Re: [GIT PULL] overlay filesystem v25

[Miklos Szeredi]

[Paul McKenney added to CC]

On Sat, Oct 25, 2014 at 7:06 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Oct 25, 2014 at 11:53:52AM +0200, Miklos Szeredi wrote:
>
>> Yes, but it's not about race with copy-up (which the ovl_path_upper()
>> protects against), but race of two fsync calls with each other.  If
>> there's no synchronization between them, then that od->upperfile does
>> indeed count as lockless access, no matter that the assignment was
>> done under lock.
>
>         p = global;
>         if (!p) {       // outside of lock
>                 p = alloc();
>                 grab lock
>                 if (!global) {
>                         global = p;
>                 } else {
>                         destroy(p);
>                         p = global;
>                 }
>                 drop lock
>         }
> is a very common pattern, especially if you look for cases when lock is
> a spinlock and allocation is blocking (in those cases you'll often see
> destroy() part done after dropping the lock; that's where what I fucked up in
> what I'd originally pushed.  And it wasn't even needed - fput() under
> ->i_mutex is OK...)

Being a very common pattern does not automatically make it correct...

My understanding of these issues is very limited, but it's not clear
to me what will order initialization of members of p with the storing
of p into global.  E.g. we start out with global == NULL and p->foo ==
0.

CPU1:
  p->foo = 1
  grab lock
  if (!global)
      global = p

CPU1:
  p = global
  if (p)
     q = p->foo

Is it guaranteed that the above sequence (as is, without any barriers
or ACCESS_ONCE() other than the lock acquisition) will result in q ==
1 if p != NULL?

Thanks,
Miklos
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