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Re: [PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read
From: |
Roman Kagan |
Subject: |
Re: [PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read |
Date: |
Fri, 12 Nov 2021 22:04:54 +0300 |
On Fri, Nov 12, 2021 at 12:24:06PM +0400, Marc-André Lureau wrote:
> Hi
>
> On Thu, Nov 11, 2021 at 7:44 PM Roman Kagan <rvkagan@yandex-team.ru> wrote:
>
> > As its name suggests, ChardevClass.chr_sync_read is supposed to do a
> > blocking read. The only implementation of it, tcp_chr_sync_read, does
> > set the underlying io channel to the blocking mode indeed.
> >
> > Therefore a failure return with EAGAIN is not expected from this call.
> >
> > So do not retry it in qemu_chr_fe_read_all; instead place an assertion
> > that it doesn't fail with EAGAIN.
> >
>
> The code was introduced in :
> commit 7b0bfdf52d694c9a3a96505aa42ce3f8d63acd35
> Author: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
> Date: Tue May 27 15:03:48 2014 +0300
>
> Add chardev API qemu_chr_fe_read_all
Right, but at that point chr_sync_read wasn't made to block. It
happened later in
commit bcdeb9be566ded2eb35233aaccf38742a21e5daa
Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Date: Thu Jul 6 19:03:53 2017 +0200
chardev: block during sync read
A sync read should block until all requested data is
available (instead of retrying in qemu_chr_fe_read_all). Change the
channel to blocking during sync_read.
> > @@ -68,13 +68,10 @@ int qemu_chr_fe_read_all(CharBackend *be, uint8_t
> > *buf, int len)
> > }
> >
> > while (offset < len) {
> > - retry:
> > res = CHARDEV_GET_CLASS(s)->chr_sync_read(s, buf + offset,
> > len - offset);
> > - if (res == -1 && errno == EAGAIN) {
> > - g_usleep(100);
> > - goto retry;
> > - }
> > + /* ->chr_sync_read should block */
> > + assert(!(res < 0 && errno == EAGAIN));
> >
> >
> While I agree with the rationale to clean this code a bit, I am not so sure
> about replacing it with an assert(). In the past, when we did such things
> we had unexpected regressions :)
Valid point, qemu may be run against some OS where a blocking call may
sporadically return -EAGAIN, and it would be hard to reliably catch this
with testing.
> A slightly better approach perhaps is g_warn_if_fail(), although it's not
> very popular in qemu.
I think the first thing to decide is whether -EAGAIN from a blocking
call isn't broken enough, and justifies (unlimited) retries. I'm
tempted to just remove any special handling of -EAGAIN and treat it as
any other error, leaving up to the caller to handle (most probably to
fail the call and initiate a recovery, if possible).
Does this make sense?
Thanks,
Roman.
- Re: [PATCH 05/10] vhost-backend: avoid overflow on memslots_limit, (continued)
[PATCH 07/10] vhost-vdpa: stick to -errno error return convention, Roman Kagan, 2021/11/11
[PATCH 02/10] chardev/char-socket: tcp_chr_recv: don't clobber errno, Roman Kagan, 2021/11/11
[PATCH 09/10] vhost: stick to -errno error return convention, Roman Kagan, 2021/11/11
[PATCH 10/10] vhost-user-blk: propagate error return from generic vhost, Roman Kagan, 2021/11/11
[PATCH 08/10] vhost-user: stick to -errno error return convention, Roman Kagan, 2021/11/11
[PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read, Roman Kagan, 2021/11/11
Re: [PATCH 00/10] vhost: stick to -errno error return convention, Michael S. Tsirkin, 2021/11/11
Re: [PATCH 00/10] vhost: stick to -errno error return convention, Michael S. Tsirkin, 2021/11/28