> Date: Sat, 1 Jun 2019 17:32:52 -0300
> From: Martin Pieuchot <m...@openbsd.org>
>
> Currently it isn't safe to call mtx_enter_try(9) if you're already
> holding the mutex. That means it isn't safe to call that function
> in hardclock(9), like with `windup_mtx'. That's why the mutex needs
> to be initialized as IPL_CLOCK.
>
> I'm working on removing the SCHED_LOCK() from inside hardclock(9).
> That leads me to wonder if I should initialize all mutexes to IPL_SCHED,
> possibly blocking clock interrupts, or if we should change the mutex API
> to allow mtx_enter_try(9) to deal with recursion.
>
> The diff below removes the recursion check for mtx_enter_try(9).
>
> Comments? Oks?
My initial reaction is that if you're trying to lock when you already
have the lock, there is something wrong with your locking strategy and
that this is something we don't want.
> Index: kern/kern_lock.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_lock.c,v
> retrieving revision 1.69
> diff -u -p -r1.69 kern_lock.c
> --- kern/kern_lock.c 23 Apr 2019 13:35:12 -0000 1.69
> +++ kern/kern_lock.c 1 Jun 2019 18:26:39 -0000
> @@ -251,6 +251,8 @@ __mtx_init(struct mutex *mtx, int wantip
> }
>
> #ifdef MULTIPROCESSOR
> +int _mtx_enter_try(struct mutex *, int);
> +
> void
> mtx_enter(struct mutex *mtx)
> {
> @@ -263,7 +265,7 @@ mtx_enter(struct mutex *mtx)
> LOP_EXCLUSIVE | LOP_NEWORDER, NULL);
>
> spc->spc_spinning++;
> - while (mtx_enter_try(mtx) == 0) {
> + while (_mtx_enter_try(mtx, 0) == 0) {
> CPU_BUSY_CYCLE();
>
> #ifdef MP_LOCKDEBUG
> @@ -278,7 +280,7 @@ mtx_enter(struct mutex *mtx)
> }
>
> int
> -mtx_enter_try(struct mutex *mtx)
> +_mtx_enter_try(struct mutex *mtx, int try)
> {
> struct cpu_info *owner, *ci = curcpu();
> int s;
> @@ -292,7 +294,7 @@ mtx_enter_try(struct mutex *mtx)
>
> owner = atomic_cas_ptr(&mtx->mtx_owner, NULL, ci);
> #ifdef DIAGNOSTIC
> - if (__predict_false(owner == ci))
> + if (!try && __predict_false(owner == ci))
> panic("mtx %p: locking against myself", mtx);
> #endif
> if (owner == NULL) {
> @@ -310,6 +312,12 @@ mtx_enter_try(struct mutex *mtx)
> splx(s);
>
> return (0);
> +}
> +
> +int
> +mtx_enter_try(struct mutex *mtx)
> +{
> + return _mtx_enter_try(mtx, 1);
> }
> #else
> void
>
>
