[PULL 06/19] gluster: Do not move coroutine into BDS context

[Kevin Wolf <kwolf@redhat.com>]

From: Hanna Czenczek <hreitz@redhat.com>

The request coroutine may not run in the BDS AioContext.  We should wake
it in its own context, not move it.

With that, we can remove GlusterAIOCB.aio_context.

Also add a comment why aio_co_schedule() is safe to use in this way.

**Note:** Due to a lack of a gluster set-up, I have not tested this
commit.  It seemed safe enough to send anyway, just maybe not to
qemu-stable.  To be clear, I don’t know of any user-visible bugs that
would arise from the state without this patch; the request coroutine is
moved into the main BDS AioContext, so guest device completion code will
run in a different context than where the request started, which can’t
be good, but I haven’t actually confirmed any bugs (due to not being
able to test it).

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20251110154854.151484-7-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/gluster.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/block/gluster.c b/block/gluster.c
index 89abd40f31..4fb25b2c6d 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -56,7 +56,6 @@ typedef struct GlusterAIOCB {
     int64_t size;
     int ret;
     Coroutine *coroutine;
-    AioContext *aio_context;
 } GlusterAIOCB;
 
 typedef struct BDRVGlusterState {
@@ -743,7 +742,17 @@ static void gluster_finish_aiocb(struct glfs_fd *fd, ssize_t ret,
         acb->ret = -EIO; /* Partial read/write - fail it */
     }
 
-    aio_co_schedule(acb->aio_context, acb->coroutine);
+    /*
+     * Safe to call: The coroutine will yield exactly once awaiting this
+     * scheduling, and the context is its own context, so it will be scheduled
+     * once it does yield.
+     *
+     * (aio_co_wake() would call qemu_get_current_aio_context() to check whether
+     * we are in the same context, but we are not in a qemu thread, so we cannot
+     * do that.  Use aio_co_schedule() directly.)
+     */
+    aio_co_schedule(qemu_coroutine_get_aio_context(acb->coroutine),
+                    acb->coroutine);
 }
 
 static void qemu_gluster_parse_flags(int bdrv_flags, int *open_flags)
@@ -1006,7 +1015,6 @@ static coroutine_fn int qemu_gluster_co_pwrite_zeroes(BlockDriverState *bs,
     acb.size = bytes;
     acb.ret = 0;
     acb.coroutine = qemu_coroutine_self();
-    acb.aio_context = bdrv_get_aio_context(bs);
 
     ret = glfs_zerofill_async(s->fd, offset, bytes, gluster_finish_aiocb, &acb);
     if (ret < 0) {
@@ -1184,7 +1192,6 @@ static coroutine_fn int qemu_gluster_co_rw(BlockDriverState *bs,
     acb.size = size;
     acb.ret = 0;
     acb.coroutine = qemu_coroutine_self();
-    acb.aio_context = bdrv_get_aio_context(bs);
 
     if (write) {
         ret = glfs_pwritev_async(s->fd, qiov->iov, qiov->niov, offset, 0,
@@ -1251,7 +1258,6 @@ static coroutine_fn int qemu_gluster_co_flush_to_disk(BlockDriverState *bs)
     acb.size = 0;
     acb.ret = 0;
     acb.coroutine = qemu_coroutine_self();
-    acb.aio_context = bdrv_get_aio_context(bs);
 
     ret = glfs_fsync_async(s->fd, gluster_finish_aiocb, &acb);
     if (ret < 0) {
@@ -1299,7 +1305,6 @@ static coroutine_fn int qemu_gluster_co_pdiscard(BlockDriverState *bs,
     acb.size = 0;
     acb.ret = 0;
     acb.coroutine = qemu_coroutine_self();
-    acb.aio_context = bdrv_get_aio_context(bs);
 
     ret = glfs_discard_async(s->fd, offset, bytes, gluster_finish_aiocb, &acb);
     if (ret < 0) {
-- 
2.51.1