linux-kernel - RE: [PATCH v2 1/2] ia64: implement atomic64_dec_if

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Date:   Mon, 12 Sep 2016 18:26:07 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] ia64: implement atomic64_dec_if_positive

+static __inline__ long atomic64_dec_if_positive(atomic64_t *v)
+{
+	long c, old, dec;
+	c = atomic64_read(v);
+	for (;;) {
+		dec = c - 1;
+		if (unlikely(dec < 0))
+			break;
+		old = atomic64_cmpxchg((v), c, dec);
+		if (likely(old == c))
+			break;
+		c = old;
+	}
+	return dec;
+}

I was about to say "add a cpu_relax()" in the bottom of that loop. But none of the other
atomic ops that spin on a cmpxchg do that ... so:

Acked-by: Tony Luck <tony.luck@...el.com>

-Tony