linux-kernel - Re: [PATCH] lib/extable.c: use bsearch() library function in search

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Message-Id: <7E09691B-2059-4551-98CE-143C834928CA@m3y3r.de>
Date:   Fri, 2 Jun 2017 16:41:24 +0200
From:   Thomas Meyer <thomas@...3r.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/extable.c: use bsearch() library function in search_extable()



With kind regards
Thomas

> Am 02.06.2017 um 01:40 schrieb Rasmus Villemoes <linux@...musvillemoes.dk>:
> 
>> On Wed, May 31 2017, Thomas Meyer <thomas@...3r.de> wrote:
>> 
>> Signed-off-by: Thomas Meyer <thomas@...3r.de>
>> ---
>>  lib/extable.c | 30 +++++++++++++-----------------
>>  1 file changed, 13 insertions(+), 17 deletions(-)
>> 
>> diff --git a/lib/extable.c b/lib/extable.c
>> index 62968da..eb16cb3 100644
>> --- a/lib/extable.c
>> +++ b/lib/extable.c
>> @@ -9,6 +9,7 @@
>>   * 2 of the License, or (at your option) any later version.
>>   */
>>  
>> +#include <linux/bsearch.h>
>>  #include <linux/module.h>
>>  #include <linux/init.h>
>>  #include <linux/sort.h>
>> @@ -51,7 +52,7 @@ static void swap_ex(void *a, void *b, int size)
>>   * This is used both for the kernel exception table and for
>>   * the exception tables of modules that get loaded.
>>   */
>> -static int cmp_ex(const void *a, const void *b)
>> +static int cmp_ex_sort(const void *a, const void *b)
>>  {
>>      const struct exception_table_entry *x = a, *y = b;
>>  
>> @@ -67,7 +68,7 @@ void sort_extable(struct exception_table_entry *start,
>>            struct exception_table_entry *finish)
>>  {
>>      sort(start, finish - start, sizeof(struct exception_table_entry),
>> -         cmp_ex, swap_ex);
>> +         cmp_ex_sort, swap_ex);
>>  }
>>  
>>  #ifdef CONFIG_MODULES
>> @@ -93,6 +94,13 @@ void trim_init_extable(struct module *m)
>>  #endif /* !ARCH_HAS_SORT_EXTABLE */
>>  
>>  #ifndef ARCH_HAS_SEARCH_EXTABLE
>> +
>> +static int cmp_ex_search(const void *key, const void *elt)
>> +{
>> +    const struct exception_table_entry * _elt = elt;
>> +    return *(unsigned long*) key - ex_to_insn(_elt);
>> +}
>> +
> 
> This triggers a pet peeve of mine: Don't return the result of a
> subtraction as a comparison result; you may have unsigned longs x, y for
> which x < y but (int)(x-y) > 0; e.g. 0x20000000 and 0xb0000000. On a 32
> bit machine, all the addresses you're comparing are likely to be within
> one half of the address space, so the differences do fit in a signed 32
> bit int, but when BITS_PER_LONG=64, this will cause problems.
> 
> So do it carefully, just as it's done in cmp_ex.
> 
> You should be able to get away with just passing value by, well, value,
> so that this just needs to be (unsigned long)key and (void*)value in the
> bsearch call, though I don't know how much that saves.

Good hint, will change it.
> 
> 
>>  /*
>>   * Search one exception table for an entry corresponding to the
>>   * given instruction address, and return the address of the entry,
>> @@ -105,21 +113,9 @@ search_extable(const struct exception_table_entry *first,
>>             const struct exception_table_entry *last,
>>             unsigned long value)
>>  {
>> -    while (first <= last) {
>> -        const struct exception_table_entry *mid;
>> +    if(last < first)
>> +        return NULL;
>>  
>> -        mid = ((last - first) >> 1) + first;
>> -        /*
>> -         * careful, the distance between value and insn
>> -         * can be larger than MAX_LONG:
>> -         */
>> -        if (ex_to_insn(mid) < value)
>> -            first = mid + 1;
>> -        else if (ex_to_insn(mid) > value)
>> -            last = mid - 1;
>> -        else
>> -            return mid;
>> -    }
>> -    return NULL;
>> +    return bsearch(&value, first, last - first + 1, sizeof(struct exception_table_entry), cmp_ex_search);
> 
> It does seem to make sense to use the library routine, but maybe the
> binary search is open-coded for performance, to avoid the cost of the
> callbacks?

As far as I understand this search shouldn't be performance critical as this code path is the seldom error case.

And if it is really performance critical we should add an comment to make it clear why the bsearch library function is not used.
> 
> If we do this, could you also get rid of the silly plus/minus 1 thing in
> a followup patch (there aren't that many search_extable callers).

Okay will change and send a v2 the next days.

> 
> Rasmus

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