On Thu, 16 Aug 2007, Paul Mackerras wrote:

quoted text
> Herbert Xu writes:
> 
> > See sk_stream_mem_schedule in net/core/stream.c:
> > 
> >         /* Under limit. */
> >         if (atomic_read(sk->sk_prot->memory_allocated) < sk->sk_prot->sysctl_mem[0]) {
> >                 if (*sk->sk_prot->memory_pressure)
> >                         *sk->sk_prot->memory_pressure = 0;
> >                 return 1;
> >         }
> > 
> >         /* Over hard limit. */
> >         if (atomic_read(sk->sk_prot->memory_allocated) > sk->sk_prot->sysctl_mem[2]) {
> >                 sk->sk_prot->enter_memory_pressure();
> >                 goto suppress_allocation;
> >         }
> > 
> > We don't need to reload sk->sk_prot->memory_allocated here.
> 
> Are you sure?  How do you know some other CPU hasn't changed the value
> in between?

I can't speak for this particular case, but there could be similar code
examples elsewhere, where we do the atomic ops on an atomic_t object
inside a higher-level locking scheme that would take care of the kind of
problem you're referring to here. It would be useful for such or similar
code if the compiler kept the value of that atomic object in a register.
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