Re: Latches with weak memory ordering (Re: max_wal_senders must die)

Robert Haas <robertmhaas@gmail.com> writes:
> So what DO we need to guard against here?

I think the general problem can be stated as "process A changes two or
more values in shared memory in a fairly short span of time, and process
B, which is concurrently examining the same variables, sees those
changes occur in a different order than A thought it made them in".

In practice we do not need to worry about changes made with a kernel
call in between, as any sort of context swap will cause the kernel to
force cache synchronization.

Also, the intention is that the locking primitives will take care of
this for any shared structures that are protected by a lock.  (There
were some comments upthread suggesting maybe our lock code is not
bulletproof; but if so that's something to fix in the lock code, not
a logic error in code using the locks.)

So what this boils down to is being an issue for shared data structures
that we access without using locks.  As, for example, the latch
structures.

The other case that I can think of offhand is the signal multiplexing
flags.  I think we're all right there so far as the flags themselves are
concerned because only one atomic update is involved on each side:
there's no possibility of inconsistency due to cache visibility skew.
But we'd be at some risk if we were using any such flag as a cue to go
look at some other shared-memory state.
        regards, tom lane