On 10/15/2012 05:37 PM, Maarten Lankhorst wrote:
Op 15-10-12 14:27, Thomas Hellstrom schreef:
On 10/12/2012 12:09 PM, Maarten Lankhorst wrote:
Op 12-10-12 07:57, Thomas Hellstrom schreef:
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?
Lockdep works on classes of lock, not necessarily individual locks.
Doing 2 bo_reserve's of any bo would count as possible deadlock, no matter if you
always take them in a certain order or not.
So you mean that if I bo_reserve A and then bo_reserve B (which is used only when binding A to the GPU), lockdep will complain even if nobody ever bo_reserves B before A? That will make it impossible to use BOs as page tables for GPU binding for example.
As far as I tell can nobody does it like that, page tables are simply initialized on channel
creation, pinned in memory and kept like that while the host serializes with their own vm locking.
I don't think the fact that nobody's using a feature yet is a valid
argument to say it will never be used.
With that argument a lot of code could go away. Including reservation
objects... ;)
Reserving multiple buffer objects in a pre-determined order is a
perfectly valid thing to do. For example an
execbuf implementation could ditch the ticketed reserve and instead do a
quick sort of the buffer objects
in pointer value order. And FWIW vmware have a couple of patches pending
for a future "hardware" revision
that implement GPU bind using buffer objects for page tables; the code
becomes neat and we can use
the delayed delete mechanism to avoid stalling at unbind time.
So from my pow, if lockdep fails to handle that situation, the lockdep
implementation is incomplete.
To make multi-object reservation work, the fix is to add a ticket "lock" of which all the
reservation objects are a nested lock of. Since in this case the ticket lock would prevent
deadlocks, this is acceptable. Having 2 ticket 'locks' at the same time would count as
deadlock, rightfully. If you hold a reservation from a ticket, then try to reserve without
a ticket, it counts as deadlock too. See below for some examples I was using to test.
But if a ticket lock can be used to abstract a locking sequence that we know will never deadlock,
why can't we abstract locking from a list with atomic list removal with another "lock", and make sure we get the order right between them?
No, see the test below, lockdep won't be fooled by your diversions that easily!! :-)
It's not a diversion, it's an attempt to abstract a valid locking scenario.
I think you got me wrong. In the analogy of mutex_lock_nested(), (see
http://www.mjmwired.net/kernel/Documentation/lockdep-design.txt
line 153
enum reservation_class {
RES_TICKET,
RES_LRU,
RES_DDESTROY
};
/Thomas
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