[RFC PATCH] mpqemu: Remove unlock/lock of iothread in mpqemu-link send and recv functions

[Alexander Duyck]

From: Alexander Duyck <alexanderduyck@fb.com>

When I run Multi-process QEMU with an e1000 as the remote device and SMP
enabled I see the combination lock up and become unresponsive. The QEMU build
is a fairly standard x86_64-softmmu setup. After doing some digging I tracked
the lockup down to the what appears to be a race with the mpqemu-link msg_send
and msg_receive functions and the reacquisition of the lock.

I am assuming the issue is some sort of lock inversion though I haven't
identified exactly what the other lock involved is yet. For now removing
the logic to unlock the iothread and then reacquire the lock seems to
resolve the issue. I am assuming the releasing of the lock was some form of
optimization but I am not certain so I am submitting this as an RFC.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
---
 hw/remote/mpqemu-link.c |   25 -------------------------
 1 file changed, 25 deletions(-)

diff --git a/hw/remote/mpqemu-link.c b/hw/remote/mpqemu-link.c
index 9bd98e82197e..3e7818f54a63 100644
--- a/hw/remote/mpqemu-link.c
+++ b/hw/remote/mpqemu-link.c
@@ -33,7 +33,6 @@
  */
 bool mpqemu_msg_send(MPQemuMsg *msg, QIOChannel *ioc, Error **errp)
 {
-    bool iolock = qemu_mutex_iothread_locked();
     bool iothread = qemu_in_iothread();
     struct iovec send[2] = {};
     int *fds = NULL;
@@ -57,16 +56,6 @@ bool mpqemu_msg_send(MPQemuMsg *msg, QIOChannel *ioc, Error 
**errp)
      */
     assert(qemu_in_coroutine() || !iothread);
 
-    /*
-     * Skip unlocking/locking iothread lock when the IOThread is running
-     * in co-routine context. Co-routine context is asserted above
-     * for IOThread case.
-     * Also skip lock handling while in a co-routine in the main context.
-     */
-    if (iolock && !iothread && !qemu_in_coroutine()) {
-        qemu_mutex_unlock_iothread();
-    }
-
     if (!qio_channel_writev_full_all(ioc, send, G_N_ELEMENTS(send),
                                     fds, nfds, 0, errp)) {
         ret = true;
@@ -74,11 +63,6 @@ bool mpqemu_msg_send(MPQemuMsg *msg, QIOChannel *ioc, Error 
**errp)
         trace_mpqemu_send_io_error(msg->cmd, msg->size, nfds);
     }
 
-    if (iolock && !iothread && !qemu_in_coroutine()) {
-        /* See above comment why skip locking here. */
-        qemu_mutex_lock_iothread();
-    }
-
     return ret;
 }
 
@@ -96,7 +80,6 @@ static ssize_t mpqemu_read(QIOChannel *ioc, void *buf, size_t 
len, int **fds,
                            size_t *nfds, Error **errp)
 {
     struct iovec iov = { .iov_base = buf, .iov_len = len };
-    bool iolock = qemu_mutex_iothread_locked();
     bool iothread = qemu_in_iothread();
     int ret = -1;
 
@@ -106,16 +89,8 @@ static ssize_t mpqemu_read(QIOChannel *ioc, void *buf, 
size_t len, int **fds,
      */
     assert(qemu_in_coroutine() || !iothread);
 
-    if (iolock && !iothread && !qemu_in_coroutine()) {
-        qemu_mutex_unlock_iothread();
-    }
-
     ret = qio_channel_readv_full_all_eof(ioc, &iov, 1, fds, nfds, errp);
 
-    if (iolock && !iothread && !qemu_in_coroutine()) {
-        qemu_mutex_lock_iothread();
-    }
-
     return (ret <= 0) ? ret : iov.iov_len;
 }