Re: [patch 09/14] syslets: x86, add move_user_context() method

On Thu, 15 Feb 2007, Ingo Molnar wrote:

/*
+ * Move user-space context from one kernel thread to another.
+ * This includes registers and FPU state. Callers must make
+ * sure that neither task is running user context at the moment:
+ */
+void
+move_user_context(struct task_struct *new_task, struct task_struct *old_task)
+{
+ struct pt_regs *old_regs = task_pt_regs(old_task);
+ struct pt_regs *new_regs = task_pt_regs(new_task);
+ union i387_union *tmp;
+
+ *new_regs = *old_regs;
+ /*
+ * Flip around the FPU state too:
+ */
+ tmp = new_task->thread.i387;
+ new_task->thread.i387 = old_task->thread.i387;
+ old_task->thread.i387 = tmp;
+}

Let's say that old_task ("prev" at the incoming schedule) has TS_USEDFPU
set. Its context gets moved to the new_task (the one returning to
userspace) *before* the __unlazy_fpu() done in __switch_to(). The
__unlazy_fpu() at the following schedule will save the state to the old
new_task context, and that fine as far as the going-to-sleep task goes.
The next fault happening in new_task (return to userspace one) will reload
a non up2date context (the one we got from old_task, but never hit by
the __unlazy_fpu() flush). Right?



- Davide

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