linux-kernel - Re: [PATCH v5 5/9] IB/isert: Replace semaphore sem with completion

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Message-ID: <31503532.zqWpFShUDU@wuerfel>
Date:   Mon, 21 Nov 2016 15:50:59 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Binoy Jayan <binoy.jayan@...aro.org>,
        Doug Ledford <dledford@...hat.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        Sagi Grimberg <sagig@...lanox.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Nicholas Bellinger <nab@...ux-iscsi.org>,
        Jenny Derzhavetz <jennyf@...lanox.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steve Wise <swise@...ngridcomputing.com>,
        Mark Bloch <markb@...lanox.com>,
        Tatyana E Nikolova <tatyana.e.nikolova@...el.com>,
        Matan Barak <matanb@...lanox.com>,
        Lijun Ou <oulijun@...wei.com>,
        "Wei Hu(Xavier)" <xavier.huwei@...wei.com>,
        Faisal Latif <faisal.latif@...el.com>,
        Mustafa Ismail <mustafa.ismail@...el.com>,
        Mark Brown <broonie@...nel.org>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org
Subject: Re: [PATCH v5 5/9] IB/isert: Replace semaphore sem with completion

On Monday, November 21, 2016 2:33:02 PM CET Sagi Grimberg wrote:
> >>> diff --git a/drivers/infiniband/ulp/isert/ib_isert.c b/drivers/infiniband/ulp/isert/ib_isert.c
> >>> index 6dd43f6..de80f56 100644
> >>> --- a/drivers/infiniband/ulp/isert/ib_isert.c
> >>> +++ b/drivers/infiniband/ulp/isert/ib_isert.c
> >>> @@ -619,7 +619,7 @@
> >>>       mutex_unlock(&isert_np->mutex);
> >>>
> >>>       isert_info("np %p: Allow accept_np to continue\n", isert_np);
> >>> -     up(&isert_np->sem);
> >>> +     complete(&isert_np->comp);
> >>>  }
> >>>
> >>>  static void
> >>> @@ -2311,7 +2311,7 @@ struct rdma_cm_id *
> >>>               isert_err("Unable to allocate struct isert_np\n");
> >>>               return -ENOMEM;
> >>>       }
> >>> -     sema_init(&isert_np->sem, 0);
> >>> +     init_completion(&isert_np->comp);
> >>
> >> This is still racy, a connect event can complete just before we
> >> init the completion and *will* get lost...
> >>
> >> This code started off with a waitqueue which exposes the same
> >> problem, see:
> >> 531b7bf4bd79 Target/iser: Fix iscsit_accept_np and rdma_cm racy flow
> >>
> >> So, still NAK from me...
> >
> > I don't see what you mean here. The code using a waitqueue had no
> > counter but the completion does.
> 
> The problem here is that init_completion sets the counter to zero
> and between this thread wakes up and until it initializes comp->done
> we might have another connect event completing it and I fail to
> see how this is handled correctly.

But the sema_init()/init_completion() is done right after the
structure is allocated, so nothing should have a pointer to it
at this time.

Later when we do the down()/wait_for_completion(), that will
just decrement the counter rather than initialize, and converting
from one to the other doesn't change anything here.

	Arnd