Re: [RFC PATCH v2 5/8] block.c: wrlock in bdrv_replace_child_noperm

[Paolo Bonzini]

On 4/26/22 10:51, Emanuele Giuseppe Esposito wrote:
> The only problem here is ->attach and ->detach callbacks
> could call bdrv_{un}apply_subtree_drain(), which itself
> will use a rdlock to navigate through all nodes.
> To avoid deadlocks, take the lock only outside the drains,
> and if we need to both attach and detach, do it in a single
> critical section.
>
> Therefore change ->attach and ->detach to return true if they
> are modifying the lock, so that we don't take it twice or release
> temporarly.

An alternative way to implement this could be

struct GraphLockState {
    bool taken;
}

void bdrv_graph_ensure_wrlock(GraphLockState *state)
{
    if (state->taken) {
        return;
    }
    bdrv_graph_wrlock();
    state->taken = true;
}

void bdrv_graph_ensure_wrunlock(GraphLockState *state)
{
    ...
}

and pass the GraphLockState* to ->attach and ->detach.

Paolo