On Fri, Jan 04, 2019 at 12:39:12PM -0500, bfields@xxxxxxxxxxxx wrote: > I wonder if there's a race here independent of that change: > > svc_xprt_enqueue() callers all do something like: > > 1. change some condition > 2. call svc_xprt_enqueue() to check whether the xprt should > now be enqueued. > > where the conditions are settings of the xpt_flags, or socket wspace, or > xpt_nr_rqsts. > > In theory if we miss some concurrent change we're OK because whoever's > making that change will then also call svc_xprt_enqueue. But that's not > enough; e.g.: > > task 1 task 2 > ------ ------ > set XPT_DATA > atomic_dec(xpt_nr_rqsts) > > check XPT_DATA && check xpt_nr_rqsts > > check XPT_DATA && check xpt_nr_rqsts > > If the tasks only see their local changes, then neither see both > conditions true, so the socket doesn't get enqueued. (And a request > that was ready to be processed will sit around until someone else comes > calls svc_xprt_enqueue() on that xprt.) So maybe we actually need static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt) { + mb(); if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE))) return true; if (xprt->xpt_flags & ((1<<XPT_DATA)|(1<<XPT_DEFERRED))) { Then whichever memory barrier executes second guarantees that the following check sees the result of both the XPT_DATA and xpt_nr_rqsts changes. I think.... --b.