Re: [Qemu-devel] [PATCH] block: Fix race in gluster_finish_aiocb

[Bharata B Rao]

On Wed, Aug 21, 2013 at 05:40:11PM +0200, Paolo Bonzini wrote:
> 
> We could just use a bottom half, too.  Add a bottom half to acb,
> schedule it in gluster_finish_aiocb, delete it in the bottom half's own
> callback.

I tried this approach (the patch at the end of this mail), but see this
occasional errors, doesn't happen always. Any clues on what am I missing here ?

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffcbfff700 (LWP 11301)]
0x000055555597156b in event_notifier_set (e=0xa4)
    at util/event_notifier-posix.c:97
97              ret = write(e->wfd, &value, sizeof(value));
(gdb) bt
#0  0x000055555597156b in event_notifier_set (e=0xa4)
    at util/event_notifier-posix.c:97
#1  0x00005555555dd3cd in aio_notify (ctx=0x0) at async.c:233
#2  0x00005555555dd00f in qemu_bh_schedule (bh=0x7fff300008e0) at async.c:128
#3  0x00005555556002da in gluster_finish_aiocb (fd=0x5555562ae5d0, ret=4096, 
    arg=0x7fffd00419c0) at block/gluster.c:409
#4  0x00007ffff5e70cdc in glfs_io_async_cbk (ret=4096, frame=0x0, data=
    0x555556443ee0) at glfs-fops.c:567
#5  0x00007ffff5c2843e in synctask_wrap (old_task=0x555556365940)
    at syncop.c:133
#6  0x00007ffff3b03370 in ?? () from /lib64/libc.so.6
#7  0x0000000000000000 in ?? ()
(gdb) up
#1  0x00005555555dd3cd in aio_notify (ctx=0x0) at async.c:233
233         event_notifier_set(&ctx->notifier);
(gdb) up
#2  0x00005555555dd00f in qemu_bh_schedule (bh=0x7fff300008e0) at async.c:128
128         aio_notify(bh->ctx);
(gdb) p *bh
$1 = {ctx = 0x0, cb = 0x5555555ffdcd <qemu_gluster_aio_bh>, opaque = 
    0x7fffd00419c0, next = 0x555556345e70, scheduled = false, idle = false, 
  deleted = true}


gluster: Use BH mechanism for AIO completion

Replace the existing pipe based AIO completion handing by BH based method.

Signed-off-by: Bharata B Rao <address@hidden>
---
 block/gluster.c |   69 ++++++-------------------------------------------------
 1 file changed, 8 insertions(+), 61 deletions(-)

diff --git a/block/gluster.c b/block/gluster.c
index 46f36f8..598b335 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -30,7 +30,6 @@ typedef struct GlusterAIOCB {
 
 typedef struct BDRVGlusterState {
     struct glfs *glfs;
-    int fds[2];
     struct glfs_fd *fd;
     int event_reader_pos;
     GlusterAIOCB *event_acb;
@@ -231,12 +230,13 @@ out:
     return NULL;
 }
 
-static void qemu_gluster_complete_aio(GlusterAIOCB *acb, BDRVGlusterState *s)
+static void qemu_gluster_aio_bh(void *opaque)
 {
     int ret;
+    GlusterAIOCB *acb = opaque;
     bool *finished = acb->finished;
     BlockDriverCompletionFunc *cb = acb->common.cb;
-    void *opaque = acb->common.opaque;
+    void *cb_opaque = acb->common.opaque;
 
     if (!acb->ret || acb->ret == acb->size) {
         ret = 0; /* Success */
@@ -246,33 +246,15 @@ static void qemu_gluster_complete_aio(GlusterAIOCB *acb, 
BDRVGlusterState *s)
         ret = -EIO; /* Partial read/write - fail it */
     }
 
+    qemu_bh_delete(acb->bh);
     qemu_aio_release(acb);
-    cb(opaque, ret);
+
+    cb(cb_opaque, ret);
     if (finished) {
         *finished = true;
     }
 }
 
-static void qemu_gluster_aio_event_reader(void *opaque)
-{
-    BDRVGlusterState *s = opaque;
-    ssize_t ret;
-
-    do {
-        char *p = (char *)&s->event_acb;
-
-        ret = read(s->fds[GLUSTER_FD_READ], p + s->event_reader_pos,
-                   sizeof(s->event_acb) - s->event_reader_pos);
-        if (ret > 0) {
-            s->event_reader_pos += ret;
-            if (s->event_reader_pos == sizeof(s->event_acb)) {
-                s->event_reader_pos = 0;
-                qemu_gluster_complete_aio(s->event_acb, s);
-            }
-        }
-    } while (ret < 0 && errno == EINTR);
-}
-
 /* TODO Convert to fine grained options */
 static QemuOptsList runtime_opts = {
     .name = "gluster",
@@ -329,17 +311,7 @@ static int qemu_gluster_open(BlockDriverState *bs,  QDict 
*options,
     s->fd = glfs_open(s->glfs, gconf->image, open_flags);
     if (!s->fd) {
         ret = -errno;
-        goto out;
-    }
-
-    ret = qemu_pipe(s->fds);
-    if (ret < 0) {
-        ret = -errno;
-        goto out;
     }
-    fcntl(s->fds[GLUSTER_FD_READ], F_SETFL, O_NONBLOCK);
-    qemu_aio_set_fd_handler(s->fds[GLUSTER_FD_READ],
-        qemu_gluster_aio_event_reader, NULL, s);
 
 out:
     qemu_opts_del(opts);
@@ -417,31 +389,10 @@ static const AIOCBInfo gluster_aiocb_info = {
 static void gluster_finish_aiocb(struct glfs_fd *fd, ssize_t ret, void *arg)
 {
     GlusterAIOCB *acb = (GlusterAIOCB *)arg;
-    BlockDriverState *bs = acb->common.bs;
-    BDRVGlusterState *s = bs->opaque;
-    int retval;
 
     acb->ret = ret;
-    retval = qemu_write_full(s->fds[GLUSTER_FD_WRITE], &acb, sizeof(acb));
-    if (retval != sizeof(acb)) {
-        /*
-         * Gluster AIO callback thread failed to notify the waiting
-         * QEMU thread about IO completion.
-         *
-         * Complete this IO request and make the disk inaccessible for
-         * subsequent reads and writes.
-         */
-        error_report("Gluster failed to notify QEMU about IO completion");
-
-        qemu_mutex_lock_iothread(); /* We are in gluster thread context */
-        acb->common.cb(acb->common.opaque, -EIO);
-        qemu_aio_release(acb);
-        close(s->fds[GLUSTER_FD_READ]);
-        close(s->fds[GLUSTER_FD_WRITE]);
-        qemu_aio_set_fd_handler(s->fds[GLUSTER_FD_READ], NULL, NULL, NULL);
-        bs->drv = NULL; /* Make the disk inaccessible */
-        qemu_mutex_unlock_iothread();
-    }
+    acb->bh = qemu_bh_new(qemu_gluster_aio_bh, acb);
+    qemu_bh_schedule(acb->bh);
 }
 
 static BlockDriverAIOCB *qemu_gluster_aio_rw(BlockDriverState *bs,
@@ -592,10 +543,6 @@ static void qemu_gluster_close(BlockDriverState *bs)
 {
     BDRVGlusterState *s = bs->opaque;
 
-    close(s->fds[GLUSTER_FD_READ]);
-    close(s->fds[GLUSTER_FD_WRITE]);
-    qemu_aio_set_fd_handler(s->fds[GLUSTER_FD_READ], NULL, NULL, NULL);
-
     if (s->fd) {
         glfs_close(s->fd);
         s->fd = NULL;