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The original intent of vfio_container.group_lock is to protect vfio_container.group_list, however over time it's become a crutch to prevent changes in container composition any time we call into the iommu driver backend. This introduces problems when we start to have more complex interactions, for example when a user's DMA unmap request triggers a notification to an mdev vendor driver, who responds by attempting to unpin mappings within that request, re-entering the iommu backend. We incorrectly assume that the use of read-locks here allow for this nested locking behavior, but a poorly timed write-lock could in fact trigger a deadlock. The current use of group_lock seems to fall into the trap of locking code, not data. Correct that by removing uses of group_lock that are not directly related to group_list. Note that the vfio type1 iommu backend has its own mutex, vfio_iommu.lock, which it uses to protect itself for each of these interfaces anyway. The group_lock appears to be a redundancy for these interfaces and type1 even goes so far as to release its mutex to allow for exactly the re-entrant code path above. Reported-by: Chuanxiao Dong <chuanxiao.dong@intel.com> Signed-off-by: Alex Williamson <alex.williamson@redhat.com> Acked-by: Alexey Kardashevskiy <aik@ozlabs.ru> Cc: stable@vger.kernel.org # v4.10+
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.