On 07/04/2026 17:19, Ashutosh Bapat wrote:
Hi Heikki,
CallShmemCallbacksAfterStartup() holds ShmemIndexLock while invoking
init_fn/attach_fn callbacks. That looks wrong. Before this commit,
init or attach code was not run with the lock held. Any reason the
lock is held while calling init and attach callbacks. Since these
function can come from extensions, we don't have control on what goes
in those functions, and thus looks problematic. Further, it will
serialize all the attach_fn executions across backends, since each
will be run under the lock.
This was intentional, I added a note in the docs about it:
When <function>RegisterShmemCallbacks()</function> is called after
startup, it will immediately call the appropriate callbacks,
depending
on whether the requested memory areas were already initialized by
another backend. The callbacks will be called while holding an
internal
lock, which prevents concurrent two backends from initializing the
memory area concurrently.
That "internal lock" is ShmemIndexLock. I piggybacked on that since the
code needs to acquire it anyway for the hash table lookups.
With the old ShmemInitStruct() interface, extensions needed to do the
locking themselves, usually by holding AddinShmemInitLock.
(Now that I read that again, the grammar on the last sentence sounds
awkward...)
In my case, the init_fn was performing ShmemIndex lookup which
deadlocked. It's questionable whether init function should lookup
ShmemIndex but, it's not something that needs to be prohibited
either.
Yeah I'm curious what the use case is. We could easily introduce another
lock or reuse AddinShmemInitLock for this.
- Heikki
