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From 19651bdbf15a4ce03d6fc6e3a6be514a3f46a118 Mon Sep 17 00:00:00 2001
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From: Fam Zheng <famz@redhat.com>
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Date: Thu, 18 May 2017 09:21:21 +0200
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Subject: [PATCH 08/18] serial: change retry logic to avoid concurrency
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RH-Author: Fam Zheng <famz@redhat.com>
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Message-id: <20170518092131.16571-9-famz@redhat.com>
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Patchwork-id: 75300
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O-Subject: [RHEL-7.4 qemu-kvm PATCH v3 08/18] serial: change retry logic to avoid concurrency
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Bugzilla: 1451470
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Eduardo Habkost <ehabkost@redhat.com>
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From: Kirill Batuzov <batuzovk@ispras.ru>
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Whenever serial_xmit fails to transmit a byte it adds a watch that would
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call it again when the "line" becomes ready. This results in a retry
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chain:
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  serial_xmit -> add_watch -> serial_xmit
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Each chain is able to transmit one character, and for every character
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passed to serial by the guest driver a new chain is spawned.
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The problem lays with the fact that a new chain is spawned even when
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there is one already waiting on the watch. So there can be several retry
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chains waiting concurrently on one "line". Every chain tries to transmit
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current character, so character order is not messed up. But also every
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chain increases retry counter (tsr_retry). If there are enough
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concurrent chains this counter will hit MAX_XMIT_RETRY value and
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the character will be dropped.
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To reproduce this bug you need to feed serial output to some program
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consuming it slowly enough. A python script from bug #1335444
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description is an example of such program.
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This commit changes retry logic in the following way to avoid
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concurrency: instead of spawning a new chain for each character being
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transmitted spawn only one and make it transmit characters until FIFO is
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empty.
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The change consists of two parts:
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 - add a do {} while () loop in serial_xmit (diff is a bit erratic
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   for this part, diff -w will show actual change),
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 - do not call serial_xmit from serial_ioport_write if there is one
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   waiting on the watch already.
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This should fix another issue causing bug #1335444.
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Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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(cherry picked from commit f702e62a193e9ddb41cef95068717e5582b39a64)
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Signed-off-by: Fam Zheng <famz@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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 hw/char/serial.c | 59 +++++++++++++++++++++++++++++++-------------------------
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 1 file changed, 33 insertions(+), 26 deletions(-)
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diff --git a/hw/char/serial.c b/hw/char/serial.c
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index add4738..33e06fb 100644
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--- a/hw/char/serial.c
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+++ b/hw/char/serial.c
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@@ -223,37 +223,42 @@ static gboolean serial_xmit(GIOChannel *chan, GIOCondition cond, void *opaque)
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 {
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     SerialState *s = opaque;
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-    if (s->tsr_retry <= 0) {
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-        if (s->fcr & UART_FCR_FE) {
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-            if (fifo8_is_empty(&s->xmit_fifo)) {
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+    do {
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+        if (s->tsr_retry <= 0) {
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+            if (s->fcr & UART_FCR_FE) {
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+                if (fifo8_is_empty(&s->xmit_fifo)) {
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+                    return FALSE;
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+                }
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+                s->tsr = fifo8_pop(&s->xmit_fifo);
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+                if (!s->xmit_fifo.num) {
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+                    s->lsr |= UART_LSR_THRE;
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+                }
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+            } else if ((s->lsr & UART_LSR_THRE)) {
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                 return FALSE;
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-            }
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-            s->tsr = fifo8_pop(&s->xmit_fifo);
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-            if (!s->xmit_fifo.num) {
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+            } else {
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+                s->tsr = s->thr;
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                 s->lsr |= UART_LSR_THRE;
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+                s->lsr &= ~UART_LSR_TEMT;
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             }
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-        } else if ((s->lsr & UART_LSR_THRE)) {
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-            return FALSE;
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-        } else {
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-            s->tsr = s->thr;
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-            s->lsr |= UART_LSR_THRE;
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-            s->lsr &= ~UART_LSR_TEMT;
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         }
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-    }
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-    if (s->mcr & UART_MCR_LOOP) {
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-        /* in loopback mode, say that we just received a char */
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-        serial_receive1(s, &s->tsr, 1);
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-    } else if (qemu_chr_fe_write(s->chr, &s->tsr, 1) != 1) {
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-        if (s->tsr_retry >= 0 && s->tsr_retry < MAX_XMIT_RETRY &&
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-            qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP, serial_xmit, s) > 0) {
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-            s->tsr_retry++;
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-            return FALSE;
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+        if (s->mcr & UART_MCR_LOOP) {
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+            /* in loopback mode, say that we just received a char */
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+            serial_receive1(s, &s->tsr, 1);
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+        } else if (qemu_chr_fe_write(s->chr, &s->tsr, 1) != 1) {
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+            if (s->tsr_retry >= 0 && s->tsr_retry < MAX_XMIT_RETRY &&
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+                qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,
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+                                      serial_xmit, s) > 0) {
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+                s->tsr_retry++;
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+                return FALSE;
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+            }
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+            s->tsr_retry = 0;
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+        } else {
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+            s->tsr_retry = 0;
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         }
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-        s->tsr_retry = 0;
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-    } else {
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-        s->tsr_retry = 0;
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-    }
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+        /* Transmit another byte if it is already available. It is only
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+           possible when FIFO is enabled and not empty. */
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+    } while ((s->fcr & UART_FCR_FE) && !fifo8_is_empty(&s->xmit_fifo));
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     s->last_xmit_ts = qemu_get_clock_ns(vm_clock);
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@@ -293,7 +298,9 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
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             s->thr_ipending = 0;
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             s->lsr &= ~UART_LSR_THRE;
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             serial_update_irq(s);
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-            serial_xmit(NULL, G_IO_OUT, s);
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+            if (s->tsr_retry <= 0) {
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+                serial_xmit(NULL, G_IO_OUT, s);
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+            }
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         }
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         break;
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     case 1:
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-- 
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1.8.3.1
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