On Tue, May 21, 2024 at 12:57:19PM +0000, Brendan Jackman wrote: > These fields are written by memory hotplug under mem_hotplug_lock but > read without any lock. It seems like reader code is robust against the > value being stale or "from the future", but we also need to account > for: > > 1. Load/store tearing (according to Linus[1], this really happens, > even when everything is aligned as you would hope). > > 2. Invented loads[2] - the compiler can spill and re-read these fields > ([2] calls this "invented loads") and assume that they have not > changed. > > Note we don't need READ_ONCE in paths that have the mem_hotplug_lock > for write, but we still need WRITE_ONCE to prevent store-tearing. > > [1] https://lore.kernel.org/all/CAHk-=wj2t+GK+DGQ7Xy6U7zMf72e7Jkxn4_-kGyfH3WFEoH+YQ@xxxxxxxxxxxxxx/T/#u > As discovered via the original big-bad article[2] > [2] https://lwn.net/Articles/793253/ > > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx> Oh, from a quick look it seems cma_pages would need this too. present_early_pages seems fine. I'll wait a few days in case anyone points out this whole thing is garbage, then check more carefully and send a v2.
