[PATCH v2] powerpc: Call restore_sprs() before _switch()

[PATCH v2] powerpc: Call restore_sprs() before _switch()

[Anton Blanchard]

commit 152d523e6307 ("powerpc: Create context switch helpers save_sprs()
and restore_sprs()") moved the restore of SPRs after the call to _switch().

There is an issue with this approach - new tasks do not return through
_switch(), they are set up by copy_thread() to directly return through
ret_from_fork() or ret_from_kernel_thread(). This means restore_sprs() is
not getting called for new tasks.

Fix this by moving restore_sprs() before _switch().

Signed-off-by: Anton Blanchard <anton@samba.org>
Fixes: 152d523e6307 ("powerpc: Create context switch helpers save_sprs() and restore_sprs()")

fold
---
 arch/powerpc/kernel/process.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 58194c3..6f5c392 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -964,14 +964,17 @@ struct task_struct *__switch_to(struct task_struct *prev,
 
 	tm_recheckpoint_new_task(new);
 
-	last = _switch(old_thread, new_thread);
-
-	/* Need to recalculate these after calling _switch() */
-	old_thread = &last->thread;
-	new_thread = &current->thread;
-
+	/*
+	 * Call restore_sprs() before calling _switch(). If we move it after
+	 * _switch() then we miss out on calling it for new tasks. The reason
+	 * for this is we manually create a stack frame for new tasks that
+	 * directly returns through ret_from_fork() or
+	 * ret_from_kernel_thread(). See copy_thread() for details.
+	 */
 	restore_sprs(old_thread, new_thread);
 
+	last = _switch(old_thread, new_thread);
+
 #ifdef CONFIG_PPC_BOOK3S_64
 	if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
 		current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
-- 
2.5.0

Re: [v2] powerpc: Call restore_sprs() before _switch()

[Michael Ellerman]

On Thu, 2015-10-12 at 09:44:39 UTC, Anton Blanchard wrote:
> commit 152d523e6307 ("powerpc: Create context switch helpers save_sprs()
> and restore_sprs()") moved the restore of SPRs after the call to _switch().
> 
> There is an issue with this approach - new tasks do not return through
> _switch(), they are set up by copy_thread() to directly return through
> ret_from_fork() or ret_from_kernel_thread(). This means restore_sprs() is
> not getting called for new tasks.
> 
> Fix this by moving restore_sprs() before _switch().
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>
> Fixes: 152d523e6307 ("powerpc: Create context switch helpers save_sprs() and restore_sprs()")

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/20dbe67062062c2a790832f0

cheers