From: Sebastian Andrzej Siewior Date: Tue, 24 Jul 2018 14:48:55 +0200 Subject: [PATCH 019/353] x86/efi: drop task_lock() from efi_switch_mm() Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=3d393f40689a14bc6d2304d35d22407d42a7f4ba efi_switch_mm() is a wrapper around switch_mm() which saves current's ->active_mm, sets the requests mm as ->active_mm and invokes switch_mm(). I don't think that task_lock() is required during that procedure. It protects ->mm which isn't changed here. It needs to be mentioned that during the whole procedure (switch to EFI's mm and back) the preemption needs to be disabled. A context switch at this point would reset the cr3 value based on current->mm. Also, this function may not be invoked at the same time on a different CPU because it would overwrite the efi_scratch.prev_mm information. Remove task_lock() and also update the comment to reflect it. Signed-off-by: Sebastian Andrzej Siewior --- arch/x86/platform/efi/efi_64.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c index 77d05b56089a..cd7e4bfb43cf 100644 --- a/arch/x86/platform/efi/efi_64.c +++ b/arch/x86/platform/efi/efi_64.c @@ -619,18 +619,16 @@ void __init efi_dump_pagetable(void) /* * Makes the calling thread switch to/from efi_mm context. Can be used - * for SetVirtualAddressMap() i.e. current->active_mm == init_mm as well - * as during efi runtime calls i.e current->active_mm == current_mm. - * We are not mm_dropping()/mm_grabbing() any mm, because we are not - * losing/creating any references. + * in a kernel thread and user context. Preemption needs to remain disabled + * while the EFI-mm is borrowed. mmgrab()/mmdrop() is not used because the mm + * can not change under us. + * It should be ensured that there are no concurent calls to this function. */ void efi_switch_mm(struct mm_struct *mm) { - task_lock(current); efi_scratch.prev_mm = current->active_mm; current->active_mm = mm; switch_mm(efi_scratch.prev_mm, mm, NULL); - task_unlock(current); } #ifdef CONFIG_EFI_MIXED