On 11/20, Alan Stern wrote:
>
> @@ -158,6 +199,11 @@ void synchronize_srcu(struct srcu_struct
>
> [... snip ...]
>
> +#ifdef SMP__STORE_MB_LOAD_WORKS /* The fast path */
> + if (srcu_readers_active_idx(sp, idx) == 0)
> + goto done;
> +#endif
I guess this is connected to another message from you,
> But of course it _is_ needed for the fastpath to work. In fact, it might
> not be good enough, depending on the architecture. Here's what the
> fastpath ends up looking like (using c[idx] is essentially the same as
> using hardluckref):
>
> WRITER READER
> ------ ------
> dataptr = &(new data) atomic_inc(&hardluckref)
> mb mb
> while (hardluckref > 0) ; access *dataptr
>
> Notice the pattern: Each CPU does store-mb-load. It is known that on
> some architectures each CPU can end up loading the old value (the value
> from before the other CPU's store). This would mean the writer would see
> hardluckref == 0 right away and the reader would see the old dataptr.
So, if we have global A == B == 0,
CPU_0 CPU_1
A = 1; B = 2;
mb(); mb();
b = B; a = A;
It could happen that a == b == 0, yes? Isn't this contradicts with definition
of mb?
By definition, when CPU_0 issues 'b = B', 'A = 1' should be visible to other
CPUs, yes? Now, b == 0 means that CPU_1 did not read 'a = A' yet, otherwise
'B = 2' should be visible to all CPUs (by definition again).
Could you please clarify this?
Btw, this is funny, but I was going to suggest _exactly_ same cleanup for
srcu_read_lock :)
Oleg.
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