linux-kernel - [PATCH] tile: comment assumption about __insn

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Date:	Tue, 9 Apr 2013 12:33:07 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Mark Salter <msalter@...hat.com>,
	Aurelien Jacquiot <a-jacquiot@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christian Ruppert <christian.ruppert@...lis.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, <linux-c6x-dev@...ux-c6x.org>
Subject: [PATCH] tile: comment assumption about __insn_mtspr for <asm/irqflags.h>

The arch_local_irq_save(), etc., routines are required to function
as compiler barriers.  They do, but it's subtle and requires knowing
that the gcc builtin __insn_mtspr() is marked as a memory clobber.
Provide a comment explaining the assumption.

Signed-off-by: Chris Metcalf <cmetcalf@...era.com>
---
 arch/tile/include/asm/irqflags.h |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

Linus wrote:
> Some other architectures (tile) have such subtle implementations
> (where is __insn_mtspr() defined?) that I have a hard time judging.

diff --git a/arch/tile/include/asm/irqflags.h b/arch/tile/include/asm/irqflags.h
index 241c0bb..c96f9bb 100644
--- a/arch/tile/include/asm/irqflags.h
+++ b/arch/tile/include/asm/irqflags.h
@@ -40,7 +40,15 @@
 #include <asm/percpu.h>
 #include <arch/spr_def.h>
 
-/* Set and clear kernel interrupt masks. */
+/*
+ * Set and clear kernel interrupt masks.
+ *
+ * NOTE: __insn_mtspr() is a compiler builtin marked as a memory
+ * clobber.  We rely on it being equivalent to a compiler barrier in
+ * this code since arch_local_irq_save() and friends must act as
+ * compiler barriers.  This compiler semantic is baked into enough
+ * places that the compiler will maintain it going forward.
+ */
 #if CHIP_HAS_SPLIT_INTR_MASK()
 #if INT_PERF_COUNT < 32 || INT_AUX_PERF_COUNT < 32 || INT_MEM_ERROR >= 32
 # error Fix assumptions about which word various interrupts are in
-- 
1.7.10.3

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