locking spinlocks during copy_to_user, copy_from_user

[anish kumar]

On Fri, 2013-01-25 at 10:00 -0300, Pablo Pessolani wrote:
> Hi:
>    It is well know that is not recomended to keep a spinlock locked
> during operations that can be preempted. Some of these operations are
> copy_to_user, copy_from_user.
> Below is the code of the write_lock() as a sample.
>  
> 340 static inline void __write_lock(rwlock_t *lock)
>  341{
>  342        preempt_disable(); <<<<<< THE ISSUE IS HERE <<<<<<<<<<<<<<
>  343        rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_);
>  344        LOCK_CONTENDED(lock, _raw_write_trylock, _raw_write_lock);
>  345}
> 
> On write_unlock() the preemtion is enable.
>  
> My question is: Is there any know consequence if I enable preemption
> before copy_to_user/copy_from user (keeping the spinlock locked) and
why do you want to do this?
>  then disable preemption again after the copy?????
> i.e.:
> write_lock(&lock);
> .......
>  preempt_enable(); 
>  copy_to_user(.......);
>   preempt_disable(); 
>  .......
> write_unlock(&lock);
> 
> Thanks in advance.
> PAP
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