Re: atomic read write

[Alexander Bluhm]

On Thu, Mar 10, 2022 at 04:39:49PM +0100, Alexander Bluhm wrote:
> > Below is a patch that shows how to accomplish release semantics with
> > the file and refcnt APIs. (The added memory barriers ensure that the
> > CPU completes its loads and stores to the object before dropping the
> > reference. Another CPU might delete the object immediately after.
> > The barrier might not be strictly necessary with refcnt_finalize(),
> > though.)

The enter and exit membars that protect the critical section should
be symmetric.  Maybe this diff is better.

And to avoid memory barriers the nobody understands we should convert
FRELE to refcnt_rele.

bluhm

Index: kern/kern_synch.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/kern/kern_synch.c,v
retrieving revision 1.183
diff -u -p -r1.183 kern_synch.c
--- kern/kern_synch.c   10 Mar 2022 15:21:08 -0000      1.183
+++ kern/kern_synch.c   10 Mar 2022 18:11:39 -0000
@@ -805,6 +805,7 @@ void
 refcnt_init(struct refcnt *r)
 {
        atomic_store_int(&r->r_refs, 1);
+       membar_enter_after_atomic();
 }
 
 void
@@ -818,6 +819,7 @@ refcnt_take(struct refcnt *r)
 #else
        atomic_inc_int(&r->r_refs);
 #endif
+       membar_enter_after_atomic();
 }
 
 int
@@ -825,6 +827,7 @@ refcnt_rele(struct refcnt *r)
 {
        u_int refcnt;
 
+       membar_exit_before_atomic();
        refcnt = atomic_dec_int_nv(&r->r_refs);
        KASSERT(refcnt != ~0);
 
@@ -844,6 +847,7 @@ refcnt_finalize(struct refcnt *r, const 
        struct sleep_state sls;
        u_int refcnt;
 
+       membar_exit_before_atomic();
        refcnt = atomic_dec_int_nv(&r->r_refs);
        while (refcnt) {
                sleep_setup(&sls, r, PWAIT, wmesg, 0);
@@ -856,11 +860,13 @@ void
 cond_init(struct cond *c)
 {
        atomic_store_int(&c->c_wait, 1);
+       membar_enter_after_atomic();
 }
 
 void
 cond_signal(struct cond *c)
 {
+       membar_exit_before_atomic();
        atomic_store_int(&c->c_wait, 0);
 
        wakeup_one(c);
@@ -872,9 +878,11 @@ cond_wait(struct cond *c, const char *wm
        struct sleep_state sls;
        unsigned int wait;
 
+       membar_exit_before_atomic();
        wait = atomic_load_int(&c->c_wait);
        while (wait) {
                sleep_setup(&sls, c, PWAIT, wmesg, 0);
+               membar_exit_before_atomic();
                wait = atomic_load_int(&c->c_wait);
                sleep_finish(&sls, wait);
        }