On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote: > > Replacing down_read_trylock() with the new routine loses an acquire > > fence. That alone is not a problem, but see below. > > Hmm. I think this acquire fence is actually necessary. We don't want > the later vm_lock_seq check to be reordered and happen before we take > the refcount. Otherwise this might happen: > > reader writer > if (vm_lock_seq == mm_lock_seq) // check got reordered > return false; > vm_refcnt += VMA_LOCK_OFFSET > vm_lock_seq == mm_lock_seq > vm_refcnt -= VMA_LOCK_OFFSET > if (!__refcount_inc_not_zero_limited()) > return false; > > Both reader's checks will pass and the reader would read-lock a vma > that was write-locked. Hmm, you're right. That acquire does matter here.
