[SPARC64] mm: context switch ptlock

sparc64 is unique among architectures in taking the page_table_lock in
its context switch (well, cris does too, but erroneously, and it's not
yet SMP anyway).

This seems to be a private affair between switch_mm and activate_mm,
using page_table_lock as a per-mm lock, without any relation to its uses
elsewhere.  That's fine, but comment it as such; and unlock sooner in
switch_mm, more like in activate_mm (preemption is disabled here).

There is a block of "if (0)"ed code in smp_flush_tlb_pending which would
have liked to rely on the page_table_lock, in switch_mm and elsewhere;
but its comment explains how dup_mmap's flush_tlb_mm defeated it.  And
though that could have been changed at any time over the past few years,
now the chance vanishes as we push the page_table_lock downwards, and
perhaps split it per page table page.  Just delete that block of code.

Which leaves the mysterious spin_unlock_wait(&oldmm->page_table_lock)
in kernel/fork.c copy_mm.  Textual analysis (supported by Nick Piggin)
suggests that the comment was written by DaveM, and that it relates to
the defeated approach in the sparc64 smp_flush_tlb_pending.  Just delete
this block too.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc64/kernel/smp.c

@@ -883,34 +883,13 @@ void smp_flush_tlb_pending(struct mm_struct *mm, unsigned long nr, unsigned long
 	u32 ctx = CTX_HWBITS(mm->context);
 	int cpu = get_cpu();
 
-	if (mm == current->active_mm && atomic_read(&mm->mm_users) == 1) {
+	if (mm == current->active_mm && atomic_read(&mm->mm_users) == 1)
 		mm->cpu_vm_mask = cpumask_of_cpu(cpu);
-		goto local_flush_and_out;
-	} else {
-		/* This optimization is not valid.  Normally
-		 * we will be holding the page_table_lock, but
-		 * there is an exception which is copy_page_range()
-		 * when forking.  The lock is held during the individual
-		 * page table updates in the parent, but not at the
-		 * top level, which is where we are invoked.
-		 */
-		if (0) {
-			cpumask_t this_cpu_mask = cpumask_of_cpu(cpu);
-
-			/* By virtue of running under the mm->page_table_lock,
-			 * and mmu_context.h:switch_mm doing the same, the
-			 * following operation is safe.
-			 */
-			if (cpus_equal(mm->cpu_vm_mask, this_cpu_mask))
-				goto local_flush_and_out;
-		}
-	}
-
-	smp_cross_call_masked(&xcall_flush_tlb_pending,
-			      ctx, nr, (unsigned long) vaddrs,
-			      mm->cpu_vm_mask);
+	else
+		smp_cross_call_masked(&xcall_flush_tlb_pending,
+				      ctx, nr, (unsigned long) vaddrs,
+				      mm->cpu_vm_mask);
 
-local_flush_and_out:
 	__flush_tlb_pending(ctx, nr, vaddrs);
 
 	put_cpu();

include/asm-sparc64/mmu_context.h

@@ -87,37 +87,35 @@ extern void __flush_tlb_mm(unsigned long, unsigned long);
 static inline void switch_mm(struct mm_struct *old_mm, struct mm_struct *mm, struct task_struct *tsk)
 {
 	unsigned long ctx_valid;
+	int cpu;
 
+	/* Note: page_table_lock is used here to serialize switch_mm
+	 * and activate_mm, and their calls to get_new_mmu_context.
+	 * This use of page_table_lock is unrelated to its other uses.
+	 */ 
 	spin_lock(&mm->page_table_lock);
-	if (CTX_VALID(mm->context))
-		ctx_valid = 1;
-        else
-		ctx_valid = 0;
+	ctx_valid = CTX_VALID(mm->context);
+	if (!ctx_valid)
+		get_new_mmu_context(mm);
+	spin_unlock(&mm->page_table_lock);
 
 	if (!ctx_valid || (old_mm != mm)) {
-		if (!ctx_valid)
-			get_new_mmu_context(mm);
-
 		load_secondary_context(mm);
 		reload_tlbmiss_state(tsk, mm);
 	}
 
-	{
-		int cpu = smp_processor_id();
-
-		/* Even if (mm == old_mm) we _must_ check
-		 * the cpu_vm_mask.  If we do not we could
-		 * corrupt the TLB state because of how
-		 * smp_flush_tlb_{page,range,mm} on sparc64
-		 * and lazy tlb switches work. -DaveM
-		 */
-		if (!ctx_valid || !cpu_isset(cpu, mm->cpu_vm_mask)) {
-			cpu_set(cpu, mm->cpu_vm_mask);
-			__flush_tlb_mm(CTX_HWBITS(mm->context),
-				       SECONDARY_CONTEXT);
-		}
+	/* Even if (mm == old_mm) we _must_ check
+	 * the cpu_vm_mask.  If we do not we could
+	 * corrupt the TLB state because of how
+	 * smp_flush_tlb_{page,range,mm} on sparc64
+	 * and lazy tlb switches work. -DaveM
+	 */
+	cpu = smp_processor_id();
+	if (!ctx_valid || !cpu_isset(cpu, mm->cpu_vm_mask)) {
+		cpu_set(cpu, mm->cpu_vm_mask);
+		__flush_tlb_mm(CTX_HWBITS(mm->context),
+			       SECONDARY_CONTEXT);
 	}
-	spin_unlock(&mm->page_table_lock);
 }
 
 #define deactivate_mm(tsk,mm)	do { } while (0)
@@ -127,6 +125,10 @@ static inline void activate_mm(struct mm_struct *active_mm, struct mm_struct *mm
 {
 	int cpu;
 
+	/* Note: page_table_lock is used here to serialize switch_mm
+	 * and activate_mm, and their calls to get_new_mmu_context.
+	 * This use of page_table_lock is unrelated to its other uses.
+	 */ 
 	spin_lock(&mm->page_table_lock);
 	if (!CTX_VALID(mm->context))
 		get_new_mmu_context(mm);

kernel/fork.c

@@ -470,13 +470,6 @@ static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)
 	if (clone_flags & CLONE_VM) {
 		atomic_inc(&oldmm->mm_users);
 		mm = oldmm;
-		/*
-		 * There are cases where the PTL is held to ensure no
-		 * new threads start up in user mode using an mm, which
-		 * allows optimizing out ipis; the tlb_gather_mmu code
-		 * is an example.
-		 */
-		spin_unlock_wait(&oldmm->page_table_lock);
 		goto good_mm;
 	}