[Qemu-devel] [PATCH 1/1] migration: fix ram decompression race deadlock

["Denis V. Lunev" <den@openvz.org>]

From: Maxim Nestratov <mnestratov@virtuozzo.com>

There is a race in between do_data_decompress and start_decompression.

do_data_decompress()
    while (!quit_decomp_thread) {
        qemu_mutex_lock(&param->mutex);
        while (!param->start && !quit_decomp_thread) {
            qemu_cond_wait(&param->cond, &param->mutex);
            ...
            param->start = false;
        }
        qemu_mutex_unlock(&param->mutex);
        [ preempted here, start_decompression() is executed ]
    }

start_decompression()
{
    qemu_mutex_lock(&param->mutex);
    param->start = true;
    qemu_cond_signal(&param->cond);
    qemu_mutex_unlock(&param->mutex);
}

In this case do_data_decompress will never enter inner loop again and
will eat 100% CPU. The patch fixes this problem by correcting while loop
where we wait for condition only and other actions are moved out of it.

Signed-off-by: Maxim Nestratov <mnestratov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>
---
 migration/ram.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 3f05738..579bfc0 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2193,18 +2193,18 @@ static void *do_data_decompress(void *opaque)
         qemu_mutex_lock(&param->mutex);
         while (!param->start && !quit_decomp_thread) {
             qemu_cond_wait(&param->cond, &param->mutex);
-            pagesize = TARGET_PAGE_SIZE;
-            if (!quit_decomp_thread) {
-                /* uncompress() will return failed in some case, especially
-                 * when the page is dirted when doing the compression, it's
-                 * not a problem because the dirty page will be retransferred
-                 * and uncompress() won't break the data in other pages.
-                 */
-                uncompress((Bytef *)param->des, &pagesize,
-                           (const Bytef *)param->compbuf, param->len);
-            }
-            param->start = false;
         }
+        pagesize = TARGET_PAGE_SIZE;
+        if (!quit_decomp_thread) {
+           /* uncompress() will return failed in some case, especially
+            * when the page is dirted when doing the compression, it's
+            * not a problem because the dirty page will be retransferred
+            * and uncompress() won't break the data in other pages.
+            */
+            uncompress((Bytef *)param->des, &pagesize,
+                       (const Bytef *)param->compbuf, param->len);
+        }
+        param->start = false;
         qemu_mutex_unlock(&param->mutex);
     }
 
-- 
2.1.4