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tile: comment assumption about __insn_mtspr for <asm/irqflags.h>

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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@tilera.com>
[ This came about from me wondering about the synchronization rules of
  __insn_mtspr()   - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Chris Metcalf 2013-04-09 12:33:07 -04:00 committed by Linus Torvalds
parent 84ee9805bc
commit 3e2e0d2c22
1 changed files with 9 additions and 1 deletions
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10
arch/tile/include/asm/irqflags.h
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@ -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