> On Feb 17, 2020, at 5:31 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > > I'm confused. AFAICS both sides hold mmap_sem on write: > > - vm_mmap_pgoff() takes mmap_sem for the write on the write side > > - do_mprotect_pkey() takes mmap_sem for the write on the read side > > > What do I miss? Ah, good catch. I missed the locking for the read there. This is interesting because Marco did confirmed that the concurrency could happen, https://lore.kernel.org/lkml/20191025173511.181416-1-elver@xxxxxxxxxx/ If that means KCSAN is not at fault, then I could think of two things, 1) someone downgrades the lock. I don’t think that a case here. Only __do_munmap() will do that but I did not see how it will affect us here. 2) the reader and writer are two different processes. So, they held a different mmap_sem, but I can’t see how could two processes shared the same vm_area_struct. Also, file->f_mapping->i_mmap was also stored in the writer, but I can’t see how it was also loaded in the reader. Any ideas?
