Messages in this thread | | | Subject | Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes | From | Thomas Hellstrom <> | Date | Thu, 14 Jun 2018 13:54:15 +0200 |
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On 06/14/2018 01:36 PM, Peter Zijlstra wrote: > On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote: > >> __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) >> { >> struct mutex_waiter *cur; >> + unsigned int is_wait_die = ww_ctx->ww_class->is_wait_die; >> >> lockdep_assert_held(&lock->wait_lock); >> >> @@ -310,13 +348,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) >> if (!cur->ww_ctx) >> continue; >> >> - if (cur->ww_ctx->acquired > 0 && >> + if (is_wait_die && cur->ww_ctx->acquired > 0 && >> __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) { >> debug_mutex_wake_waiter(lock, cur); >> wake_up_process(cur->task); >> } >> >> - break; >> + if (is_wait_die || __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx)) >> + break; >> } >> } > I ended up with: > > > static void __sched > __ww_mutex_check_waiters(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) > { > bool is_wait_die = ww_ctx->ww_class->is_wait_die; > struct mutex_waiter *cur; > > lockdep_assert_held(&lock->wait_lock); > > list_for_each_entry(cur, &lock->wait_list, list) { > if (!cur->ww_ctx) > continue; > > if (is_wait_die) { > /* > * Because __ww_mutex_add_waiter() and > * __ww_mutex_check_stamp() wake any but the earliest > * context, this can only affect the first waiter (with > * a context). > */ > if (cur->ww_ctx->acquired > 0 && > __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) { > debug_mutex_wake_waiter(lock, cur); > wake_up_process(cur->task); > } > > break; > } > > if (__ww_mutex_wound(lock, cur->ww_ctx, ww_ctx)) > break; > } > }
Looks OK to me.
> > Currently you don't allow mixing WD and WW contexts (which is not > immediately obvious from the above code), and the above hard relies on > that. Are there sensible use cases for mixing them? IOW will your > current restriction stand without hassle?
Contexts _must_ agree on the algorithm used to resolve deadlocks. With Wait-Die, for example, older transactions will wait if a lock is held by a younger transaction and with Wound-Wait, younger transactions will wait if a lock is held by an older transaction so there is no way of mixing them.
Thanks,
/Thomas
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