Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

David Howells
2019-05-31T12:02:32Z

Peter Zijlstra <peterz@infradead.org> wrote:

> Can you re-iterate the exact problem? I konw we talked about this in the
> past, but I seem to have misplaced those memories :/

Take this for example:

void afs_put_call(struct afs_call *call)
{
struct afs_net *net = call->net;
int n = atomic_dec_return(&call->usage);
int o = atomic_read(&net->nr_outstanding_calls);

trace_afs_call(call, afs_call_trace_put, n + 1, o,
__builtin_return_address(0));

ASSERTCMP(n, >=, 0);
if (n == 0) {
...
}
}

I am printing the usage count in the afs_call tracepoint so that I can use it
to debug refcount bugs. If I do it like this:

void afs_put_call(struct afs_call *call)
{
int n = refcount_read(&call->usage);
int o = atomic_read(&net->nr_outstanding_calls);

trace_afs_call(call, afs_call_trace_put, n, o,
__builtin_return_address(0));

if (refcount_dec_and_test(&call->usage)) {
...
}
}

then there's a temporal gap between the usage count being read and the actual
atomic decrement in which another CPU can alter the count. This can be
exacerbated by an interrupt occurring, a softirq occurring or someone enabling
the tracepoint.

I can't do the tracepoint after the decrement if refcount_dec_and_test()
returns false unless I save all the values from the object that I might need
as the object could be destroyed any time from that point on. In this
particular case, that's just call->debug_id, but it could be other things in
other cases.

Note that I also can't touch the afs_net object in that situation either, and
the outstanding calls count that I record will potentially be out of date -
but there's not a lot I can do about that.

David