From dbe97cff7dd9f0f75c524afdd55ad46be3d15295 Mon Sep 17 00:00:00 2001 From: Demi Marie Obenour Date: Tue, 21 Jun 2022 22:27:26 -0400 Subject: xen/gntdev: Avoid blocking in unmap_grant_pages() unmap_grant_pages() currently waits for the pages to no longer be used. In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a deadlock against i915: i915 was waiting for gntdev's MMU notifier to finish, while gntdev was waiting for i915 to free its pages. I also believe this is responsible for various deadlocks I have experienced in the past. Avoid these problems by making unmap_grant_pages async. This requires making it return void, as any errors will not be available when the function returns. Fortunately, the only use of the return value is a WARN_ON(), which can be replaced by a WARN_ON when the error is detected. Additionally, a failed call will not prevent further calls from being made, but this is harmless. Because unmap_grant_pages is now async, the grant handle will be sent to INVALID_GRANT_HANDLE too late to prevent multiple unmaps of the same handle. Instead, a separate bool array is allocated for this purpose. This wastes memory, but stuffing this information in padding bytes is too fragile. Furthermore, it is necessary to grab a reference to the map before making the asynchronous call, and release the reference when the call returns. It is also necessary to guard against reentrancy in gntdev_map_put(), and to handle the case where userspace tries to map a mapping whose contents have not all been freed yet. Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in use") Cc: stable@vger.kernel.org Signed-off-by: Demi Marie Obenour Reviewed-by: Juergen Gross Link: https://lore.kernel.org/r/20220622022726.2538-1-demi@invisiblethingslab.com Signed-off-by: Juergen Gross --- drivers/xen/gntdev-common.h | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'drivers/xen/gntdev-common.h') diff --git a/drivers/xen/gntdev-common.h b/drivers/xen/gntdev-common.h index 20d7d059dadb..40ef379c28ab 100644 --- a/drivers/xen/gntdev-common.h +++ b/drivers/xen/gntdev-common.h @@ -16,6 +16,7 @@ #include #include #include +#include struct gntdev_dmabuf_priv; @@ -56,6 +57,7 @@ struct gntdev_grant_map { struct gnttab_unmap_grant_ref *unmap_ops; struct gnttab_map_grant_ref *kmap_ops; struct gnttab_unmap_grant_ref *kunmap_ops; + bool *being_removed; struct page **pages; unsigned long pages_vm_start; @@ -73,6 +75,11 @@ struct gntdev_grant_map { /* Needed to avoid allocation in gnttab_dma_free_pages(). */ xen_pfn_t *frames; #endif + + /* Number of live grants */ + atomic_t live_grants; + /* Needed to avoid allocation in __unmap_grant_pages */ + struct gntab_unmap_queue_data unmap_data; }; struct gntdev_grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count, -- cgit