hirez.programming.kicks-ass.net>
Date:   Thu, 31 Jan 2019 09:51:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, jannh@...gle.com, paulmck@...ux.ibm.com,
        will.deacon@....com, mingo@...hat.com, netdev@...r.kernel.org,
        kernel-team@...com, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v5 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Thu, Jan 31, 2019 at 09:49:52AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 30, 2019 at 01:34:19PM -0800, Alexei Starovoitov wrote:
> > On Wed, Jan 30, 2019 at 10:05:29PM +0100, Peter Zijlstra wrote:

> > > +static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
> > > +{
> > > +	atomic_t *l = (void *)lock;
> > > +	do {
> > > +		atomic_cond_read_relaxed(l, !VAL);
> > 
> > wow. that's quite a macro magic.
> 
> Yeah, C sucks for not having lambdas, this was the best we could come up
> with.
> 
> This basically allows architectures to optimize the
> wait-for-variable-to-change thing. Currently only ARM64 does that, I
> have a horrible horrible patch that makes x86 use MONITOR/MWAIT for
> this, and I suppose POWER should use it but doesn't.

Nick, do you guys want something like this?

---
diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index fbe8df433019..111984c5670d 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h
@@ -99,6 +99,20 @@ do {									\
 #define barrier_nospec()
 #endif /* CONFIG_PPC_BARRIER_NOSPEC */
 
+#define smp_cond_load_relaxed(ptr, cond_expr) ({		\
+	typeof(ptr) __PTR = (ptr);				\
+	typeof(*ptr) VAL = READ_ONCE(*__PTR);			\
+	if (unlikely(!(cond_expr))) {				\
+		spin_begin();					\
+		do {						\
+			spin_cpu_relax();			\
+			VAL = READ_ONCE(*__PTR);		\
+		} while (!(cond_expr));				\
+		spin_end();					\
+	}							\
+	VAL;							\
+})
+
 #include <asm-generic/barrier.h>
 
 #endif /* _ASM_POWERPC_BARRIER_H */