[RFC] [PATCH] lazy TLB: possible bug, some notes (2.3.12)

Manfred Spraul (manfreds@colorfullife.com)
Fri, 30 Jul 1999 19:34:01 +0200

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1) I've attached a small patch which should fix 2 possible
problems on the i386.
* mm->cpu_vm_mask was not set on crossing TLB flushes.
* there was a race in flush_tlb_current_task() between setting
mm->cpu_vm_mask and the IPI which could modify cpu_vm_mask.

Since flush_tlb_current_task() is now identical to
flush_tlb_mm(current->mm), I'd remove flush_tlb_current_task()
and change the macro for flush_tlb(). [in include/asm/pgtable.h]

2) I think it should be possible to call start_lazy_tlb()
even if the thread is already in the lazy-tlb mode.
i.e. multiple calls to start_lazy_tlb() should be possible
as long as you call end_lazy_tlb() for every start_lazy_tlb()
call.
Curently, this would oops.

3) I noticed that 2.3.12 breaks the PPC mm implementation:
mmu_context_overflow() assumes that it can get to all
mm's by looking at tsk->mm for all tasks in the system.
(arch/ppc/mm/init.c)

This is not correct any more: if a user thread executes
sys_bdflush(), then this mm will not be referenced
from tsk->mm.
[I don't have a PPC, but it looks wrong]

--
	Manfred
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--- 2.3/arch/i386/kernel/smp.c	Thu Jul 29 15:23:06 1999
+++ build-2.3/arch/i386/kernel/smp.c	Fri Jul 30 19:27:02 1999
@@ -1617,6 +1617,8 @@
 			if (test_bit(cpu, &smp_invalidate_needed)) {
 				clear_bit(cpu, &smp_invalidate_needed);
 				local_flush_tlb();
+				if(current->mm)
+					atomic_set_mask(1 << cpu, &current->mm->cpu_vm_mask);
 			}
 			--stuck;
 			if (!stuck) {
@@ -1637,14 +1639,16 @@
  */	
 void flush_tlb_current_task(void)
 {
-	unsigned long vm_mask = 1 << current->processor;
 	struct mm_struct *mm = current->mm;
+	unsigned long vm_mask = 1 << current->processor;
+	unsigned long cpu_mask = mm->cpu_vm_mask & ~vm_mask;
 
-	if (mm->cpu_vm_mask != vm_mask) {
-		flush_tlb_others(mm->cpu_vm_mask & ~vm_mask);
+	mm->cpu_vm_mask = 0;
+	if (current->active_mm == mm) {
 		mm->cpu_vm_mask = vm_mask;
+		local_flush_tlb();
 	}
-	local_flush_tlb();
+	flush_tlb_others(cpu_mask);
 }
 
 void flush_tlb_mm(struct mm_struct * mm)








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