On Wed, Jan 15, 2025 at 11:48:41AM +0100, Peter Zijlstra wrote: > 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. Notably, it means refcount_t is entirely unsuitable for anything SLAB_TYPESAFE_BY_RCU, since they all will need secondary validation conditions after the refcount succeeds. And this is probably fine, but let me ponder this all a little more.
