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Re: [PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read

From: Marc-André Lureau
Subject: Re: [PATCH 04/10] chardev/char-fe: don't allow EAGAIN from blocking read
Date: Fri, 12 Nov 2021 12:24:06 +0400


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) {

Marc-André Lureau

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