linux-kernel - Re: [PATCH] coredump: Replace opencoded set_mask

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Date:	Fri, 07 Aug 2015 20:14:03 +0530
From:	Vineet Gupta <vgupta@...opsys.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coredump: Replace opencoded set_mask_bits()

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@...hat.com>

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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