Messages in this thread |  | | From | Suren Baghdasaryan <> | | Date | Wed, 15 Jan 2025 07:00:37 -0800 | | Subject | Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count |
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On Wed, Jan 15, 2025 at 3:13 AM Peter Zijlstra <peterz@infradead.org> wrote: > > 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.
Thanks for reviewing, Peter! Yes, I'm changing the code to use atomic_t instead of refcount_t and it comes out quite nicely I think. I had to add two small helper functions: vm_refcount_inc() - similar to refcount_add_not_zero() but with an acquired fence. vm_refcnt_sub() - similar to refcount_sub_and_test(). I could use atomic_sub_and_test() but that would add unnecessary acquire fence in the pagefault path, so I'm using refcount_sub_and_test() logic instead.
For SLAB_TYPESAFE_BY_RCU I think we are ok with the __vma_enter_locked()/__vma_exit_locked() transition in the vma_mark_detached() before freeing the vma and would not need secondary validation. In __vma_enter_locked(), vm_refcount gets VMA_LOCK_OFFSET set, which prevents readers from taking the refcount. In __vma_exit_locked() vm_refcnt transitions to 0, so again that prevents readers from taking the refcount. IOW, the readers won't get to the secondary validation and will fail early on __refcount_inc_not_zero_limited(). I think this transition correctly serves the purpose of waiting for current temporary readers to exit and preventing new readers from read-locking and using the vma.
> > And this is probably fine, but let me ponder this all a little more.
Thanks for taking the time!
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