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Date: Thu, 10 Nov 2022 14:49:53 +0000 From: Matthew Auld <matthew.auld@...el.com> To: Mani Milani <mani@...omium.org>, LKML <linux-kernel@...r.kernel.org> Cc: Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Hellström <thomas.hellstrom@...ux.intel.com>, Chris Wilson <chris@...is-wilson.co.uk>, Christian König <christian.koenig@....com>, Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, Jani Nikula <jani.nikula@...ux.intel.com>, Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, Niranjana Vishwanathapura <niranjana.vishwanathapura@...el.com>, Nirmoy Das <nirmoy.das@...el.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org Subject: Re: [PATCH] drm/i915: Fix unhandled deadlock in grab_vma() On 10/11/2022 05:31, Mani Milani wrote: > At present, the gpu thread crashes at times when grab_vma() attempts to > acquire a gem object lock when in a deadlock state. > > Problems: > I identified the following 4 issues in the current code: > 1. Since grab_vma() calls i915_gem_object_trylock(), which consequently > calls ww_mutex_trylock(), to acquire lock, it does not perform any > -EDEADLK handling; And -EALREADY handling is also unreliable, > according to the description of ww_mutex_trylock(). > 2. Since the return value of grab_vma() is a boolean showing > success/failure, it does not provide any extra information on the > failure reason, and therefore does not provide any mechanism to its > caller to take any action to fix a potential deadlock. > 3. Current grab_vma() implementation produces inconsistent behaviour > depending on the refcount value, without informing the caller. If > refcount is already zero, grab_vma() neither acquires lock nor > increments the refcount, but still returns 'true' for success! This > means that grab_vma() returning true (for success) does not always > mean that the gem obj is actually safely accessible. > 4. Currently, calling "i915_gem_object_lock(obj,ww)" is meant to be > followed by a consequent "i915_gem_object_unlock(obj)" ONLY if the > original 'ww' object pointer was NULL, or otherwise not be called and > leave the houskeeping to "i915_gem_ww_ctx_fini(ww)". There are a few > issues with this: > - This is not documented anywhere in the code (that I could find), > but only explained in an older commit message. > - This produces an inconsistent usage of the lock/unlock functions, > increasing the chance of mistakes and issues. > - This is not a clean design as it requires any new code that calls > these lock/unlock functions to know their internals, as well as the > internals of the functions calling the new code being added. > > Fix: > To fix the issues above, this patch: > 1. Changes grab_vma() to call i915_gem_object_lock() instead of > i915_gem_object_trylock(), to handle -EDEADLK and -EALREADY cases. > This should not cause any issue since the PIN_NONBLOCK flag is > checked beforehand in the 2 cases grab_vma() is called. > 2. Changes grab_vma() to return the actual error code, instead of bool. > 3. Changes grab_vma() to behave consistently when returning success, by > both incrementing the refcount and acquiring lock at all times. > 4. Changes i915_gem_object_unlock() to pair with i915_gem_object_lock() > nicely in all cases and do the housekeeping without the need for the > caller to do anything other than simply calling lock and unlock. > 5. Ensures the gem obj->obj_link is initialized and deleted from the ww > list such that it can be tested for emptiness using list_empty(). > > Signed-off-by: Mani Milani <mani@...omium.org> > --- > > drivers/gpu/drm/i915/gem/i915_gem_object.c | 2 + > drivers/gpu/drm/i915/gem/i915_gem_object.h | 10 ++++- > drivers/gpu/drm/i915/i915_gem_evict.c | 48 ++++++++++++---------- > drivers/gpu/drm/i915/i915_gem_ww.c | 8 ++-- > 4 files changed, 41 insertions(+), 27 deletions(-) > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c > index 369006c5317f..69d013b393fb 100644 > --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c > @@ -78,6 +78,8 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj, > > INIT_LIST_HEAD(&obj->mm.link); > > + INIT_LIST_HEAD(&obj->obj_link); > + > INIT_LIST_HEAD(&obj->lut_list); > spin_lock_init(&obj->lut_lock); > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h > index 1723af9b0f6a..7e7a61bdf52c 100644 > --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h > @@ -219,7 +219,7 @@ static inline bool i915_gem_object_trylock(struct drm_i915_gem_object *obj, > return ww_mutex_trylock(&obj->base.resv->lock, &ww->ctx); > } > > -static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj) > +static inline void __i915_gem_object_unlock(struct drm_i915_gem_object *obj) > { > if (obj->ops->adjust_lru) > obj->ops->adjust_lru(obj); > @@ -227,6 +227,14 @@ static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj) > dma_resv_unlock(obj->base.resv); > } > > +static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj) > +{ > + if (list_empty(&obj->obj_link)) > + __i915_gem_object_unlock(obj); > + else > + i915_gem_ww_unlock_single(obj); > +} > + > static inline void > i915_gem_object_set_readonly(struct drm_i915_gem_object *obj) > { > diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c > index f025ee4fa526..3eb514b4eddc 100644 > --- a/drivers/gpu/drm/i915/i915_gem_evict.c > +++ b/drivers/gpu/drm/i915/i915_gem_evict.c > @@ -55,29 +55,33 @@ static int ggtt_flush(struct intel_gt *gt) > return intel_gt_wait_for_idle(gt, MAX_SCHEDULE_TIMEOUT); > } > > -static bool grab_vma(struct i915_vma *vma, struct i915_gem_ww_ctx *ww) > +static int grab_vma(struct i915_vma *vma, struct i915_gem_ww_ctx *ww) > { > + int err; > + > + /* Dead objects don't need pins */ > + if (dying_vma(vma)) > + atomic_and(~I915_VMA_PIN_MASK, &vma->flags); > + > + err = i915_gem_object_lock(vma->obj, ww); AFAIK the issue here is that we are already holding the vm->mutex, so this can potentially deadlock, which I guess is why this was trylock. We typically grab a bunch of object locks during execbuf, and then grab the vm->mutex, before binding the vma for each object. So vm->mutex is always our inner lock, and the object lock is the outer one. Using a full lock here then inverts that locking AFAICT. Like say if one process is holding object A + vm->mutex and then tries to grab object B here in grab_vma(), but another process is already holding object B + waiting to grab vm->mutex? > + > /* > * We add the extra refcount so the object doesn't drop to zero until > - * after ungrab_vma(), this way trylock is always paired with unlock. > + * after ungrab_vma(), this way lock is always paired with unlock. > */ > - if (i915_gem_object_get_rcu(vma->obj)) { > - if (!i915_gem_object_trylock(vma->obj, ww)) { > - i915_gem_object_put(vma->obj); > - return false; > - } > - } else { > - /* Dead objects don't need pins */ > - atomic_and(~I915_VMA_PIN_MASK, &vma->flags); > - } > + if (!err) > + i915_gem_object_get(vma->obj); > > - return true; > + return err; > } > > static void ungrab_vma(struct i915_vma *vma) > { > - if (dying_vma(vma)) > + if (dying_vma(vma)) { > + /* Dead objects don't need pins */ > + atomic_and(~I915_VMA_PIN_MASK, &vma->flags); > return; > + } > > i915_gem_object_unlock(vma->obj); > i915_gem_object_put(vma->obj); > @@ -93,10 +97,11 @@ mark_free(struct drm_mm_scan *scan, > if (i915_vma_is_pinned(vma)) > return false; > > - if (!grab_vma(vma, ww)) > + if (grab_vma(vma, ww)) > return false; > > list_add(&vma->evict_link, unwind); > + > return drm_mm_scan_add_block(scan, &vma->node); > } > > @@ -284,10 +289,12 @@ i915_gem_evict_something(struct i915_address_space *vm, > vma = container_of(node, struct i915_vma, node); > > /* If we find any non-objects (!vma), we cannot evict them */ > - if (vma->node.color != I915_COLOR_UNEVICTABLE && > - grab_vma(vma, ww)) { > - ret = __i915_vma_unbind(vma); > - ungrab_vma(vma); > + if (vma->node.color != I915_COLOR_UNEVICTABLE) { > + ret = grab_vma(vma, ww); > + if (!ret) { > + ret = __i915_vma_unbind(vma); > + ungrab_vma(vma); > + } > } else { > ret = -ENOSPC; > } > @@ -382,10 +389,9 @@ int i915_gem_evict_for_node(struct i915_address_space *vm, > break; > } > > - if (!grab_vma(vma, ww)) { > - ret = -ENOSPC; > + ret = grab_vma(vma, ww); > + if (ret) > break; > - } > > /* > * Never show fear in the face of dragons! > diff --git a/drivers/gpu/drm/i915/i915_gem_ww.c b/drivers/gpu/drm/i915/i915_gem_ww.c > index 3f6ff139478e..937b279f50fc 100644 > --- a/drivers/gpu/drm/i915/i915_gem_ww.c > +++ b/drivers/gpu/drm/i915/i915_gem_ww.c > @@ -19,16 +19,14 @@ static void i915_gem_ww_ctx_unlock_all(struct i915_gem_ww_ctx *ww) > struct drm_i915_gem_object *obj; > > while ((obj = list_first_entry_or_null(&ww->obj_list, struct drm_i915_gem_object, obj_link))) { > - list_del(&obj->obj_link); > - i915_gem_object_unlock(obj); > - i915_gem_object_put(obj); > + i915_gem_ww_unlock_single(obj); > } > } > > void i915_gem_ww_unlock_single(struct drm_i915_gem_object *obj) > { > - list_del(&obj->obj_link); > - i915_gem_object_unlock(obj); > + list_del_init(&obj->obj_link); > + __i915_gem_object_unlock(obj); > i915_gem_object_put(obj); > } >
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