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SubjectRe: [PATCH v2] ftrace: simplify ftrace hash lookup code
On Mon,  9 Sep 2019 08:31:59 +0800
Changbin Du <changbin.du@gmail.com> wrote:

> Function ftrace_lookup_ip() will check empty hash table. So we don't
> need extra check outside.
>
> Signed-off-by: Changbin Du <changbin.du@gmail.com>
>
> ---
> v2: fix incorrect code remove.
> ---
> kernel/trace/ftrace.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index f9821a3374e9..92aab854d3b1 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1463,8 +1463,7 @@ static bool hash_contains_ip(unsigned long ip,
> */
> return (ftrace_hash_empty(hash->filter_hash) ||
> __ftrace_lookup_ip(hash->filter_hash, ip)) &&
> - (ftrace_hash_empty(hash->notrace_hash) ||
> - !__ftrace_lookup_ip(hash->notrace_hash, ip));
> + !ftrace_lookup_ip(hash->notrace_hash, ip);

I don't care for this part. I've nacked this change in the past. Why?
let's compare the changes:

return (ftrace_hash_empty(hash->filter_hash) ||
__ftrace_lookup_ip(hash->filter_hash, ip)) &&
(ftrace_hash_empty(hash->notrace_hash) ||
!__ftrace_lookup_ip(hash->notrace_hash, ip));

vs:

return (ftrace_hash_empty(hash->filter_hash) ||
__ftrace_lookup_ip(hash->filter_hash, ip)) &&
!ftrace_lookup_ip(hash->notrace_hash, ip);

The issue I have with this is that it abstracts out the difference
between the filter_hash and the notrace_hash. Sometimes open coded
works better if it is compared to something that is similar.

The current code I see:

Return true if (filter_hash is empty or ip exists in filter_hash
and notrace_hash is empty or it does not exist in notrace_hash

With your update I see:

Return true if filter_hash is empty or ip exists in filter_hash
and ip does not exist in notrace_hash

It makes it not easy to see if what happens if notrace_hash is empty

Hmm, come to think of it, perhaps we should change ftrace_lookup_ip()
to include what to do on empty.

Maybe:

bool ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
{
if (ftrace_hash_empty(hash))
return empty_result;

return __ftrace_lookup_ip(hash, ip);
}

Then we can change the above to:

return ftrace_lookup_ip(hash->filter_hash, ip, true) &&
!ftrace_lookup_ip(hash->notrace_hash, ip, false);

That would probably work better.

Want to send that update?

-- Steve


> }
>
> /*
> @@ -6036,11 +6035,7 @@ clear_func_from_hash(struct ftrace_init_func
> *func, struct ftrace_hash *hash) {
> struct ftrace_func_entry *entry;
>
> - if (ftrace_hash_empty(hash))
> - return;
> -
> - entry = __ftrace_lookup_ip(hash, func->ip);
> -
> + entry = ftrace_lookup_ip(hash, func->ip);
> /*
> * Do not allow this rec to match again.
> * Yeah, it may waste some memory, but will be removed

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