On Mon, 16 Jan 2023 15:08:48 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx>
> On Mon, Jan 16, 2023 at 6:07 AM Hillf Danton <hdanton@xxxxxxxx> wrote:
> > On Mon, 9 Jan 2023 12:53:36 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > 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. 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
> > 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?
> 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.
> 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?
