powerpc/pseries/vas: Hold mmap_mutex after mmap lock during window close

Commit 8ef7b9e1765a ("powerpc/pseries/vas: Close windows with DLPAR
core removal") unmaps the window paste address and issues HCALL to
close window in the hypervisor for migration or DLPAR core removal
events. So holds mmap_mutex and then mmap lock before unmap the
paste address. But if the user space issue mmap paste address at
the same time with the migration event, coproc_mmap() is called
after holding the mmap lock which can trigger deadlock when trying
to acquire mmap_mutex in coproc_mmap().

t1: mmap() call to mmap t2: Migration event
window paste address

do_mmap2() migration_store()
ksys_mmap_pgoff() pseries_migrate_partition()
vm_mmap_pgoff() vas_migration_handler()
Acquire mmap lock reconfig_close_windows()
do_mmap() lock mmap_mutex
mmap_region() Acquire mmap lock
call_mmap() //Wait for mmap lock
coproc_mmap() unmap vma
lock mmap_mutex update window status
//wait for mmap_mutex Release mmap lock
mmap vma unlock mmap_mutex
update window status
unlock mmap_mutex
...
Release mmap lock

Fix this deadlock issue by holding mmap lock first before mmap_mutex
in reconfig_close_windows().

Fixes: 8ef7b9e1765a ("powerpc/pseries/vas: Close windows with DLPAR core removal")
Signed-off-by: Haren Myneni <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/platforms/pseries/vas.c

···
 		}
 
 		task_ref = &win->vas_win.task_ref;
+		/*
+		 * VAS mmap (coproc_mmap()) and its fault handler
+		 * (vas_mmap_fault()) are called after holding mmap lock.
+		 * So hold mmap mutex after mmap_lock to avoid deadlock.
+		 */
+		mmap_write_lock(task_ref->mm);
 		mutex_lock(&task_ref->mmap_mutex);
 		vma = task_ref->vma;
 		/*
···
 		 */
 		win->vas_win.status |= flag;
 
-		mmap_write_lock(task_ref->mm);
 		/*
 		 * vma is set in the original mapping. But this mapping
 		 * is done with mmap() after the window is opened with ioctl.
···
 		if (vma)
 			zap_vma_pages(vma);
 
-		mmap_write_unlock(task_ref->mm);
 		mutex_unlock(&task_ref->mmap_mutex);
+		mmap_write_unlock(task_ref->mm);
 		/*
 		 * Close VAS window in the hypervisor, but do not
 		 * free vas_window struct since it may be reused