Subject: dcache aliasing problem on fork

From: Atsushi Nemoto <[email-masked]>
Date: Fri, 04 Feb 2005 18:38:13 +0900 (JST)

There is a dcache aliasing problem on preempt kernel (or SMP kernel,
perhaps) when a multi-threaded program calls fork().

1. Now there is a process containing two thread (T1 and T2).  The
   thread T1 call fork().  dup_mmap() function called on T1 context.

static inline int dup_mmap(struct mm_struct * mm, struct mm_struct * oldmm)
{
	...
	flush_cache_mm(current->mm);
	/* A */
	...
	(write-protect all Copy-On-Write pages)
	...
	/* B */
	flush_tlb_mm(current->mm);
	...
}

2. When preemption happens between A and B (or on SMP kernel), the
   thread T2 can run and modify data on COW pages without page fault
   (modified data will stay in cache).

3. Some time after fork() completed, the thread T2 may cause page
   fault by write-protect on COW pages .

4. Then data of the COW page will be copied to newly allocated
   physical page (copy_cow_page()).  It reads data via kernel mapping.
   The kernel mapping can have different 'color' with user space
   mapping of the thread T2 (dcache aliasing).  Therefore
   copy_cow_page() will copy stale data.  Then the modified data in
   cache will be lost.

How should we fix this problem?  Any idea?

---
Atsushi Nemoto

---

From: Jun Sun <[email-masked]>
Date: Fri, 4 Feb 2005 09:44:10 -0800

> How should we fix this problem?  Any idea?

It seems to me a naive solution is to introduce a spinlock to make all
three operation automic.  you flush tlb first and make relavent tlb fault
handling sync with this spinlock as well.

At in theory it should fix the problem, but the spinlock might be held
for too long this dup_mmap().

BTW, is this problem real or hypothetic?

Jun

---

From: Atsushi Nemoto <[email-masked]>
Date: Sat, 05 Feb 2005 21:06:48 +0900 (JST)

>>>>> On Fri, 4 Feb 2005 09:44:10 -0800, Jun Sun <[email-masked]> said:

jsun> It seems to me a naive solution is to introduce a spinlock to
jsun> make all three operation automic.  you flush tlb first and make
jsun> relavent tlb fault handling sync with this spinlock as well.

jsun> At in theory it should fix the problem, but the spinlock might
jsun> be held for too long this dup_mmap().

Yes, it may be too long.  Also dup_mmap might sleep via alloc_pages,
cond_resched_lock, etc. therefore the spinlock can not be held
entirely.  Now I think fixing copy_cow_page() might be a way to go.

jsun> BTW, is this problem real or hypothetic?

Yes.  This is a real problem.  Using fork() in multi-thread program
should be legal and perhaps only way to call external program
(system() will use fork() internally).  It will not be a special case.

---
Atsushi Nemoto