On Tue, Aug 25, 2015 at 11:56:38AM +0200, Ingo Molnar wrote:
> +void get_vmalloc_info(struct vmalloc_info *vmi)
> +{
> + unsigned int cache_gen, gen;
I see you dropped the u64 thing, good, ignore that previous email.
> +
> + /*
> + * The common case is that the cache is valid, so first
> + * read it, then check its validity.
> + *
> + * The two read barriers make sure that we read
> + * 'cache_gen', 'vmap_info_cache' and 'gen' in
> + * precisely that order:
> + */
> + cache_gen = vmap_info_cache_gen;
> + smp_rmb();
> + *vmi = vmap_info_cache;
> + smp_rmb();
> + gen = vmap_info_gen;
> +
> + /*
> + * If the generation counter of the cache matches that of
> + * the vmalloc generation counter then return the cache:
> + */
> + if (cache_gen == gen)
> + return;
There is one aspect of READ_ONCE() that is not replaced with smp_rmb(),
and that is that READ_ONCE() should avoid split loads.
Without READ_ONCE() the compiler is free to turn the loads into separate
and out of order byte loads just because its insane, thereby also making
the WRITE_ONCE() pointless.
Now I'm fairly sure it all doesn't matter much, the info can change the
moment we've copied it, so the read is inherently racy.
And by that same argument I feel this is all somewhat over engineered.
> +
> + /* Make sure 'gen' is read before the vmalloc info: */
> + smp_rmb();
> + calc_vmalloc_info(vmi);
> +
> + /*
> + * All updates to vmap_info_cache_gen go through this spinlock,
> + * so when the cache got invalidated, we'll only mark it valid
> + * again if we first fully write the new vmap_info_cache.
> + *
> + * This ensures that partial results won't be used and that the
> + * vmalloc info belonging to the freshest update is used:
> + */
> + spin_lock(&vmap_info_lock);
> + if ((int)(gen-vmap_info_cache_gen) > 0) {
> + vmap_info_cache = *vmi;
> + /*
> + * Make sure the new cached data is visible before
> + * the generation counter update:
> + */
> + smp_wmb();
> + vmap_info_cache_gen = gen;
> + }
> + spin_unlock(&vmap_info_lock);
> +}
> +
> +#endif /* CONFIG_PROC_FS */
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