Displaying 4 results from an estimated 4 matches for "futex_atomic_cmpxchg_inatomic".
2019 May 07
4
[PATCH RFC] vhost: don't use kmap() to log dirty pages
...to a userspace bitmap through GUP and
kmap_atomic() since kernel doesn't have a set_bit_to_user()
helper. This will cause issues for the arch that has virtually tagged
caches. The way to fix is to keep using userspace virtual address.
Fortunately, futex has a cmpxchg to userspace memory helper
futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the
userspace bitmap with zero, set the bit and then write it back through
put_user().
Note: there're archs (few non popular ones) that don't implement
futex helper, we can't log dirty pages. We can fix them on top or
simply disable LOG_ALL features o...
2019 May 07
4
[PATCH RFC] vhost: don't use kmap() to log dirty pages
...to a userspace bitmap through GUP and
kmap_atomic() since kernel doesn't have a set_bit_to_user()
helper. This will cause issues for the arch that has virtually tagged
caches. The way to fix is to keep using userspace virtual address.
Fortunately, futex has a cmpxchg to userspace memory helper
futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the
userspace bitmap with zero, set the bit and then write it back through
put_user().
Note: there're archs (few non popular ones) that don't implement
futex helper, we can't log dirty pages. We can fix them on top or
simply disable LOG_ALL features o...
2019 May 08
0
[PATCH RFC] vhost: don't use kmap() to log dirty pages
...GUP and
> kmap_atomic() since kernel doesn't have a set_bit_to_user()
> helper. This will cause issues for the arch that has virtually tagged
> caches. The way to fix is to keep using userspace virtual address.
>
> Fortunately, futex has a cmpxchg to userspace memory helper
> futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the
> userspace bitmap with zero, set the bit and then write it back through
> put_user().
>
> Note: there're archs (few non popular ones) that don't implement
> futex helper, we can't log dirty pages. We can fix them on top or
> sim...
2019 May 07
0
[PATCH RFC] vhost: don't use kmap() to log dirty pages
...set_bit_to_user(int nr, u32 __user *addr)
> {
> unsigned long log = (unsigned long)addr;
> struct page *page;
> + u32 old_log;
> int r;
>
> r = get_user_pages_fast(log, 1, 1, &page);
> if (r < 0)
> return r;
> BUG_ON(r != 1);
> +
> + r = futex_atomic_cmpxchg_inatomic(&old_log, addr, 0, 0);
> + if (r < 0)
> + return r;
> +
> + old_log |= 1 << nr;
> + r = put_user(old_log, addr);
> + if (r < 0)
> + return r;
And this just looks odd to me. Why do we need the futex call to
replace a 0 value with 0? Why does it still duplica...