From 019a4a61f0173e09a532d47c0979367cae365d0c Mon Sep 17 00:00:00 2001
From: patacongo <patacongo@42af7a65-404d-4744-a932-0658087f49c3>
Date: Fri, 5 Nov 2010 00:29:37 +0000
Subject: Update some comments

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3073 42af7a65-404d-4744-a932-0658087f49c3
---
 nuttx/arch/avr/src/avr32/up_fullcontextrestore.S | 22 ++++++++++++++++++++++
 nuttx/arch/avr/src/avr32/up_sigdeliver.c         | 17 +++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

(limited to 'nuttx/arch/avr/src')

diff --git a/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S b/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
index c40239307..c883c58c7 100755
--- a/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
+++ b/nuttx/arch/avr/src/avr32/up_fullcontextrestore.S
@@ -53,6 +53,28 @@
 /****************************************************************************
  * Name: up_fullcontextrestore
  *
+ * Descripion:
+ *   Restore the full-running contex of a thread.
+ *
+ *   NOTE: Thus function must handle one very strange case.  That is when
+ *   this function is called with up_sigdeliver().  That case is strange in
+ *   two ways:
+ *
+ *   1. It is not a context switch between threads.  Rather, up_fullcontextrestore
+ *      must behave more it more like a longjmp within the same task, using the
+ *      same stack.
+ *   2. In this case, this function is called with r12 pointing to a register
+ *      save area on the stack to be destroyed.  This is dangerous for two
+ *      reasons: (a) there is a period of time where the stack contents still
+ *      contain valid data, but are outside of range protected by the stack
+ *      pointer (hence, interrupts must be disabled), and (b) there is the
+ *      very real possibility that the new stack pointer might overlap with
+ *      the register save area and stack usage in this function might corrupt
+ *      the register save data before the state is restored.
+ *
+ * Input Parameters:
+ *   r12 = A pointer to the register save area of the thread to be restored.
+ *
  * C Prototype:
  *  void up_fullcontextrestore(uint32_t *regs);
  *
diff --git a/nuttx/arch/avr/src/avr32/up_sigdeliver.c b/nuttx/arch/avr/src/avr32/up_sigdeliver.c
index aa3940b79..b000f118a 100644
--- a/nuttx/arch/avr/src/avr32/up_sigdeliver.c
+++ b/nuttx/arch/avr/src/avr32/up_sigdeliver.c
@@ -82,7 +82,7 @@
 void up_sigdeliver(void)
 {
   _TCB  *rtcb = (_TCB*)g_readytorun.head;
-  uint32_t regs[XCPTCONTEXT_REGS];
+  uint32_t regs[XCPTCONTEXT_REGS+3];  /* Why +3? See below */
   sig_deliver_t sigdeliver;
 
   /* Save the errno.  This must be preserved throughout the signal handling
@@ -128,7 +128,20 @@ void up_sigdeliver(void)
   (void)irqsave();
   rtcb->pterrno = saved_errno;
 
-  /* Then restore the correct state for this thread of execution. */
+  /* Then restore the correct state for this thread of execution. This is an
+   * unusual case that must be handled by up_fullcontextresore. This case is
+   * unusal in two ways:
+   *
+   *   1. It is not a context switch between threads.  Rather, up_fullcontextrestore
+   *      must behave more it more like a longjmp within the same task, using
+   *      he same stack.
+   *   2. In this case, up_fullcontextrestore is called with r12 pointing to
+   *      a register save area on the stack to be destroyed.  This is
+   *      dangerous because there is the very real possibility that the new
+   *      stack pointer might overlap with the register save area and hat stack
+   *      usage in up_fullcontextrestore might corrupt the register save data
+   *      before the state is restored.
+   */
 
   up_ledoff(LED_SIGNAL);
   up_fullcontextrestore(regs);
-- 
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