[PATCH] cxl: Fix possible deadlock when processing page faults from cxllib

[Vaibhav Jain]

Frederic Barrat <fbarrat at linux.vnet.ibm.com> writes:

> cxllib_handle_fault() is called by an external driver when it needs to
> have the host process page faults for a buffer which may cover several
> pages. Currently the function holds the mm->mmap_sem semaphore with
> read access while iterating over the buffer, since it could spawn
> several VMAs. When calling a lower-level function to handle the page
> fault for a single page, the semaphore is accessed again in read
> mode. That is wrong and can lead to deadlocks if a writer tries to
> sneak in while a buffer of several pages is being processed.
>
> The fix is to release the semaphore once cxllib_handle_fault() got the
> information it needs from the current vma. The address space/VMAs
> could evolve while we iterate over the full buffer, but in the
> unlikely case where we miss a page, the driver will raise a new page
> fault when retrying.
>
> Fixes: 3ced8d730063 ("cxl: Export library to support IBM XSL")
> Cc: stable at vger.kernel.org # 4.13+
> Signed-off-by: Frederic Barrat <fbarrat at linux.vnet.ibm.com>

Reviewed-by: Vaibhav Jain <vaibhav at linux.vnet.ibm.com>