[lttng-dev] [RFC] adding into middle of RCU list

[Paul E. McKenney]

On Sun, Sep 01, 2013 at 01:42:10PM -0700, Josh Triplett wrote:
> On Sat, Aug 31, 2013 at 02:32:28PM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 29, 2013 at 07:16:37PM -0700, Josh Triplett wrote:
> > > On Thu, Aug 29, 2013 at 05:57:33PM -0700, Paul E. McKenney wrote:
> > > > On Fri, Aug 23, 2013 at 02:08:22PM -0700, Paul E. McKenney wrote:
> > > > > On Fri, Aug 23, 2013 at 01:16:53PM -0400, Mathieu Desnoyers wrote:
> > > > > > #define __rcu_assign_pointer(p, v, space) \
> > > > > >         do { \
> > > > > >                 smp_wmb(); \
> > > > > >                 (p) = (typeof(*v) __force space *)(v); \
> > > > > >         } while (0)
> > > > > 
> > > > > Or I need to fix this one as well.  ;-)
> > > > 
> > > > In that vein...  Is there anything like typeof() that also preserves
> > > > sparse's notion of address space?  Wrapping an ACCESS_ONCE() around
> > > > "p" in the assignment above results in sparse errors.
> > > 
> > > typeof() will preserve sparse's notion of address space as long as you
> > > do typeof(p), not typeof(*p):
> > > 
> > > $ cat test.c
> > > #define as(n) __attribute__((address_space(n),noderef))
> > > #define __force __attribute__((force))
> > > 
> > > int main(void)
> > > {
> > >     int target = 0;
> > >     int as(1) *foo = (__force typeof(target) as(1) *) ⌖
> > >     typeof(foo) bar = foo;
> > >     return *bar;
> > > }
> > > $ sparse test.c
> > > test.c:9:13: warning: dereference of noderef expression
> > > 
> > > Notice that sparse didn't warn on the assignment of foo to bar (because
> > > typeof propagated the address space of 1), and warned on the dereference
> > > of bar (because typeof propagated noderef).
> > 
> > Thank you for the info!
> > 
> > Suppose that I want to do something like this:
> > 
> > #define __rcu_assign_pointer(p, v, space) \
> >         do { \
> >                 smp_wmb(); \
> >                 ACCESS_ONCE(p) = (typeof(*v) __force space *)(v); \
> >         } while (0)
> > 
> > Now, this does typeof(*p), so as you noted above sparse complains about
> > address-space mismatches.  Thus far, I haven't been able to come up with
> > something that (1) does sparse address-space checking, (2) does C type
> > checking, and (3) forces the assignment to be volatile.
> > 
> > Any thoughts on how to do this?
> 
> First of all, if p and v had compatible types *including* address
> spaces, you wouldn't need the "space" argument; the following
> self-contained test case passes both sparse and GCC typechecking:
> 
> #define as(n) __attribute__((address_space(n),noderef))
> #define __force __attribute__((force))
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> extern void smp_wmb(void);
> 
> #define rcu_assign_pointer(p, v) \
>     do { \
>         smp_wmb(); \
>         ACCESS_ONCE(p) = (v); \
>     } while (0)
> 
> struct foo;
> 
> int main(void)
> {
>     struct foo as(1) *dest;
>     struct foo as(1) *src = (void *)0;
> 
>     rcu_assign_pointer(dest, src);
> 
>     return 0;
> }
> 
> 
> 
> But in this case, you want dest and src to have compatible types except
> that dest must have the __rcu address space and src might not.  So,
> let's change the types of dest and src, and add the appropriate cast.
> The following also passes both GCC and sparse:
> 
> #define __rcu __attribute__((address_space(4),noderef))
> #define __force __attribute__((force))
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> extern void smp_wmb(void);
> 
> #define rcu_assign_pointer(p, v) \
>     do { \
>         smp_wmb(); \
>         ACCESS_ONCE(p) = (typeof(*(v)) __rcu __force *)(v); \
>     } while (0)
> 
> struct foo { int x; };
> 
> int main(void)
> {
>     struct foo __rcu *dest;
>     struct foo *src = (void *)0;
> 
>     rcu_assign_pointer(dest, src);
> 
>     return 0;
> }
> 
> 
> However, that cast forces the source to have the __rcu address space
> without checking what address space it started out with.  If you want to
> verify that the source has the kernel address space, you can cast to
> that address space first, *without* __force, which will warn if the
> source doesn't start out with that address space:
> 
> #define __kernel __attribute__((address_space(0)))
> #define __user __attribute__((address_space(1),noderef))
> #define __rcu __attribute__((address_space(4),noderef))
> #define __force __attribute__((force))
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> extern void smp_wmb(void);
> 
> #define rcu_assign_pointer(p, v) \
>     do { \
>         smp_wmb(); \
>         ACCESS_ONCE(p) = (typeof(*(v)) __rcu __force *)(typeof(*(v)) __kernel *)(v); \
>     } while (0)
> 
> struct foo { int x; };
> 
> int main(void)
> {
>     struct foo __rcu *dest;
>     struct foo *src = (void *)0;
>     struct foo __user *badsrc = (void *)0;
> 
>     rcu_assign_pointer(dest, src);
>     rcu_assign_pointer(dest, badsrc);
> 
>     return 0;
> }
> 
> 
> This produces a warning on the line using badsrc:
> 
> test.c:23:5: warning: cast removes address space of expression
> 
> However, that doesn't seem like the most obvious warning, since
> rcu_assign_pointer doesn't look like a cast, and since it doesn't print
> the full types involved like most address space warnings do.  So,
> instead, let's add and use a __chk_kernel_ptr function, similar to
> __chk_user_ptr in compiler.h:
> 
> #define __kernel __attribute__((address_space(0)))
> #define __user __attribute__((address_space(1),noderef))
> #define __rcu __attribute__((address_space(4),noderef))
> #define __force __attribute__((force))
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> extern void __chk_kernel_ptr(const volatile void *);
> extern void smp_wmb(void);
> 
> #define rcu_assign_pointer(p, v) \
>     do { \
>         smp_wmb(); \
>         __chk_kernel_ptr(v); \
>         ACCESS_ONCE(p) = (typeof(*(v)) __rcu __force *)(v); \
>     } while (0)
> 
> struct foo { int x; };
> 
> int main(void)
> {
>     struct foo __rcu *dest;
>     struct foo *src = (void *)0;
>     struct foo __user *badsrc = (void *)0;
> 
>     rcu_assign_pointer(dest, src);
>     rcu_assign_pointer(dest, badsrc);
> 
>     return 0;
> }
> 
> 
> This produces a somewhat better warning:
> 
> test.c:25:5: warning: incorrect type in argument 1 (different address spaces)
> test.c:25:5:    expected void const volatile *<noident>
> test.c:25:5:    got struct foo [noderef] <asn:1>*badsrc
> 
> That at least shows the full type of badsrc, but it still seems
> suboptimal for two reasons: it says it expects "void const volatile *"
> rather than the actual type it wants, and it says "in argument 1" (of
> __chk_kernel_ptr), which seems unnecessarily confusing when the type
> error actually applies to argument 2 of rcu_assign_pointer.  We can do
> better by declaring a fake local function for checking, instead:
> 
> #define __kernel __attribute__((address_space(0)))
> #define __user __attribute__((address_space(1),noderef))
> #define __rcu __attribute__((address_space(4),noderef))
> #define __force __attribute__((force))
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> extern void smp_wmb(void);
> 
> #define rcu_assign_pointer(p, v) \
>     do { \
>         smp_wmb(); \
>         extern void __rcu_assign_pointer_typecheck(int, typeof(*(v)) __kernel *); \
>         __rcu_assign_pointer_typecheck(0, v); \
>         ACCESS_ONCE(p) = (typeof(*(v)) __rcu __force *)(v); \
>     } while (0)
> 
> struct foo { int x; };
> 
> int main(void)
> {
>     struct foo __rcu *dest;
>     struct foo *src = (void *)0;
>     struct foo __user *badsrc = (void *)0;
> 
>     rcu_assign_pointer(dest, src);
>     rcu_assign_pointer(dest, badsrc);
> 
>     return 0;
> }
> 
> 
> This last approach produces a very clear warning:
> 
> test.c:25:5: warning: incorrect type in argument 2 (different address spaces)
> test.c:25:5:    expected struct foo *<noident>
> test.c:25:5:    got struct foo [noderef] <asn:1>*badsrc
> 
> If you want, you can even add an argument name for the second argument
> of __rcu_assign_pointer_typecheck, and it'll replace the <noident> in
> the second line of the warning.
> 
> So, that last approach meets all the criteria you mentioned:
> > something that (1) does sparse address-space checking, (2) does C type
> > checking, and (3) forces the assignment to be volatile.
> 
> Will that work for all the use cases you have in mind?  If so, I'll
> submit a patch changing rcu_assign_pointer to use that approach.

Looks like it does the right thing, thank you!

Would it also be possible for the call to __rcu_assign_pointer_typecheck()
to be only present when building under sparse?

							Thanx, Paul