On Thu, May 13, 2021 at 11:08:25AM -0500, Scott Cheloha wrote:
> > On May 13, 2021, at 10:57 AM, Visa Hankala <[email protected]> wrote:
> >
> > On Thu, May 13, 2021 at 12:04:57AM -0500, Scott Cheloha wrote:
> >> The funky locking dance in softclock() and softclock_thread() is
> >> needed to keep from violating the locking hierarchy. The
> >> timeout_mutex is above the kernel lock, so we need to leave the
> >> timeout_mutex, then drop the kernel lock, and then reenter the
> >> timeout_mutex to start running TIMEOUT_MPSAFE timeouts.
> >
> > Are you sure that dropping the kernel lock in softclock(), in
> > non-process context, is a good idea? That makes assumptions how the
> > layers above work.
>
> I figured it was the most obvious way forward.
>
> When you say "layers above," do you mean softintr_dispatch()?
> Or something else?
I mean softintr_dispatch() and anything else in the dispatch path.
> > At the minimum, I think the soft interrupt code has to be changed so
> > that it is possible to register MP-safe handlers.
>
> Some platforms (e.g. arm64) have softintr_establish_flags(), but this
> is not universally available.
>
> Do I need to unlock all the softintr internals before proceeding
> with this? Or do I just need to bring softrintr_establish_flags()
> to every platform without it?
I think softrintr_establish() could register an MP-safe handler if
IPL_MPSAFE is ORed in the ipl parameter.
Dropping the kernel lock has various implications. At least the code
has to ensure that at most one CPU can run a given handler at a time.
In addition, softintr_disestablish() should probably block until the
handler has stopped (at the moment, this is not guaranteed on the
platforms that have softintr_establish_flags()).
Below is a hasty sketch for amd64 that I am not suggesting to commit.
It is probably wrong in more ways than one.
Index: arch/amd64/amd64/softintr.c
===================================================================
RCS file: src/sys/arch/amd64/amd64/softintr.c,v
retrieving revision 1.10
diff -u -p -r1.10 softintr.c
--- arch/amd64/amd64/softintr.c 11 Sep 2020 09:27:09 -0000 1.10
+++ arch/amd64/amd64/softintr.c 13 May 2021 17:28:51 -0000
@@ -37,6 +37,7 @@
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/malloc.h>
+#include <sys/smr.h>
#include <machine/intr.h>
@@ -85,7 +86,6 @@ softintr_dispatch(int which)
floor = ci->ci_handled_intr_level;
ci->ci_handled_intr_level = ci->ci_ilevel;
- KERNEL_LOCK();
for (;;) {
mtx_enter(&si->softintr_lock);
sih = TAILQ_FIRST(&si->softintr_q);
@@ -95,14 +95,20 @@ softintr_dispatch(int which)
}
TAILQ_REMOVE(&si->softintr_q, sih, sih_q);
sih->sih_pending = 0;
-
- uvmexp.softs++;
-
mtx_leave(&si->softintr_lock);
- (*sih->sih_fn)(sih->sih_arg);
+ atomic_inc_int(&uvmexp.softs);
+
+ if (sih->sih_flags & IPL_MPSAFE) {
+ mtx_enter(&sih->sih_mtx);
+ (*sih->sih_fn)(sih->sih_arg);
+ mtx_leave(&sih->sih_mtx);
+ } else {
+ KERNEL_LOCK();
+ (*sih->sih_fn)(sih->sih_arg);
+ KERNEL_UNLOCK();
+ }
}
- KERNEL_UNLOCK();
ci->ci_handled_intr_level = floor;
}
@@ -117,7 +123,10 @@ softintr_establish(int ipl, void (*func)
{
struct x86_soft_intr *si;
struct x86_soft_intrhand *sih;
- int which;
+ int flags, which;
+
+ flags = ipl & IPL_MPSAFE;
+ ipl &= ~IPL_MPSAFE;
switch (ipl) {
case IPL_SOFTCLOCK:
@@ -141,9 +150,11 @@ softintr_establish(int ipl, void (*func)
sih = malloc(sizeof(*sih), M_DEVBUF, M_NOWAIT);
if (__predict_true(sih != NULL)) {
+ mtx_init(&sih->sih_mtx, IPL_NONE);
sih->sih_intrhead = si;
sih->sih_fn = func;
sih->sih_arg = arg;
+ sih->sih_flags = flags;
sih->sih_pending = 0;
}
return (sih);
@@ -167,5 +178,8 @@ softintr_disestablish(void *arg)
}
mtx_leave(&si->softintr_lock);
+ /* Wait for any running handlers to finish. */
+ smr_barrier();
+
free(sih, M_DEVBUF, sizeof(*sih));
}
Index: arch/amd64/include/intr.h
===================================================================
RCS file: src/sys/arch/amd64/include/intr.h,v
retrieving revision 1.32
diff -u -p -r1.32 intr.h
--- arch/amd64/include/intr.h 17 Jun 2020 06:14:52 -0000 1.32
+++ arch/amd64/include/intr.h 13 May 2021 17:28:51 -0000
@@ -232,13 +232,16 @@ extern void (*ipifunc[X86_NIPI])(struct
#ifndef _LOCORE
#include <sys/queue.h>
+#include <sys/mutex.h>
struct x86_soft_intrhand {
TAILQ_ENTRY(x86_soft_intrhand)
sih_q;
+ struct mutex sih_mtx;
struct x86_soft_intr *sih_intrhead;
void (*sih_fn)(void *);
void *sih_arg;
+ int sih_flags;
int sih_pending;
};
