[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>

diff --git a/arch/sparc64/kernel/smp.c b/arch/sparc64/kernel/smp.c
index b137fd6..a9089e2 100644 (file)
--- a/arch/sparc64/kernel/smp.c
+++ b/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();

diff --git a/include/asm-sparc64/mmu_context.h b/include/asm-sparc64/mmu_context.h
index 87c43c6..08ba72d 100644 (file)
--- a/include/asm-sparc64/mmu_context.h
+++ b/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);

diff --git a/kernel/fork.c b/kernel/fork.c
index efac2c5..158710d 100644 (file)
--- a/kernel/fork.c
+++ b/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;
        }