the preparing of content before execution is interesting idea, that can be used more. The almost queries and plans are not too big, so when the size of content is not too big - less than 1MB, then can be used one DSM for all backends.
When size of content is bigger than limit, then DSM will be allocated specially for this content. The pointer to DSM and offset can be stored in requested process slot. The reading and writing to requested slot should be protected by spinlock, but it should block only two related processes for short time (copy memory).
Sorry, I don't think this will fly.
The whole idea is that a backend publishes the plan of a query just before running it and it doesn't care which other backend(s) might be reading it, how many times and in which order. The only required locking (implicit) is contained in the code for dsm_attach/detach().
Other possibility is showing the size of content in requested process slot. Then the requester can preallocate enough size of shared memory. But this doesn't solve a issues related to waiting requester for content. So first variant is pretty simple, and should be preferred. The disadvantage is clear - it can enforce maybe significant slowdown of fast queries.
Both of these approaches have just too many synchronization problems, IMO.
