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[SPARC64] mm: context switch ptlock

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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>
This commit is contained in:
Hugh Dickins
2005-11-07 14:09:01 -08:00
committed by David S. Miller
parent b8ae48656d
commit dedeb0029b
3 changed files with 29 additions and 55 deletions
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31
arch/sparc64/kernel/smp.c
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@@ -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);
else
smp_cross_call_masked(&xcall_flush_tlb_pending,
ctx, nr, (unsigned long) vaddrs,
mm->cpu_vm_mask);
/* 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);
local_flush_and_out:
__flush_tlb_pending(ctx, nr, vaddrs);
put_cpu();