CVS commit: src/sys/kern

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Mon May  1 12:17:56 UTC 2023

Modified Files:
        src/sys/kern: kern_mutex.c

Log Message:
mutex(9): Omit needless membar_consumer.

In practical terms, this is not necessary because MUTEX_SET_WAITERS
already issues MUTEX_MEMBAR_ENTER, which on all architectures is a
sequential consistency barrier, i.e., read/write-before-read/write,
subsuming membar_consumer.

In theoretical terms, MUTEX_MEMBAR_ENTER might imply only
write-before-read/write, so one might imagine that the
read-before-read ordering of membar_consumer _could_ be necessary.
However, the memory operations that are significant here are:

1. load owner := mtx->mtx_owner
2. store mtx->mtx_owner := owner | MUTEX_BIT_WAITERS
3. load owner->l_cpu->ci_curlwp to test if equal to owner

(1) is program-before (2) and at the same memory location,
mtx->mtx_owner, so (1) happens-before (2).

And (2) is separated in program order by MUTEX_MEMBAR_ENTER from (3),
so (2) happens-before (3).

So even if the membar_consumer were intended to guarantee that (1)
happens-before (3), it's not necessary, because we can already prove
it from MUTEX_MEMBAR_ENTER.

But actually, we don't really need (1) happens-before (3), exactly;
what we really need is (2) happens-before (3), since this is a little
manifestation of Dekker's algorithm between cpu_switchto and
mutex_exit, where each CPU sets one flag and must ensure it is
visible to the other CPUs before testing the other flag -- one flag
here is the MUTEX_BIT_WAITERS bit, and the other `flag' here is the
condition owner->l_cpu->ci_curlwp == owner; the corresponding logic,
in cpu_switchto, is:

1'. store owner->l_cpu->ci_curlwp := owner
2'. load mtx->mtx_owner to test if MUTEX_BIT_WAITERS set


To generate a diff of this commit:
cvs rdiff -u -r1.105 -r1.106 src/sys/kern/kern_mutex.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.