Re: git: 02ea6033020e - main - LinuxKPI: Allow spin_lock_irqsave to be called within a critical section

On 1/19/22 23:02, Vladimir Kondratyev wrote:
> On 19.01.2022 12:50, Hans Petter Selasky wrote:
>> On 1/18/22 22:35, Vladimir Kondratyev wrote:
>>>
>>> Any ideas how to avoid it in a generic way?
>>
>> Hi,
>>
>> On a non-SMP system this will lead to deadlock.
>>
>> Is it possible you can pre-lock this spin lock earlier, so that it is 
>> already locked, so instead of
>>
>> while(trylock());
>>
>> You have:
>>
>> assert (trylock() XXX)
>>
> 
> Unfortunately, vulnerable functions are called in too many code paths to 
> patch them all.
> 
>> Or else,
>>
>> convert this particular lock to a native FreeBSD spinlock mutex.
>>
> It can be done for wake_up() but not for dma_fence_signal() which 
> suffers from this problem too. Some code that uses that lock expect it 
> to be spinlock_t
> 
> I think we can just drop critical section in seqlock-related part of 
> dma-buf code and replace it with rwlock as kib@ and mjg@ suggested. 
> Leave seqlock for actual locking to preserve semantics as much as 
> possible and add rwlock to implement reader's blocking. Following 
> snippets show code conversion required for this change:
> 
> 
> Lock seqlock as reader:
> 
> retry:
>      seq = read_seqcount_begin(&obj->seq);
> 
> ... reader payload ...
> 
>      if (read_seqcount_retry(&obj->seq, seq)) {
> #ifdef __FreeBSD__
>          /* Wait for rwlock to be released by writer */
>          rw_rlock(&obj->rwlock);
>          rw_runlock(&obj->rwlock);
> #endif
>          goto retry;
>      }
> 
> 
> Lock seqlock as writer:
> 
> #ifdef __linux__
>      preempt_disable();
> #elif defined (__FreeBSD__)
>      rw_wlock(&obj->rwlock);
> #endif
>      write_seqcount_begin(&obj->seq);
> 
> ... writer payload ...
> 
>      write_seqcount_end(&obj->seq);
> #ifdef __linux__
>      preempt_enable();
> #elif defined (__FreeBSD__)
>      rw_wunlock(&obj->rwlock);
> #endif
> 

Hi Vladimir,

Waiting for a differential revision.

--HPS