mirrors/qemu
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

graph-lock: Unlock the AioContext while polling

Browse Source 
If the caller keeps the AioContext lock for a block node in an iothread,
polling in bdrv_graph_wrlock() deadlocks if the condition isn't
fulfilled immediately.

Now that all callers make sure to actually have the AioContext locked
when they call bdrv_replace_child_noperm() like they should, we can
change bdrv_graph_wrlock() to take a BlockDriverState whose AioContext
lock the caller holds (NULL if it doesn't) and unlock it temporarily
while polling.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-11-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2023-06-05 10:57:10 +02:00
parent 22dd940544
commit 31b2ddfea3
3 changed files with 28 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
block.c
View File

@ -2854,7 +2854,7 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
* Replaces the node that a BdrvChild points to without updating permissions.
*
* If @new_bs is non-NULL, the parent of @child must already be drained through
* @child.
* @child and the caller must hold the AioContext lock for @new_bs.
*/
static void bdrv_replace_child_noperm(BdrvChild *child,
BlockDriverState *new_bs)
@ -2893,7 +2893,7 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
}
/* TODO Pull this up into the callers to avoid polling here */
bdrv_graph_wrlock();
bdrv_graph_wrlock(new_bs);
if (old_bs) {
if (child->klass->detach) {
child->klass->detach(child);

23
block/graph-lock.c
View File

@ -110,8 +110,10 @@ static uint32_t reader_count(void)
}
#endif
void bdrv_graph_wrlock(void)
void bdrv_graph_wrlock(BlockDriverState *bs)
{
AioContext *ctx = NULL;
GLOBAL_STATE_CODE();
/*
* TODO Some callers hold an AioContext lock when this is called, which
@ -120,7 +122,22 @@ void bdrv_graph_wrlock(void)
*/
#if 0
assert(!qatomic_read(&has_writer));
#endif
/*
* Release only non-mainloop AioContext. The mainloop often relies on the
* BQL and doesn't lock the main AioContext before doing things.
*/
if (bs) {
ctx = bdrv_get_aio_context(bs);
if (ctx != qemu_get_aio_context()) {
aio_context_release(ctx);
} else {
ctx = NULL;
}
}
#if 0
/* Make sure that constantly arriving new I/O doesn't cause starvation */
bdrv_drain_all_begin_nopoll();
@ -150,6 +167,10 @@ void bdrv_graph_wrlock(void)
bdrv_drain_all_end();
#endif
if (ctx) {
aio_context_acquire(bdrv_get_aio_context(bs));
}
}
void bdrv_graph_wrunlock(void)

6
include/block/graph-lock.h
View File

@ -111,10 +111,12 @@ void unregister_aiocontext(AioContext *ctx);
* The wrlock can only be taken from the main loop, with BQL held, as only the
* main loop is allowed to modify the graph.
*
* If @bs is non-NULL, its AioContext is temporarily released.
*
* This function polls. Callers must not hold the lock of any AioContext other
* than the current one.
* than the current one and the one of @bs.
*/
void bdrv_graph_wrlock(void) TSA_ACQUIRE(graph_lock) TSA_NO_TSA;
void bdrv_graph_wrlock(BlockDriverState *bs) TSA_ACQUIRE(graph_lock) TSA_NO_TSA;
/*
* bdrv_graph_wrunlock: