On Mon, Jun 27, 2022 at 02:10:02PM -0400, Demi Marie Obenour wrote: > commit dbe97cff7dd9f0f75c524afdd55ad46be3d15295 upstream > > 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@xxxxxxxxxxxxxxx > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx> > Reviewed-by: Juergen Gross <jgross@xxxxxxxx> > Link: https://lore.kernel.org/r/20220622022726.2538-1-demi@xxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Juergen Gross <jgross@xxxxxxxx> > --- > drivers/xen/gntdev-common.h | 7 ++ > drivers/xen/gntdev.c | 142 +++++++++++++++++++++++++----------- > 2 files changed, 106 insertions(+), 43 deletions(-) All now queued up, thanks. greg k-h
