From: Lukas Straub <lukasstraub2@web.de>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>,
"Daniel P. Berrangé" <berrange@redhat.com>,
qemu-block <qemu-block@nongnu.org>,
"Juan Quintela" <quintela@redhat.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
qemu-devel <qemu-devel@nongnu.org>,
"Marc-André Lureau" <marcandre.lureau@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Max Reitz" <mreitz@redhat.com>
Subject: Re: [PATCH v4 2/4] block/nbd.c: Add yank feature
Date: Fri, 19 Jun 2020 20:07:34 +0200 [thread overview]
Message-ID: <20200619200734.12fca8d3@luklap> (raw)
In-Reply-To: <20200617150909.GL1728005@stefanha-x1.localdomain>
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On Wed, 17 Jun 2020 16:09:09 +0100
Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Mon, May 25, 2020 at 05:44:26PM +0200, Lukas Straub wrote:
> > @@ -1395,6 +1407,15 @@ static int nbd_client_reopen_prepare(BDRVReopenState *state,
> > return 0;
> > }
> >
> > +static void nbd_yank(void *opaque)
> > +{
> > + BlockDriverState *bs = opaque;
> > + BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
> > +
> > + qio_channel_shutdown(QIO_CHANNEL(s->sioc), QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
>
> qio_channel_shutdown() is not guaranteed to be thread-safe. Please
> document new assumptions that are being introduced.
>
> Today we can more or less get away with it (although TLS sockets are a
> little iffy) because it boils down the a shutdown(2) system call. I
> think it would be okay to update the qio_channel_shutdown() and
> .io_shutdown() documentation to clarify that this is thread-safe.
Good idea, especially since the migration code already assumes this. I will do this in the next version.
> > + atomic_set(&s->state, NBD_CLIENT_QUIT);
>
> docs/devel/atomics.rst says:
>
> No barriers are implied by ``atomic_read`` and ``atomic_set`` in either Linux
> or QEMU.
>
> Other threads might not see the latest value of s->state because this is
> a weakly ordered memory access.
>
> I haven't audited the NBD code in detail, but if you want the other
> threads to always see NBD_CLIENT_QUIT then s->state should be set before
> calling qio_channel_shutdown() using a stronger atomics API like
> atomic_load_acquire()/atomic_store_release().
You are right, I will change that in the next version.
Thanks,
Lukas Straub
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next prev parent reply other threads:[~2020-06-19 18:11 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-25 15:44 [PATCH v4 0/4] Introduce 'yank' oob qmp command to recover from hanging qemu Lukas Straub
2020-05-25 15:44 ` [PATCH v4 1/4] Introduce yank feature Lukas Straub
2020-06-16 14:39 ` Daniel P. Berrangé
2020-06-19 14:23 ` Lukas Straub
2020-06-19 16:53 ` Daniel P. Berrangé
2020-06-16 14:49 ` Daniel P. Berrangé
2020-06-17 15:12 ` Stefan Hajnoczi
2020-06-19 16:29 ` Lukas Straub
2020-06-19 16:52 ` Daniel P. Berrangé
2020-06-19 16:59 ` Lukas Straub
2020-05-25 15:44 ` [PATCH v4 2/4] block/nbd.c: Add " Lukas Straub
2020-06-16 14:40 ` Daniel P. Berrangé
2020-06-16 14:44 ` Daniel P. Berrangé
2020-06-19 16:23 ` Lukas Straub
2020-06-17 15:09 ` Stefan Hajnoczi
2020-06-19 18:07 ` Lukas Straub [this message]
2020-05-25 15:44 ` [PATCH v4 3/4] chardev/char-socket.c: " Lukas Straub
2020-06-16 14:41 ` Daniel P. Berrangé
2020-05-25 15:44 ` [PATCH v4 4/4] migration: " Lukas Straub
2020-06-16 14:42 ` Daniel P. Berrangé
2020-06-06 19:30 ` [PATCH v4 0/4] Introduce 'yank' oob qmp command to recover from hanging qemu Lukas Straub
2020-06-17 14:41 ` Stefan Hajnoczi
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