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

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Message-ID: <9de1c243-8299-4587-8661-7773cef31a05@huaweicloud.com>
Date: Thu, 19 Sep 2024 20:07:32 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Boqun Feng <boqun.feng@...il.com>, Alan Huang <mmpgouride@...il.com>
Cc: 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>,
 Lai Jiangshan <jiangshanlai@...il.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

Am 9/19/2024 um 8:56 AM schrieb Boqun Feng:
> On Wed, Sep 18, 2024 at 11:17:37PM +0800, Alan Huang wrote:
> [...]
>>> +#define hazptr_tryprotect(hzp, gp, field) (typeof(gp))__hazptr_tryprotect(hzp, (void **)&(gp), offsetof(typeof(*gp), field))
>>> +#define hazptr_protect(hzp, gp, field) ({ \
>>> + typeof(gp) ___p; \
>>> + \
>>> + ___p = hazptr_tryprotect(hzp, gp, field); \
>>> + BUG_ON(!___p); \
>>
>> hazptr_tryprotect might return NULL, do you need a loop here?
>>
> 
> Thanks for the review. It's me who didn't do a good job here on the
> documentation. hazptr_protect() is supposed to use for the case where
> readers know the gp won't change.
> 
> Regards,
> Boqun
> 
>>> + ___p; \
>>> +})
>>> +

Oh, disregard my other e-mail, I hadn't seen this discussion.

Do you have any specific use case of this in mind? If you know that the 
pointer can't change, I would assume you can also just read the pointer 
normally and assign to the hazard pointer without a fence, no?

have fun, jonas