Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning

[Abhinav Singh <singhabhinav9051571833@gmail.com>]

On 10/28/23 17:50, Michael S. Tsirkin wrote:
> On Sat, Oct 28, 2023 at 03:52:47PM +0530, Abhinav Singh wrote:
>> This patch fixes the warning about directly dereferencing a pointer
>> tagged with __rcu annotation.
>>
>> Dereferencing the pointers tagged with __rcu directly should
>> always be avoided according to the docs. There is a rcu helper
>> function rcu_dereference(...) to use when dereferencing a __rcu
>> pointer. This function returns the non __rcu tagged pointer which
>> can be dereferenced just like a normal pointer.
>>
>> Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
>> ---
>> v1 -> v2 : added rcu_dereference(...) at line 2694
>> v2 -> v3 : added rcu_dereference(...) at line 2695
>>
>>   kernel/fork.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 10917c3e1f03..e78649974669 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
>>   
>>   	retval = -EAGAIN;
>>   	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
>> -		if (p->real_cred->user != INIT_USER &&
>> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>>   		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>>   			goto bad_fork_cleanup_count;
>>   	}
>> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>>   			 * tasklist_lock with adding child to the process tree
>>   			 * for propagate_has_child_subreaper optimization.
>>   			 */
>> -			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
>> -							 p->real_parent->signal->is_child_subreaper;
>> -			list_add_tail(&p->sibling, &p->real_parent->children);
>> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
>> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
>> +			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
>>   			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>>   			attach_pid(p, PIDTYPE_TGID);
>>   			attach_pid(p, PIDTYPE_PGID);
>> -- 
>> 2.39.2
> 
> 
> You seem to just ignore review comments. NAK. I'm not going to review this anymore.
> 
> 
I m really sorry for ignorance and careless behaviour. This is 
completely my fault, a maintainer has to do a lot of work and he cant 
correct me all the times. On my defense I will only say that I was 
really confused about a thing, instead of asking you question, I thought 
of sending in another patch with some more information is a better 
choice then a clearing my confusion by sending in a extra mail. You were 
very concise and clear about your comments but MY stupidity was on 
another level today. I m sorry for ignorant behaviour. And also thanks 
and appreciate a lot for reviewing this patch till now to all the 
maintainers.

Not sure if this patch will be reviewed again or not, but I think I 
should answer the queries.

The last patch I sent, I tested with `lockdep` on (I hope "test with 
`lockdep on`" means booting the kernel with lockdep enabled),
with these config options `CONFIG_PROVE_RCU` and `CONFIG_PROVE_LOCKING` 
enabled and it booted just fine. To confirm if lockdep was really 
enabled I found these paths inside the qemu virtual envirnoment
/proc/lockdep
/proc/lockdep_chains
/proc/lockdep_stat
/proc/locks
/proc/lock_stats

I tested the above kernel using qemu with this command
qemu-system-x86_64 \
	-m 2G \
	-smp 2 \
	-kernel /home/abhinav/linux_work/linux/arch/x86/boot/bzImage \
	-append "console=ttyS0 root=/dev/sda earlyprintk=serial   net.ifnames=0" \
	-drive file=/home/abhinav/linux_work/boot_images/bullseye.img,format=raw \
	-net user,host=10.0.2.10,hostfwd=tcp:127.0.0.1:10021-:22 \
	-net nic,model=e1000 \
	-enable-kvm \
	-nographic \
	-pidfile vm.pid \
	2>&1 | tee vm.log

I did not get warning `the run-time "suspicious rcu_dereference_check() 
usage"` as mentioned by Oleg Nesterov, which mean rcu_dereference(...) 
it called inside of rcu read side critical sections.