[Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

Christian Brauner christian.brauner at ubuntu.com
Thu Jan 30 11:45:26 UTC 2020


On January 30, 2020 12:33:41 PM GMT+01:00, Oleg Nesterov <oleg at redhat.com> wrote:
>On 01/30, Christian Brauner wrote:
>>
>> On Thu, Jan 30, 2020 at 11:50:28AM +0530,
>madhuparnabhowmik10 at gmail.com wrote:
>> > From: Madhuparna Bhowmik <madhuparnabhowmik10 at gmail.com>
>> >
>> > This patch fixes the following sparse error:
>> > kernel/exit.c:627:25: error: incompatible types in comparison
>expression
>> >
>> > And the following warning:
>> > kernel/exit.c:626:40: warning: incorrect type in assignment
>> >
>> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10 at gmail.com>
>>
>> I think the previous version was already fine but hopefully
>> RCU_INIT_POINTER() really saves some overhead. In any case:
>
>It is not about overhead, RCU_INIT_POINTER() documents the fact that we
>didn't make any changes to the new parent, we only need to change the
>pointer.

Right, I wasn't complaining.  RCU_INIT_POINTER() claims that it has less overhead than rcu_assign_pointer().
So that is an additional argument for it.

>
>And btw, I don't really understand the __rcu annotations. Say,
>according
>to sparse this code is wrong:
>
>	int __rcu *P;
>
>	void func(int *p)
>	{
>		P = p;
>	}
>
>OK, although quite possibly it is fine.
>
>However, this code
>
>	int __rcu *P;
>
>	void func(int __rcu *p)
>	{
>		*p = 10;
>	       	P = p;
>	}
>
>is almost certainly wrong but sparse is happy, asn is the same.

That's more an argument to fix sparse I guess?
The annotations themselves are rather useful I think.
They at least help me when reading the code.
It's not that rcu lifetimes are trivial and anything that helps remind me that an object wants rcu semantics I'm happy to take it. :)

>
>
>> Acked-by: Christian Brauner <christian.brauner at ubuntu.com>
>
>Acked-by: Oleg Nesterov <oleg at redhat.com>



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