| Message ID | 20171107225426.3822-1-slp@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi Sergio, On Tue, 11/07 23:54, Sergio Lopez wrote: > Commit b7a745d added a qemu_bh_cancel call to the completion function > as an optimization to prevent it from unnecessarily rescheduling itself. > > This completion function is scheduled from worker_thread, after setting > the state of a ThreadPoolElement to THREAD_DONE. > > This was considered to be safe, as the completion function restarts the > loop just after the call to qemu_bh_cancel. But, under certain access > patterns and scheduling conditions, the loop may wrongly use a > pre-fetched elem->state value, reading it as THREAD_QUEUED, and ending > the completion function without having processed a pending TPE linked at > pool->head: > > I/O thread | worker thread Isn't the left column "workder thread" and the right one "I/O thread"? Fam > ------------------------------------------------------------------------ > | speculatively read req->state > req->state = THREAD_DONE; | > qemu_bh_schedule(p->completion_bh) | > bh->scheduled = 1; | > | qemu_bh_cancel(p->completion_bh) > | bh->scheduled = 0; > | if (req->state == THREAD_DONE) > | // sees THREAD_QUEUED > > The source of the misunderstanding was that qemu_bh_cancel is now being > used by the _consumer_ rather than the producer, and therefore now needs > to have acquire semantics just like e.g. aio_bh_poll. > > In some situations, if there are no other independent requests in the > same aio context that could eventually trigger the scheduling of the > completion function, the omitted TPE and all operations pending on it > will get stuck forever. > > Signed-off-by: Sergio Lopez <slp@redhat.com> > --- > util/async.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/util/async.c b/util/async.c > index 355af73ee7..0e1bd8780a 100644 > --- a/util/async.c > +++ b/util/async.c > @@ -174,7 +174,7 @@ void qemu_bh_schedule(QEMUBH *bh) > */ > void qemu_bh_cancel(QEMUBH *bh) > { > - bh->scheduled = 0; > + atomic_mb_set(&bh->scheduled, 0); > } > > /* This func is async.The bottom half will do the delete action at the finial > -- > 2.13.6 >
Hi Fam, On Wed, Nov 8, 2017 at 2:29 AM, Fam Zheng <famz@redhat.com> wrote: >> I/O thread | worker thread > > Isn't the left column "workder thread" and the right one "I/O thread"? > Yes, you're right. I'm going to switch them back. Thanks, Sergio.
diff --git a/util/async.c b/util/async.c index 355af73ee7..0e1bd8780a 100644 --- a/util/async.c +++ b/util/async.c @@ -174,7 +174,7 @@ void qemu_bh_schedule(QEMUBH *bh) */ void qemu_bh_cancel(QEMUBH *bh) { - bh->scheduled = 0; + atomic_mb_set(&bh->scheduled, 0); } /* This func is async.The bottom half will do the delete action at the finial
Commit b7a745d added a qemu_bh_cancel call to the completion function as an optimization to prevent it from unnecessarily rescheduling itself. This completion function is scheduled from worker_thread, after setting the state of a ThreadPoolElement to THREAD_DONE. This was considered to be safe, as the completion function restarts the loop just after the call to qemu_bh_cancel. But, under certain access patterns and scheduling conditions, the loop may wrongly use a pre-fetched elem->state value, reading it as THREAD_QUEUED, and ending the completion function without having processed a pending TPE linked at pool->head: I/O thread | worker thread ------------------------------------------------------------------------ | speculatively read req->state req->state = THREAD_DONE; | qemu_bh_schedule(p->completion_bh) | bh->scheduled = 1; | | qemu_bh_cancel(p->completion_bh) | bh->scheduled = 0; | if (req->state == THREAD_DONE) | // sees THREAD_QUEUED The source of the misunderstanding was that qemu_bh_cancel is now being used by the _consumer_ rather than the producer, and therefore now needs to have acquire semantics just like e.g. aio_bh_poll. In some situations, if there are no other independent requests in the same aio context that could eventually trigger the scheduling of the completion function, the omitted TPE and all operations pending on it will get stuck forever. Signed-off-by: Sergio Lopez <slp@redhat.com> --- util/async.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)