buggy/weird behavior in ttm

Thomas Hellstrom thellstrom at vmware.com
Thu Oct 11 22:57:49 PDT 2012

On 10/11/2012 10:55 PM, Maarten Lankhorst wrote:
> Op 11-10-12 21:26, Thomas Hellstrom schreef:
>> On 10/11/2012 08:42 PM, Maarten Lankhorst wrote:
>>>> Anyway, if you plan to remove the fence lock and protect it with reserve, you must
>>>> make sure that a waiting reserve is never done in a destruction path. I think this
>>>> mostly concerns the nvidia driver.
>>> Well I don't think any lock should ever be held during destruction time,
>> What I mean is, that *something* needs to protect the fence pointer. Currently it's the
>> fence lock, and I was assuming you'd protect it with reserve. And neither TTM nor
>> Nvidia should, when a resource is about to be freed, be forced to *block* waiting for
>> reserve just to access the fence pointer. When and if you have a solution that fulfills
>> those requirements, I'm ready to review it.
> It's not blocking, cleanup_refs_or_queue will toss it on the deferred list if reservation fails,
> behavior doesn't change just because I changed the order around.

Well, I haven't looked into the code in detail yet. If you say it's 
non-blocking I believe you.
I was actually more concerned abut the Nvidia case where IIRC the wait 
was called both
with and without reservation.

>>>>> - no_wait_reserve is ignored if no_wait_gpu is false
>>>>>      ttm_bo_reserve_locked can only return true if no_wait_reserve is true, but
>>>>>      subsequently it will do a wait_unreserved if no_wait_gpu is false.
>>>>> I'm planning on removing this argument and act like it is always true, since
>>>>> nothing on the lru list should fail to reserve currently.
>>>> Yes, since all buffers that are reserved are removed from the LRU list, there
>>>> should never be a waiting reserve on them, so no_wait_reserve can be removed
>>>> from ttm_mem_evict_first, ttm_bo_evict and possibly other functions in the call chain.
>>> I suppose there will stay a small race though,
>> Hmm, where?
> When you enter the ddestroy path, you drop the lock and hope the buffer doesn't reserved
> away from under you.

Yes, that code isn't fully correct, it's missing a check for still on 
ddestroy after a waiting
reserve. However, the only chance of a waiting reserve given that the 
buffer *IS* on the
ddestroy list is if the current reserver returned early because someone 
started an
accelerated eviction which can't happen currently. The code needs fixing 
up though.

>>>>> - effectively unlimited callchain between some functions that all go through
>>>>>      ttm_mem_evict_first:
>>>>>                                        /------------------------\
>>>>> ttm_mem_evict_first - ttm_bo_evict -                          -ttm_bo_mem_space  - ttm_bo_mem_force_space - ttm_mem_evict_first
>>>>>                                        \ ttm_bo_handle_move_mem /
>>>>> I'm not surprised that there was a deadlock before, it seems to me it would
>>>>> be pretty suicidal to ever do a blocking reserve on any of those lists,
>>>>> lockdep would be all over you for this.
>>>> Well, at first this may look worse than it actually is. The driver's eviction memory order determines the recursion depth
>>>> and typically it's 0 or 1, since subsequent ttm_mem_evict_first should never touch the same LRU lists as the first one.
>>>> What would typically happen is that a BO is evicted from VRAM to TT, and if there is no space in TT, another BO is evicted
>>>> to system memory, and the chain is terminated. However a driver could set up any eviction order but that would be
>>>> a BUG.
>>>> But in essence, as you say, even with a small recursion depth, a waiting reserve could cause a deadlock.
>>>> But there should be no waiting reserves in the eviction path currently.
>>> Partially true, ttm_bo_cleanup_refs is currently capable of blocking reserve.
>>> Fixing this might mean that ttm_mem_evict_first may need to become more aggressive,
>>> since it seems all the callers of this function assume that ttm_mem_evict_first can only fail
>>> if there is really nothing more to free and blocking nested would really upset lockdep
>>> and leave you open to the same deadlocks.
>> I can't see how the waiting reserve in ttm_bo_cleanup_refs would cause a deadlock,
>> because the buffer about to be reserved is always *last* in a reservation sequence, and the
>> reservation is always released (or the buffer destroyed) before trying to reserve another buffer.
>> Technically the buffer is not looked up from a LRU list but from the delayed delete list.
>> Could you describe such a deadlock case?
> The only interesting case for this is ttm_mem_evict_first, and while it may not technically
> be a deadlock, lockdep will flag you for blocking on this anyway, since the only reason it
> would not be a deadlock is if you know the exact semantics of why.

Interesting. I guess that must be because of the previous reservation 
history for that buffer?
Let's say we were to reinitialize the lockdep history for the 
reservation object when it was put
on the ddestroy list, I assume lockdep would keep quiet, because there 
are never any other
bo reservations while such a buffer is reserved?


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