On Fri, 6 Dec 2019 08:53:27 +0100 Boris Brezillon boris.brezillon@collabora.com wrote:
On Thu, 5 Dec 2019 17:08:02 -0600 Rob Herring robh+dt@kernel.org wrote:
On Fri, Nov 29, 2019 at 8:33 AM Boris Brezillon boris.brezillon@collabora.com wrote:
On Fri, 29 Nov 2019 14:24:48 +0000 Steven Price steven.price@arm.com wrote:
On 29/11/2019 13:59, Boris Brezillon wrote:
If 2 threads change the MADVISE property of the same BO in parallel we might end up with an shmem->madv value that's inconsistent with the presence of the BO in the shrinker list.
I'm a bit worried from the point of view of user space sanity that you observed this - but clearly the kernel should be robust!
It's not something I observed, just found the race by inspecting the code, and I thought it was worth fixing it.
I'm not so sure there's a race.
I'm pretty sure there's one:
T0 T1
lock(pages) madv = 1 unlock(pages)
lock(pages) madv = 0 unlock(pages) lock(shrinker) remove_from_list(bo) unlock(shrinker)lock(shrinker) add_to_list(bo) unlock(shrinker)
You end up with madv = 0 and the BO is added to the list.
If there is, we still check madv value when purging, so it would be harmless even if the state is inconsistent.
Indeed. Note that you could also have this other situation where the BO is marked purgeable but not present in the list. In that case it will never be purged, but it's kinda user space fault anyway. I agree, none of this problems are critical, and I'm fine leaving it unfixed as long as it's documented somewhere that the race exist and is harmless.
The easiest solution to fix that is to protect the drm_gem_shmem_madvise() call with the shrinker lock.
Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support") Cc: stable@vger.kernel.org Signed-off-by: Boris Brezillon boris.brezillon@collabora.com
Reviewed-by: Steven Price steven.price@arm.com
Thanks.
drivers/gpu/drm/panfrost/panfrost_drv.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c index f21bc8a7ee3a..efc0a24d1f4c 100644 --- a/drivers/gpu/drm/panfrost/panfrost_drv.c +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c @@ -347,20 +347,19 @@ static int panfrost_ioctl_madvise(struct drm_device *dev, void *data, return -ENOENT; }
- mutex_lock(&pfdev->shrinker_lock); args->retained = drm_gem_shmem_madvise(gem_obj, args->madv);
This means we now hold the shrinker_lock while we take the pages_lock. Is lockdep happy with this change? I suspect not given all the fun I had getting lockdep happy.
I have tested with lockdep enabled and it's all good from lockdep PoV because the locks are taken in the same order in the madvise() and schinker_scan() path (first the shrinker lock, then the pages lock).
Note that patch 7 introduces a deadlock in the shrinker path, but this is unrelated to this shrinker lock being taken earlier in madvise (drm_gem_put_pages() is called while the pages lock is already held).
My bad, there's no deadlock in this version, because we don't use ->pages_use_count to retain the page table (we just use a gpu_usecount in patch 8 to prevent the purge). But I started working on a version that uses ->pages_use_count instead of introducing yet another refcount, and in this version I take/release a ref on the page table in the mmu_map()/mmu_unmap() path. This causes a deadlock when GEM mappings are teared down by the shrinker logic (because the pages lock is already taken in panfrost_gem_purge())...