Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2013-05-28 at 18:31 -0700, Paul E. McKenney wrote:
> On Tue, May 28, 2013 at 05:34:53PM -0700, Eric Dumazet wrote:
> > On Tue, 2013-05-28 at 13:10 +0400, Roman Gushchin wrote:
> > > On 28.05.2013 04:12, Eric Dumazet wrote:
> > 
> > > > Adding a barrier() is probably what we want.
> > > 
> > > I agree, inserting barrier() is also a correct and working fix.
> > 
> > Yeah, but I can not find a clean way to put it inside the "for (;;)"
> > 
> > for (barrier();;)  ->
> > 
> > error: expected expression before ‘__asm__’
> > 
> > No user currently does :
> > 
> > if (condition)
> > 	hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)
> > 
> > But who knows...
> 
> I still have my earlier question, but I suggest "({ barrier(); XXX })"
> to put the barrier into the for loop, either in the second or third
> clause, where XXX was the original second or third clause.
> 
> 	

Hmm, it doesn't work, I wanted to replace :

barrier();
for (expr1 ; expr2; expr3) {

by :

for ( some_clever_thing ; expr2; expr3) {

So barrier() should not be in second or third clause : it must be done
once and before "expr1".

Not a big deal anyway, we're not adding new
hlist_nulls_for_each_entry_rcu() users :)


About your earlier question, I really don't know why compiler would
cache a memory read if we explicitly use barrier() to prevent this from
happening.

BTW Roman patch generates a double load as in :

2cb1:       49 8b 07                mov    (%r15),%rax    
2cb4:       49 8b 07                mov    (%r15),%rax        


...
2ea2:       e8 f9 dc ff ff          callq  ba0 <sock_put>
2ea7:       8b 0c 24                mov    (%rsp),%ecx
2eaa:       e9 02 fe ff ff          jmpq   2cb1 <udp4_lib_lookup2+0x91> 

because of ACCESS_ONCE() used twice, once explicitly in
hlist_nulls_for_each_entry_rcu(), and once in rcu_dereference_raw()

While barrier();ptr = rcu_dereference(X); does generate a single load.