On February 9, 2025 5:09:51 PM PST, Rik van Riel <riel@xxxxxxxxxxx> wrote: >On Sun, 2025-02-09 at 13:57 -0800, Linus Torvalds wrote: >> >> So on x86, both read and write barriers are complete no-ops, because >> all reads are ordered, and all writes are ordered. > >Given that this thread started with a reference >to rdtsc, it may be worth keeping in mind that >rdtsc reads themselves do not always appear to >be ordered. > >Paul and I spotted some occasionaly "backwards >TSC values" from the CSD lock instrumentation code, >which went away when using ordered TSC reads: > >https://lkml.iu.edu/hypermail/linux/kernel/2410.1/03202.html > >I guess maybe a TSC read does not follow all the same >rules as a memory read, sometimes? > It probably doesn't, at least on uarches.
