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 <rvka...@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.nikol...@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.lur...@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.