Arjan van de Ven wrote:
> Pavel Emelianov wrote:
>> This causes constructions like
>>
>> down_write(&mm1->mmap_sem);
>> if (down_write_trylock(&mm2->mmap_sem)) {
>> ...
>> up_write(&mm2->mmap_sem);
>> }
>> up_write(&mm1->mmap_sem);
>>
>> generate a lockdep warning about circular locking dependence.
>
> please show me why this is safe, especially if you intermix it with
> down_read()'s... like copy_to_user and co may do.
This is safe as once the task locks the mm1->mmap_sem and gets
into '...' place it is *running* and will release booth semaphores
for sure.
> this feels like a very unsafe construct to me...
>
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