Re: [PATCH] drm/i915: Use uninterruptible mutex_lock for userptr bo creation

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Hi,

On 05/15/2015 11:42 AM, Chris Wilson wrote:
Mika encountered one pathological scenario under X where acquiring all
the mm locks (required to insert a mmu notifier) was very slow, so slow
that by the time we tried to lock the struct_mutex with the usual call
to i915_mutex_lock_interruptible(), X's signal timer had fired causing
us to restart the ioctl (and so looped indefinitely).

Indefinite loop? Are you saying userptr creation endlessly fails to manages to finish in 10ms (or is it even 100ms, forgot what timer Xorg setups up)? The __mmu_notifier_register call?

While I suspect this is the result of another bug (something leaking mm
perhaps?) we can forgo the error checking and interuptible nature of the
lock here so we only have to pay the expense once and get on with it.
This does expose the userptr creation routine to a driver livelock
though by not being interruptible.

How is this acceptable then if it can live-lock? How does that happen?

Regards,

Tvrtko



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