On Wed, Mar 06, 2024 at 10:06:21PM -0500, Mathieu Desnoyers wrote:
> On 2024-03-06 21:43, Linus Torvalds wrote:
> [...]
> >
> > Honestly, this all makes me think that we'd be *much* better off
> > showing the real "handoff" with smp_store_release() and
> > smp_load_acquire().
>
> We've done something similar in liburcu (userspace code) to allow
> Thread Sanitizer to understand the happens-before relationships
> within the RCU implementations and lock-free data structures.
>
> Moving to load-acquire/store-release (C11 model in our case)
> allowed us to provide enough happens-before relationship for
> Thread Sanitizer to understand what is happening under the
> hood in liburcu and perform relevant race detection of user
> code.
Good point!
In the kernel, though, KCSAN understands the Linux-kernel memory model,
and so we don't get that sort of false positive.
> As far as the WRITE_ONCE(x, READ_ONCE(x) + 1) pattern
> is concerned, the only valid use-case I can think of is
> split counters or RCU implementations where there is a
> single updater doing the increment, and one or more
> concurrent reader threads that need to snapshot a
> consistent value with READ_ONCE().
It is wrong here. OK, not wrong from a functional viewpoint, but it
is at best very confusing. I am applying patches to fix this. I will
push out a new "dev" branch on -rcu that will make this function appear
as shown below.
So what would you use that pattern for?
One possibility is a per-CPU statistical counter in userspace on a
fastpath, in cases where losing the occasional count is OK. Then learning
your CPU (and possibly being immediately migrated to some other CPU),
READ_ONCE() of the count, increment, and WRITE_ONCE() might (or might not)
make sense.
I suppose the same in the kernel if there was a fastpath so extreme you
could not afford to disable preemption.
At best, very niche.
Or am I suffering a failure of imagination yet again? ;-)
Thanx, Paul
------------------------------------------------------------------------
static bool
rcu_torture_pipe_update_one(struct rcu_torture *rp)
{
int i;
struct rcu_torture_reader_check *rtrcp = READ_ONCE(rp->rtort_chkp);
if (rtrcp) {
WRITE_ONCE(rp->rtort_chkp, NULL);
smp_store_release(&rtrcp->rtc_ready, 1); // Pair with smp_load_acquire().
}
i = rp->rtort_pipe_count;
if (i > RCU_TORTURE_PIPE_LEN)
i = RCU_TORTURE_PIPE_LEN;
atomic_inc(&rcu_torture_wcount[i]);
WRITE_ONCE(rp->rtort_pipe_count, i + 1);
ASSERT_EXCLUSIVE_WRITER(rp->rtort_pipe_count);
if (i + 1 >= RCU_TORTURE_PIPE_LEN) {
rp->rtort_mbtest = 0;
return true;
}
return false;
}
