On Friday 07 August 2015 05:27 PM, Oleg Nesterov wrote:
> On 08/07, Vineet Gupta wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1690,15 +1690,10 @@ EXPORT_SYMBOL(set_binfmt);
>> */
>> void set_dumpable(struct mm_struct *mm, int value)
>> {
>> - unsigned long old, new;
>> -
>> if (WARN_ON((unsigned)value > SUID_DUMP_ROOT))
>> return;
>>
>> - do {
>> - old = ACCESS_ONCE(mm->flags);
>> - new = (old & ~MMF_DUMPABLE_MASK) | value;
>> - } while (cmpxchg(&mm->flags, old, new) != old);
>> + set_mask_bits(&mm->flags, MMF_DUMPABLE_MASK, value);
>> }
>
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
I have a fundamental question though, perhaps stupid, do use cases like these
warrant the data to be atomic_t in first place. Do API like set_mask_bits() make
sense at all - or shd they be moved to atomic_* (after changing the underlying data)
See, I have such a cmpxchg loop in ARC code - originally from Peter :-)
arch/arc/kernel/smp.c. @ipi_data_ptr is NOT atomic_t
do {
new = old = ACCESS_ONCE(*ipi_data_ptr);
new |= 1U << msg;
} while (cmpxchg(ipi_data_ptr, old, new) != old);
Given that ARC (and some other RISC cores) lack native cmpxchg, we use LLSC
instructions to implement atomics including cpmxchg - the implementation itself
ensures loop is builtin making the outer loping superfluous and waste of cycles
(see e.g. cover letter @ http://www.spinics.net/lists/kernel/msg2029217.html)
So I wanted to convert that loop (and similar other cases to "some" API which
could be built conditionally based on cmpxchg or llsc. None such exist and I was
thinking of converting my case to atomic_t. Is that the right approach ?
Thx,
-Vineet
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