On Tue, Jan 17, 2023 at 10:21:28AM -0800, Suren Baghdasaryan wrote:
> static inline bool vma_read_trylock(struct vm_area_struct *vma)
> {
> int count, new;
>
> /* Check before locking. A race might cause false locked result. */
> if (READ_ONCE(vma->vm_lock->lock_seq) ==
> READ_ONCE(vma->vm_mm->mm_lock_seq))
> return false;
>
> count = atomic_read(&vma->vm_lock->count);
> for (;;) {
> /*
> * Is VMA is write-locked? Overflow might produce false
> locked result.
> * False unlocked result is impossible because we modify and check
> * vma->vm_lock_seq under vma->vm_lock protection and
> mm->mm_lock_seq
> * modification invalidates all existing locks.
> */
> if (count < 0)
> return false;
>
> new = count + 1;
> /* If atomic_t overflows, fail to lock. */
> if (new < 0)
> return false;
>
> /*
> * Atomic RMW will provide implicit mb on success to pair
> with smp_wmb in
> * vma_write_lock, on failure we retry.
> */
> new = atomic_cmpxchg(&vma->vm_lock->count, count, new);
> if (new == count)
> break;
> count = new;
> cpu_relax();
The cpu_relax() is exactly the wrong thing to do here. See this thread:
https://lore.kernel.org/linux-fsdevel/20230113184447.1707316-1-mjguzik@xxxxxxxxx/
