From: Suren Baghdasaryan <surenb@google.com>
To: Hillf Danton <hdanton@sina.com>
Cc: vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net,
peterz@infradead.org, hughd@google.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock
Date: Mon, 16 Jan 2023 20:52:45 -0800 [thread overview]
Message-ID: <CAJuCfpFq23m-KYKaDoCS2K2aM8rO7j8aPa0nVPs-_xP2Sf6GGg@mail.gmail.com> (raw)
In-Reply-To: <20230117031632.2321-1-hdanton@sina.com>
On Mon, Jan 16, 2023 at 7:16 PM Hillf Danton <hdanton@sina.com> wrote:
>
> On Mon, 16 Jan 2023 15:08:48 -0800 Suren Baghdasaryan <surenb@google.com>
> > On Mon, Jan 16, 2023 at 6:07 AM Hillf Danton <hdanton@sina.com> wrote:
> > > On Mon, 9 Jan 2023 12:53:36 -0800 Suren Baghdasaryan <surenb@google.com>
> > > > static inline bool vma_read_trylock(struct vm_area_struct *vma)
> > > > {
> > > > /* Check before locking. A race might cause false locked result. */
> > > > - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> > > > + if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> > > > return false;
> > >
> > > Add mb to pair with the above wmb like
> >
> > The wmb above is to ensure the ordering between updates of lock_seq
> > and vm_lock->count (lock_seq is updated first and vm_lock->count only
> > after that). The first access to vm_lock->count in this function is
> > atomic_inc_unless_negative() and it's an atomic RMW operation with a
> > return value. According to documentation such functions are fully
> > ordered, therefore I think we already have an implicit full memory
> > barrier between reads of lock_seq and vm_lock->count here. Am I wrong?
>
> No you are not.
I'm not wrong or the other way around? Please expand a bit.
> Revisit it in case of full mb not ensured.
>
> #ifndef arch_atomic_inc_unless_negative
> static __always_inline bool
> arch_atomic_inc_unless_negative(atomic_t *v)
> {
> int c = arch_atomic_read(v);
>
> do {
> if (unlikely(c < 0))
> return false;
> } while (!arch_atomic_try_cmpxchg(v, &c, c + 1));
>
> return true;
> }
> #define arch_atomic_inc_unless_negative arch_atomic_inc_unless_negative
> #endif
I think your point is that the full mb is not ensured here, specifically if c<0?
>
> > > int count = atomic_read(&vma->vm_lock->count);
> > > for (;;) {
> > > int new = count + 1;
> > >
> > > if (count < 0 || new < 0)
> > > return false;
> > >
> > > new = atomic_cmpxchg(&vma->vm_lock->count, count, new);
> > > if (new == count)
> > > break;
> > > count = new;
> > > cpu_relax();
> > > }
> > >
> > > (wake up waiting readers after taking the lock;
> > > but the write lock owner before this read trylock should be
> > > responsible for waking waiters up.)
> > >
> > > lock acquire for read.
> >
> > This schema might cause readers to wait, which is not an exact
>
> Could you specify a bit more on wait?
Oh, I misunderstood your intent with that for() loop. Indeed, if a
writer took the lock, the count will be negative and it terminates
with a failure. Yeah, I think that would work.
>
> > replacement for down_read_trylock(). The requirement to wake up
> > waiting readers also complicates things
>
> If the writer lock owner is preempted by a reader while releasing lock,
>
> set count to zero
> <-- preempt
> wake up waiters
>
> then lock is owned by reader but with read waiters.
>
> That is buggy if write waiter starvation is allowed in this patchset.
I don't quite understand your point here. Readers don't wait, so there
can't be "read waiters". Could you please expand with a race diagram
maybe?
>
> > and since we can always fall
> > back to mmap_lock, that complication is unnecessary IMHO. I could use
> > part of your suggestion like this:
> >
> > int new = count + 1;
> >
> > if (count < 0 || new < 0)
> > return false;
> >
> > new = atomic_cmpxchg(&vma->vm_lock->count, count, new);
> > if (new == count)
> > return false;
> >
> > Compared to doing atomic_inc_unless_negative() first, like I did
> > originally, this schema opens a bit wider window for the writer to get
> > in the middle and cause the reader to fail locking but I don't think
> > it would result in any visible regression.
>
> It is definitely fine for writer to acquire the lock while reader is
> doing trylock, no?
Yes.
>
next prev parent reply other threads:[~2023-01-17 4:53 UTC|newest]
Thread overview: 186+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-09 20:52 [PATCH 00/41] Per-VMA locks Suren Baghdasaryan
2023-01-09 20:52 ` [PATCH 01/41] maple_tree: Be more cautious about dead nodes Suren Baghdasaryan
2023-01-09 20:52 ` [PATCH 02/41] maple_tree: Detect dead nodes in mas_start() Suren Baghdasaryan
2023-01-09 20:52 ` [PATCH 03/41] maple_tree: Fix freeing of nodes in rcu mode Suren Baghdasaryan
2023-01-09 20:52 ` [PATCH 04/41] maple_tree: remove extra smp_wmb() from mas_dead_leaves() Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 05/41] maple_tree: Fix write memory barrier of nodes once dead for RCU mode Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 06/41] maple_tree: Add smp_rmb() to dead node detection Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 07/41] mm: Enable maple tree RCU mode by default Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 08/41] mm: introduce CONFIG_PER_VMA_LOCK Suren Baghdasaryan
2023-01-11 0:13 ` Davidlohr Bueso
2023-01-11 0:44 ` Suren Baghdasaryan
2023-01-11 8:23 ` Michal Hocko
2023-01-11 9:54 ` Ingo Molnar
[not found] ` <6be809f5554a4faaa22c287ba4224bd0@AcuMS.aculab.com>
2023-01-11 16:28 ` Suren Baghdasaryan
2023-01-11 16:44 ` Michal Hocko
2023-01-11 17:04 ` Suren Baghdasaryan
2023-01-11 17:37 ` Michal Hocko
2023-01-11 17:49 ` Suren Baghdasaryan
2023-01-11 18:02 ` Michal Hocko
2023-01-11 18:09 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 09/41] mm: rcu safe VMA freeing Suren Baghdasaryan
2023-01-17 14:25 ` Michal Hocko
2023-01-18 2:16 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 10/41] mm: move mmap_lock assert function definitions Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 11/41] mm: export dump_mm() Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 12/41] mm: add per-VMA lock and helper functions to control it Suren Baghdasaryan
2023-01-17 15:04 ` Michal Hocko
2023-01-17 15:12 ` Michal Hocko
2023-01-17 21:21 ` Suren Baghdasaryan
2023-01-17 21:54 ` Matthew Wilcox
2023-01-17 22:33 ` Suren Baghdasaryan
2023-01-18 9:18 ` Michal Hocko
2023-01-17 21:08 ` Suren Baghdasaryan
2023-01-17 15:07 ` Michal Hocko
2023-01-17 21:09 ` Suren Baghdasaryan
2023-01-17 18:02 ` Jann Horn
2023-01-17 21:28 ` Suren Baghdasaryan
2023-01-17 21:45 ` Jann Horn
2023-01-17 22:36 ` Suren Baghdasaryan
2023-01-17 23:15 ` Matthew Wilcox
2023-11-22 14:04 ` Alexander Gordeev
2023-01-18 12:28 ` Michal Hocko
2023-01-18 13:23 ` Jann Horn
2023-01-18 15:11 ` Michal Hocko
2023-01-18 17:36 ` Suren Baghdasaryan
2023-01-18 21:28 ` Michal Hocko
2023-01-18 21:45 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 13/41] mm: introduce vma->vm_flags modifier functions Suren Baghdasaryan
2023-01-11 15:47 ` Davidlohr Bueso
2023-01-11 17:36 ` Suren Baghdasaryan
2023-01-11 19:52 ` Davidlohr Bueso
2023-01-11 21:23 ` Suren Baghdasaryan
2023-01-17 15:09 ` Michal Hocko
2023-01-17 15:15 ` Michal Hocko
2023-01-18 2:07 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 14/41] mm: replace VM_LOCKED_CLEAR_MASK with VM_LOCKED_MASK Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 15/41] mm: replace vma->vm_flags direct modifications with modifier calls Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 16/41] mm: replace vma->vm_flags indirect modification in ksm_madvise Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 17/41] mm/mmap: move VMA locking before anon_vma_lock_write call Suren Baghdasaryan
2023-01-17 15:16 ` Michal Hocko
2023-01-18 2:01 ` Suren Baghdasaryan
2023-01-18 9:23 ` Michal Hocko
2023-01-18 18:09 ` Suren Baghdasaryan
2023-01-18 21:33 ` Michal Hocko
2023-01-18 21:48 ` Suren Baghdasaryan
2023-01-19 9:31 ` Michal Hocko
2023-01-19 18:53 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a huge page Suren Baghdasaryan
2023-01-17 15:25 ` Michal Hocko
2023-01-17 20:28 ` Jann Horn
2023-01-17 21:05 ` Suren Baghdasaryan
2023-01-18 9:40 ` Michal Hocko
2023-01-18 12:38 ` Jann Horn
2023-01-18 17:41 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 19/41] mm/mmap: write-lock VMAs before merging, splitting or expanding them Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 20/41] mm/mmap: write-lock VMAs in vma_adjust Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 21/41] mm/mmap: write-lock VMAs affected by VMA expansion Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 22/41] mm/mremap: write-lock VMA while remapping it to a new address range Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 23/41] mm: write-lock VMAs before removing them from VMA tree Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 24/41] mm: conditionally write-lock VMA in free_pgtables Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 25/41] mm/mmap: write-lock adjacent VMAs if they can grow into unmapped area Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 26/41] kernel/fork: assert no VMA readers during its destruction Suren Baghdasaryan
2023-01-17 15:42 ` Michal Hocko
2023-01-18 1:53 ` Suren Baghdasaryan
2023-01-18 9:43 ` Michal Hocko
2023-01-18 18:06 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 27/41] mm/mmap: prevent pagefault handler from racing with mmu_notifier registration Suren Baghdasaryan
2023-01-18 12:50 ` Jann Horn
2023-01-18 17:40 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from arch-specific code Suren Baghdasaryan
2023-01-17 15:47 ` Michal Hocko
2023-01-18 1:06 ` Suren Baghdasaryan
2023-01-18 2:44 ` Matthew Wilcox
2023-01-18 21:33 ` Suren Baghdasaryan
2023-01-17 21:03 ` Jann Horn
2023-01-17 23:18 ` Liam Howlett
2023-01-09 20:53 ` [PATCH 29/41] mm: fall back to mmap_lock if vma->anon_vma is not yet set Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 30/41] mm: add FAULT_FLAG_VMA_LOCK flag Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 31/41] mm: prevent do_swap_page from handling page faults under VMA lock Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock Suren Baghdasaryan
2023-01-17 19:51 ` Jann Horn
2023-01-17 20:36 ` Jann Horn
2023-01-17 20:57 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 33/41] mm: introduce per-VMA lock statistics Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 34/41] x86/mm: try VMA lock-based page fault handling first Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 35/41] arm64/mm: " Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 36/41] powerc/mm: " Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 37/41] mm: introduce mod_vm_flags_nolock Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 38/41] mm: avoid assertion in untrack_pfn Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free Suren Baghdasaryan
2023-01-17 15:57 ` Michal Hocko
2023-01-18 1:19 ` Suren Baghdasaryan
2023-01-18 9:49 ` Michal Hocko
2023-01-18 18:04 ` Suren Baghdasaryan
2023-01-18 18:34 ` Paul E. McKenney
2023-01-18 19:01 ` Suren Baghdasaryan
2023-01-18 20:20 ` Paul E. McKenney
2023-01-19 12:52 ` Michal Hocko
2023-01-19 19:17 ` Paul E. McKenney
2023-01-20 8:57 ` Michal Hocko
2023-01-20 16:08 ` Paul E. McKenney
2023-01-19 12:59 ` Michal Hocko
2023-01-19 18:52 ` Suren Baghdasaryan
2023-01-19 19:20 ` Paul E. McKenney
2023-01-19 19:47 ` Suren Baghdasaryan
2023-01-19 19:55 ` Paul E. McKenney
2023-01-20 8:52 ` Michal Hocko
2023-01-20 16:20 ` Suren Baghdasaryan
2023-01-20 16:45 ` Suren Baghdasaryan
2023-01-20 16:49 ` Matthew Wilcox
2023-01-20 17:08 ` Liam R. Howlett
2023-01-20 17:17 ` Suren Baghdasaryan
2023-01-20 17:32 ` Matthew Wilcox
2023-01-20 17:50 ` Suren Baghdasaryan
2023-01-20 19:23 ` Liam R. Howlett
2023-01-23 9:56 ` Michal Hocko
2023-01-23 16:22 ` Suren Baghdasaryan
2023-01-23 16:55 ` Michal Hocko
2023-01-23 17:07 ` Suren Baghdasaryan
2023-01-23 17:16 ` Michal Hocko
2023-01-23 17:46 ` Suren Baghdasaryan
2023-01-23 18:23 ` Matthew Wilcox
2023-01-23 18:47 ` Suren Baghdasaryan
2023-01-23 19:18 ` Michal Hocko
2023-01-23 19:30 ` Matthew Wilcox
2023-01-23 19:57 ` Suren Baghdasaryan
2023-01-23 20:00 ` Michal Hocko
2023-01-23 20:08 ` Suren Baghdasaryan
2023-01-23 20:38 ` Liam R. Howlett
2023-01-20 17:21 ` Paul E. McKenney
2023-01-20 18:42 ` Suren Baghdasaryan
2023-01-23 9:59 ` Michal Hocko
2023-01-23 17:43 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 40/41] mm: separate vma->lock from vm_area_struct Suren Baghdasaryan
2023-01-17 18:33 ` Jann Horn
2023-01-17 19:01 ` Suren Baghdasaryan
2023-01-09 20:53 ` [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Suren Baghdasaryan
2023-01-10 8:04 ` Vlastimil Babka
2023-01-10 17:05 ` Suren Baghdasaryan
2023-01-16 11:14 ` Hyeonggon Yoo
2023-01-16 22:36 ` Suren Baghdasaryan
2023-01-17 4:14 ` Matthew Wilcox
2023-01-17 4:34 ` Suren Baghdasaryan
2023-01-17 5:46 ` Matthew Wilcox
2023-01-17 5:58 ` Suren Baghdasaryan
2023-01-17 18:23 ` Matthew Wilcox
2023-01-17 18:28 ` Suren Baghdasaryan
2023-01-17 20:31 ` Michal Hocko
2023-01-17 21:00 ` Suren Baghdasaryan
[not found] ` <20230116140649.2012-1-hdanton@sina.com>
2023-01-16 23:08 ` Suren Baghdasaryan
2023-01-16 23:11 ` Suren Baghdasaryan
[not found] ` <20230117031632.2321-1-hdanton@sina.com>
2023-01-17 4:52 ` Suren Baghdasaryan [this message]
[not found] ` <20230117083355.2374-1-hdanton@sina.com>
2023-01-17 18:21 ` Suren Baghdasaryan
2023-01-17 18:27 ` Matthew Wilcox
2023-01-17 18:31 ` Suren Baghdasaryan
[not found] ` <20230118062639.2839-1-hdanton@sina.com>
2023-01-18 18:35 ` Matthew Wilcox
2023-01-17 18:11 ` Jann Horn
2023-01-17 18:26 ` Suren Baghdasaryan
2023-01-17 18:31 ` Matthew Wilcox
2023-01-17 18:36 ` Jann Horn
2023-01-17 18:49 ` Suren Baghdasaryan
2023-01-17 18:36 ` Suren Baghdasaryan
2023-01-17 18:48 ` Matthew Wilcox
2023-01-17 18:55 ` Suren Baghdasaryan
2023-01-17 18:59 ` Jann Horn
2023-01-17 19:06 ` Suren Baghdasaryan
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