Re: [PATCH] staging: lustre: lustre/ldlm: Fixed sparse warnings

[nayeem <itachi.opsrc@gmail.com>]

On Friday 16 September 2016 01:30 PM, Dilger, Andreas wrote:
> On Sep 15, 2016, at 12:33, nayeem <itachi.opsrc@gmail.com> wrote:
>> On Wednesday 14 September 2016 10:44 AM, Dilger, Andreas wrote:
>>> On Sep 12, 2016, at 04:27, Greg KH <gregkh@linuxfoundation.org> wrote:
>>>>
>>>> On Fri, Sep 09, 2016 at 08:50:35PM +0530, Nayeemahmed Badebade wrote:
>>>>> Added __acquires / __releases sparse locking annotations
>>>>> to lock_res_and_lock and unlock_res_and_lock functions in
>>>>> l_lock.c, to fix below sparse warnings:
>>>>>
>>>>> l_lock.c:47:22: warning: context imbalance in 'lock_res_and_lock' - wrong count at exit
>>>>> l_lock.c:62:6: warning: context imbalance in 'unlock_res_and_lock' - unexpected unlock
>>>>>
>>>>> Signed-off-by: Nayeemahmed Badebade <itachi.opsrc@gmail.com>
>>>>> ---
>>>>> drivers/staging/lustre/lustre/ldlm/l_lock.c | 4 ++++
>>>>> 1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/drivers/staging/lustre/lustre/ldlm/l_lock.c b/drivers/staging/lustre/lustre/ldlm/l_lock.c
>>>>> index ea8840c..c4b9612 100644
>>>>> --- a/drivers/staging/lustre/lustre/ldlm/l_lock.c
>>>>> +++ b/drivers/staging/lustre/lustre/ldlm/l_lock.c
>>>>> @@ -45,6 +45,8 @@
>>>>>   * being an atomic operation.
>>>>>   */
>>>>> struct ldlm_resource *lock_res_and_lock(struct ldlm_lock *lock)
>>>>> +				__acquires(&lock->l_lock)
>>>>> +				__acquires(lock->l_resource)
>>>>
>>>> Hm, these are tricky, I don't want to take this type of change without
>>>> an ack from the lustre developers...
>>>
>>> The "__acquires(&lock->l_lock)" line here looks correct, along with the
>>> corresponding "__releases(&lock->l_lock)" at unlock_res_and_lock().
>>>
>>> The problem, however, is that "l_resource" is not a lock, but rather a
>>> struct.  The call to "lock_res(lock->l_resource)" is actually locking
>>> "lr_lock" internally.
>>>
>>> It would be better to add "__acquires(&res->lr_lock)" at lock_res() and
>>> "__releases(&res->lr_lock)" at unlock_res().  That will also forestall
>>> any other warnings about an imbalance with lock_res()/unlock_res() or
>>> their callsites.
>>>
>>> Cheers, Andreas
>>>
>>
>> Hi Andreas,
>>
>> Thank you for your review comments. I did the change according to your comments and the diff is attached to mail. But this change doesn't seem to fix the sparse warning.
>> With this change when i compile the code "make C=2 ./drivers/staging/lustre/lustre/", sparse warning still comes:
>
>> {{{
>>   CHECK   drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.c
>> drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.c:47:22: warning: context imbalance in 'lock_res_and_lock' - wrong count at exit
>> drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.c:62:6: warning: context imbalance in 'unlock_res_and_lock' - unexpected unlock
>>   CC [M]  drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.o
>> }}}
>
> Strange, one would think that your patch should work properly.  Maybe the
> __acquires() label doesn't work on inline functions?
>

I think sparse works on inline functions.
I ran sparse on a hello world kernel module in different cases explained 
below


>> Would it be a good idea to add "__acquires(&lock->l_resource->lr_lock)" & "__acquires(&lock->l_lock)" at lock_res_and_lock() and "__releases(&lock->l_resource->lr_lock)" & "__releases(&lock->l_lock)" at unlock_res_and_lock() ?
>> Because with that change the sparse warning is fixed.
>> {{{
>>   CHECK   drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.c
>>   CC [M]  drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/l_lock.o
>> }}}
>
> This would also be possible, but then it exposes any callers of lock_res()
> and unlock() res to similar compiler warnings in the future.  I'm not
> against this in principle, but it is worthwhile to see why sparse is not
> handling this case correctly.
>
> Cheers, Andreas
>

case 1:
-------
hello.c, where spin_lock() and spin_unlock() are called indirectly via 
foo_lock() and foo_unlock() in the same function i.e "say_hello()" in 
below code.

The following code when checked with sparse doesn't give any warning

#include<linux/module.h>
#include<linux/init.h>

static DEFINE_SPINLOCK(my_lock);

static inline void foo_lock(spinlock_t *spl)
{
         spin_lock(spl);
}

static inline void foo_unlock(spinlock_t *spl)
{
         spin_unlock(spl);
}

static int __init say_hello(void)
{
         foo_lock(&my_lock);
         pr_info("Hello World!\n");
         foo_unlock(&my_lock);
         return 0;
}

static void __exit cleanup(void)
{
}

module_init(say_hello);
module_exit(cleanup);



case 2.
------
The above code when slightly modified so that, spin_lock() is called 
indirectly via foo_lock() in say_hello() and spin_unlock() via 
foo_unlock() in cleanup()

static int __init say_hello(void)
{
         foo_lock(&my_lock);
         pr_info("Hello World!\n");

         return 0;
}

static void __exit cleanup(void)
{
         foo_unlock(&my_lock);
}

Then sparse gives the warning:
{{{
test-module/hello.c:16:19: warning: context imbalance in 'say_hello' - 
wrong count at exit
test-module/hello.c:23:20: warning: context imbalance in 'cleanup' - 
unexpected unlock
}}}
To fix this if we put sparse annotations __acquires() at foo_lock() and 
__releases() at foo_unlock(), then also sparse warnings comes, which is 
exactly the case with l_lock.c in lustre code.

The warning will still be thrown if these functions are not inline.
I think this kind of case sparse is not able to handle, irrespective of 
whether function is inline or not.

case 3:
-------
Instead of putting sparse annotations at foo_lock and foo_unlock, if we 
put them at say_hello() and cleanup()

static int __init say_hello(void)
                 __acquires(&my_lock)
{
         foo_lock(&my_lock);
         pr_info("Hello World!\n");
         return 0;
}

static void __exit cleanup(void)
                 __releases(&my_lock)
{
         foo_unlock(&my_lock);
}

Then sparse seems to work properly and warning doesn't come.

So i think in case of l_lock.c in lustre, both "lock_res_and_lock()" and 
"unlock_res_and_lock" needs to have sparse annotations.

Please provide your inputs on this.

Thanks & Regards,
Nayeem