Re: [PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read
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 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
> 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
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 04/10] chardev/char-fe: don't allow EAGAIN from blocking read
Hi
On Thu, Nov 11, 2021 at 7:44 PM Roman Kagan 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
Date: Tue May 27 15:03:48 2014 +0300
Add chardev API qemu_chr_fe_read_all
Also touched later by Daniel in:
commit 53628efbc8aa7a7ab5354d24b971f4d69452151d
Author: Daniel P. Berrangé
Date: Thu Mar 31 16:29:27 2016 +0100
char: fix broken EAGAIN retry on OS-X due to errno clobbering
> Signed-off-by: Roman Kagan
> ---
> chardev/char-fe.c | 7 ++-
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/chardev/char-fe.c b/chardev/char-fe.c
> index 7789f7be9c..f94efe928e 100644
> --- a/chardev/char-fe.c
> +++ b/chardev/char-fe.c
> @@ -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 :)
A slightly better approach perhaps is g_warn_if_fail(), although it's not
very popular in qemu.
> if (res == 0) {
> break;
> --
> 2.33.1
>
>
>
--
Marc-André Lureau
[PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read
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.
Signed-off-by: Roman Kagan
---
chardev/char-fe.c | 7 ++-
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/chardev/char-fe.c b/chardev/char-fe.c
index 7789f7be9c..f94efe928e 100644
--- a/chardev/char-fe.c
+++ b/chardev/char-fe.c
@@ -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));
if (res == 0) {
break;
--
2.33.1
