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 15:18:56 +0200 |
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On 06/14/2018 02:48 PM, Thomas Hellstrom wrote: > Hi, Peter, > > On 06/14/2018 02:41 PM, Peter Zijlstra wrote: >> On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote: >>> +static bool __ww_mutex_wound(struct mutex *lock, >>> + struct ww_acquire_ctx *ww_ctx, >>> + struct ww_acquire_ctx *hold_ctx) >>> +{ >>> + struct task_struct *owner = __mutex_owner(lock); >>> + >>> + lockdep_assert_held(&lock->wait_lock); >>> + >>> + if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) && >>> + ww_ctx->acquired > 0) { >>> + hold_ctx->wounded = 1; >>> + >>> + /* >>> + * wake_up_process() paired with set_current_state() inserts >>> + * sufficient barriers to make sure @owner either sees it's >>> + * wounded or has a wakeup pending to re-read the wounded >>> + * state. >>> + * >>> + * The value of hold_ctx->wounded in >>> + * __ww_mutex_lock_check_stamp(); >>> + */ >>> + if (owner != current) >>> + wake_up_process(owner); >>> + >>> + return true; >>> + } >>> + >>> + return false; >>> +} >>> @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex >>> *lock, struct ww_acquire_ctx *ctx) >>> * and keep spinning, or it will acquire wait_lock, add itself >>> * to waiter list and sleep. >>> */ >>> - smp_mb(); /* ^^^ */ >>> + smp_mb(); /* See comments above and below. */ >>> /* >>> - * Check if lock is contended, if not there is nobody to wake up >>> + * Check if lock is contended, if not there is nobody to wake up. >>> + * We can use list_empty() unlocked here since it only compares a >>> + * list_head field pointer to the address of the list head >>> + * itself, similarly to how list_empty() can be considered >>> RCU-safe. >>> + * The memory barrier above pairs with the memory barrier in >>> + * __ww_mutex_add_waiter and makes sure lock->ctx is visible >>> before >>> + * we check for waiters. >>> */ >>> - if (likely(!(atomic_long_read(&lock->base.owner) & >>> MUTEX_FLAG_WAITERS))) >>> + if (likely(list_empty(&lock->base.wait_list))) >>> return; >> OK, so what happens is that if we see !empty list, we take wait_lock, >> if we end up in __ww_mutex_wound() we must really have !empty wait-list. >> >> It can however still see !owner because __mutex_unlock_slowpath() can >> clear the owner field. But if owner is set, it must stay valid because >> FLAG_WAITERS and we're holding wait_lock. > > If __ww_mutex_wound() is called from ww_mutex_set_context_fastpath() > owner is the current process so we can never see !owner. However if > __ww_mutex_wound() is called from __ww_mutex_add_waiter() then the > above is true.
Or actually it was intended to be true, but FLAG_WAITERS is set too late. It needs to be moved to just after we actually add the waiter to the list.
Then the hunk that replaces a FLAG_WAITERS read with a lockless list_empty() can also be ditched.
/Thomas
> >> >> So the wake_up_process() is in fact safe. >> >> Let me put that in a comment. > > > Thanks, > > Thomas > >
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