| Message ID | 20211111153354.18807-5-rvkagan@yandex-team.ru (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | vhost: stick to -errno error return convention | expand |
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 Also touched later by Daniel in: commit 53628efbc8aa7a7ab5354d24b971f4d69452151d Author: Daniel P. Berrangé <berrange@redhat.com> 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 <rvkagan@yandex-team.ru> > --- > 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 > > >
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.
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;
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 <rvkagan@yandex-team.ru> --- chardev/char-fe.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)