linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-ID: <Zux0Vyh38FK8OeJ4@boqun-archlinux>
Date: Thu, 19 Sep 2024 11:58:31 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Alan Huang <mmpgouride@...il.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
	linux-mm@...ck.org, lkmm@...ts.linux.dev,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	"Uladzislau Rezki (Sony)" <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
	Neeraj upadhyay <neeraj.upadhyay@....com>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard
 pointers

On Thu, Sep 19, 2024 at 09:57:12PM +0800, Alan Huang wrote:
[...]
> > 
> > I think you're right. (Although the node will be eventually deleted at
> > cleanup_hazptr_context(), however there could be a long-live
> > hazptr_context). It should be:
> > 
> > hazptr_t val = smp_load_acquire(&hzcp->slots[i]);
> > struct hazptr_slot_snap *snap = &hzcp->snaps[i];
> > 
> > if (val != snap->slot) { // val changed, need to update the tree node.
> > // Already in the tree, need to remove first.
> > if (!is_null_or_unused(snap->slot)) {
> > reader_del(tree, snap);
> > }
> > 
> > // use the latest snapshot.
> > snap->slot = val;
> > 
> > // Add it into tree if there is a reader
> > if (!is_null_or_unused(val))
> > reader_add(tree, snap);
> > }
> 
> It seems like that two different hazptr_context can’t be used to protect the same pointer?
> 
> Otherwise the following can happen?
> 
> thread1 					thread2  					 thread3(worker) 	      thread4
> hazptr_tryprotect(hzp1, ptr1)   hazptr_tryprotect(hzp2, ptr1) 
> 												 add ptr1 to tree

Note that we have snapshot rb_node for each hazard pointer slot, so here
thread3 actually would add two rb_nodes with ->slot == ptr1 here.

> hazptr_clear(hzp1) 
> hazptr_tryprotect(hzp1, ptr2) 
> 												 delete ptr1 from tree     unpub ptr1

Therefore, there is still one rb_node with ->slot == ptr1 in the tree
after the deletion, so updaters won't invoke ptr1's callback.

Regards,
Boqun

> 																       call_hazptr(ptr1)
> 																       oops: invoke ptr1's callback
> Or am I missing something?
> 
> > 
> > Regards,
> > Boqun
> > 
> >> I'm not so sure...
> >> 
> >> Thanks
> >> Lai
> 
>