Re: [PATCH 3/6] drm/ttm: call ttm_bo_cleanup_refs with reservation and lru lock held

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Op 28-11-12 12:54, Thomas Hellstrom schreef:
> On 11/28/2012 12:25 PM, Maarten Lankhorst wrote:
>> By removing the unlocking of lru and retaking it immediately, a race is
>> removed where the bo is taken off the swap list or the lru list between
>> the unlock and relock. As such the cleanup_refs code can be simplified,
>> it will attempt to call ttm_bo_wait non-blockingly, and if it fails
>> it will drop the locks and perform a blocking wait, or return an error
>> if no_wait_gpu was set.
>>
>> The need for looping is also eliminated, since swapout and evict_mem_first
>> will always follow the destruction path, so no new fence is allowed
>> to be attached. As far as I can see this may already have been the case,
>> but the unlocking / relocking required a complicated loop to deal with
>> re-reservation.
>>
>> The downside is that ttm_bo_cleanup_memtype_use is no longer called with
>> reservation held, so drivers must be aware that move_notify with a null
>> parameter doesn't require a reservation.
>
> Why can't we unreserve *after* ttm_bo_cleanup_memtype_use? That's not
> immediately clear from this patch.
Because we would hold the reservation while waiting and with the object still
on swap and lru lists still, that would defeat the whole purpose of keeping
the object on multiple lists, plus break current code that assumes bo's on the
those lists can always be reserved.

the if (ret && !no_wait_gpu) path has to drop the reservation and lru lock, and
isn't guaranteed to be able to retake it. Maybe it could be guaranteed now, but
I'm sure that would not be the case if the reservations were shared across
devices.

The alternatives are worse: doing a blocking reserve would rightfully anger lockdep,
and how do you want to handle tryreserve failing?

Since there is a path in which no reservation can be taken, to weed out bugs I felt it
would be better that any bugs that trigger from not holding a reservation would be
easier to find if it happens all the time, instead of only in some corner cases.

~Maarten