Re: [PATCH 1/3] dma_resv: prime lockdep annotations

From: Chris Wilson <chris@chris-wilson.co.uk>
To: DRI Development <dri-devel@lists.freedesktop.org>
Cc: "Rob Herring" <robh@kernel.org>,
	"Thomas Hellstrom" <thellstrom@vmware.com>,
	"Tomeu Vizoso" <tomeu.vizoso@collabora.com>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	"VMware Graphics" <linux-graphics-maintainer@vmware.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"Daniel Vetter" <daniel.vetter@intel.com>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Dave Airlie" <airlied@redhat.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Ben Skeggs" <bskeggs@redhat.com>
Subject: Re: [PATCH 1/3] dma_resv: prime lockdep annotations
Date: Wed, 21 Aug 2019 23:20:51 +0100	[thread overview]
Message-ID: <156642605147.20466.13019420195083819968@skylake-alporthouse-com> (raw)
In-Reply-To: <20190821215030.31660-1-daniel.vetter@ffwll.ch>

Quoting Daniel Vetter (2019-08-21 22:50:28)
> Full audit of everyone:
> 
> - i915, radeon, amdgpu should be clean per their maintainers.
> 
> - vram helpers should be fine, they don't do command submission, so
>   really no business holding struct_mutex while doing copy_*_user. But
>   I haven't checked them all.
> 
> - panfrost seems to dma_resv_lock only in panfrost_job_push, which
>   looks clean.
> 
> - v3d holds dma_resv locks in the tail of its v3d_submit_cl_ioctl(),
>   copying from/to userspace happens all in v3d_lookup_bos which is
>   outside of the critical section.
> 
> - vmwgfx has a bunch of ioctls that do their own copy_*_user:
>   - vmw_execbuf_process: First this does some copies in
>     vmw_execbuf_cmdbuf() and also in the vmw_execbuf_process() itself.
>     Then comes the usual ttm reserve/validate sequence, then actual
>     submission/fencing, then unreserving, and finally some more
>     copy_to_user in vmw_execbuf_copy_fence_user. Glossing over tons of
>     details, but looks all safe.
>   - vmw_fence_event_ioctl: No ttm_reserve/dma_resv_lock anywhere to be
>     seen, seems to only create a fence and copy it out.
>   - a pile of smaller ioctl in vmwgfx_ioctl.c, no reservations to be
>     found there.
>   Summary: vmwgfx seems to be fine too.
> 
> - virtio: There's virtio_gpu_execbuffer_ioctl, which does all the
>   copying from userspace before even looking up objects through their
>   handles, so safe. Plus the getparam/getcaps ioctl, also both safe.
> 
> - qxl only has qxl_execbuffer_ioctl, which calls into
>   qxl_process_single_command. There's a lovely comment before the
>   __copy_from_user_inatomic that the slowpath should be copied from
>   i915, but I guess that never happened. Try not to be unlucky and get
>   your CS data evicted between when it's written and the kernel tries
>   to read it. The only other copy_from_user is for relocs, but those
>   are done before qxl_release_reserve_list(), which seems to be the
>   only thing reserving buffers (in the ttm/dma_resv sense) in that
>   code. So looks safe.
> 
> - A debugfs file in nouveau_debugfs_pstate_set() and the usif ioctl in
>   usif_ioctl() look safe. nouveau_gem_ioctl_pushbuf() otoh breaks this
>   everywhere and needs to be fixed up.
> 
> v2: Thomas pointed at that vmwgfx calls dma_resv_init while it holds a
> dma_resv lock of a different object already. Christian mentioned that
> ttm core does this too for ghost objects. intel-gfx-ci highlighted
> that i915 has similar issues.
> 
> Unfortunately we can't do this in the usual module init functions,
> because kernel threads don't have an ->mm - we have to wait around for
> some user thread to do this.
> 
> Solution is to spawn a worker (but only once). It's horrible, but it
> works.
> 
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Rob Herring <robh@kernel.org>
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Eric Anholt <eric@anholt.net>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Ben Skeggs <bskeggs@redhat.com>
> Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
> Cc: Thomas Hellstrom <thellstrom@vmware.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/dma-buf/dma-resv.c | 42 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 42a8f3f11681..29988b1564c1 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -34,6 +34,7 @@
>  
>  #include <linux/dma-resv.h>
>  #include <linux/export.h>
> +#include <linux/sched/mm.h>
>  
>  /**
>   * DOC: Reservation Object Overview
> @@ -95,6 +96,28 @@ static void dma_resv_list_free(struct dma_resv_list *list)
>         kfree_rcu(list, rcu);
>  }
>  
> +#if IS_ENABLED(CONFIG_LOCKDEP)
> +struct lockdep_work {
> +       struct work_struct work;
> +       struct dma_resv obj;
> +       struct mm_struct *mm;
> +} lockdep_work;
> +
> +void lockdep_work_fn(struct work_struct *work)
> +{
> +       dma_resv_init(&lockdep_work.obj);
> +
> +       down_read(&lockdep_work.mm->mmap_sem);
> +       ww_mutex_lock(&lockdep_work.obj.lock, NULL);
> +       fs_reclaim_acquire(GFP_KERNEL);
> +       fs_reclaim_release(GFP_KERNEL);
> +       ww_mutex_unlock(&lockdep_work.obj.lock);
> +       up_read(&lockdep_work.mm->mmap_sem);
> +       
> +       mmput(lockdep_work.mm);
> +}
> +#endif

#if IS_ENABLED(CONFIG_LOCKDEP)
static void dma_resv_lockmap(void)
{
	struct mm_struct *mm = alloc_mm();
	struct dma_resv obj;

	dma_resv_init(&obj);

	down_read(&mm->mmap_sem);
	ww_mutex_lock(&obj.lock, NULL);
	fs_reclaim_acquire(GFP_KERNEL);
	fs_reclaim_release(GFP_KERNEL);
	ww_mutex_unlock(&obj.lock);
	up_read(&mm->mmap_sem);

	mmput(mm);
}
core_initcall(dma_resv_lockmap);
#endif

as a thought.
-Chris
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