powerpc: Never giveup a reclaimed thread when enabling kernel {fp, altivec, vsx}

After a thread is reclaimed from its active or suspended transactional
state the checkpointed state exists on CPU, this state (along with the
live/transactional state) has been saved in its entirety by the
reclaiming process.

There exists a sequence of events that would cause the kernel to call
one of enable_kernel_fp(), enable_kernel_altivec() or
enable_kernel_vsx() after a thread has been reclaimed. These functions
save away any user state on the CPU so that the kernel can use the
registers. Not only is this saving away unnecessary at this point, it
is actually incorrect. It causes a save of the checkpointed state to
the live structures within the thread struct thus destroying the true
live state for that thread.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -205,12 +205,23 @@ EXPORT_SYMBOL_GPL(flush_fp_to_thread);
 
 void enable_kernel_fp(void)
 {
+	unsigned long cpumsr;
+
 	WARN_ON(preemptible());
 
-	msr_check_and_set(MSR_FP);
+	cpumsr = msr_check_and_set(MSR_FP);
 
 	if (current->thread.regs && (current->thread.regs->msr & MSR_FP)) {
 		check_if_tm_restore_required(current);
+		/*
+		 * If a thread has already been reclaimed then the
+		 * checkpointed registers are on the CPU but have definitely
+		 * been saved by the reclaim code. Don't need to and *cannot*
+		 * giveup as this would save  to the 'live' structure not the
+		 * checkpointed structure.
+		 */
+		if(!msr_tm_active(cpumsr) && msr_tm_active(current->thread.regs->msr))
+			return;
 		__giveup_fpu(current);
 	}
 }
@@ -257,12 +268,23 @@ EXPORT_SYMBOL(giveup_altivec);
 
 void enable_kernel_altivec(void)
 {
+	unsigned long cpumsr;
+
 	WARN_ON(preemptible());
 
-	msr_check_and_set(MSR_VEC);
+	cpumsr = msr_check_and_set(MSR_VEC);
 
 	if (current->thread.regs && (current->thread.regs->msr & MSR_VEC)) {
 		check_if_tm_restore_required(current);
+		/*
+		 * If a thread has already been reclaimed then the
+		 * checkpointed registers are on the CPU but have definitely
+		 * been saved by the reclaim code. Don't need to and *cannot*
+		 * giveup as this would save  to the 'live' structure not the
+		 * checkpointed structure.
+		 */
+		if(!msr_tm_active(cpumsr) && msr_tm_active(current->thread.regs->msr))
+			return;
 		__giveup_altivec(current);
 	}
 }
@@ -331,12 +353,23 @@ static void save_vsx(struct task_struct *tsk)
 
 void enable_kernel_vsx(void)
 {
+	unsigned long cpumsr;
+
 	WARN_ON(preemptible());
 
-	msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
+	cpumsr = msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
 
 	if (current->thread.regs && (current->thread.regs->msr & MSR_VSX)) {
 		check_if_tm_restore_required(current);
+		/*
+		 * If a thread has already been reclaimed then the
+		 * checkpointed registers are on the CPU but have definitely
+		 * been saved by the reclaim code. Don't need to and *cannot*
+		 * giveup as this would save  to the 'live' structure not the
+		 * checkpointed structure.
+		 */
+		if(!msr_tm_active(cpumsr) && msr_tm_active(current->thread.regs->msr))
+			return;
 		if (current->thread.regs->msr & MSR_FP)
 			__giveup_fpu(current);
 		if (current->thread.regs->msr & MSR_VEC)