From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 10B8F1990AA; Thu, 3 Oct 2024 13:32:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=167.114.26.122 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727962377; cv=none; b=hzYUjOlbWJsPJhe1RkuTnehjD7MOY750x/5fqLFtNrqv5hXo5krOxPAhyX0qnw22CoLicwpEg4pdBOdWQUv0EyL7z+LKZEXn3oZW3EFfhiIC/WBLXeg4u4NUexkRfPN3Ur8oazYr+INdW5JeiCBkipmSA2xWiWTa8syD915FwH0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727962377; c=relaxed/simple; bh=YTSZ1gR4pl10kKX9+QjBQhs9v6DfHc5Ds/ZvHb/XL/g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=SuyXXXECcQS0dzyzLnoED/4P7kQS+WTURoBrazw/CL2a8aNqYNHQBtwxHItGnUdGDfcGlIvlSBn9Zh++aV9ZQcnsvJUNo7Fj/pdz0xa8OTbFO78twFMoV8R/y5Wg3QpQ3jeudqY36SEl2WdyJwfhGi02GnRWn1du8IiBxIIKmD0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b=X1GK8v/O; arc=none smtp.client-ip=167.114.26.122 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="X1GK8v/O" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727962375; bh=YTSZ1gR4pl10kKX9+QjBQhs9v6DfHc5Ds/ZvHb/XL/g=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=X1GK8v/OZtESLEHkimlfFxoawsowDAJytBMD2o7/U0FjMAMyY0u+RROxIqV1qNfcO 3VbzfSyj5e74S4TZIP+m3k83XPJdJNWh/ZRB8KvKn7KUAb6g7wvOlIjP+2ynmX6EWw I1hoTW40T0gJ8+mI7oLSWsRRU+KbCt2Lr3PndOE1bIwNRv37f57wCKuVGJ6CaybKuh 4h7vJ/CxdQBINIqr6rnGhWb9eGjU+IV3tL5x2CgUP73uxbOv13P7MtRIjp6p6RAfbj QwGn8sBl9bSG4nUAImNPfpOK1upeqFdU8V3+q3bwlJt0qieL+URAbG8uzQ1lmatJi7 RovViSGiBjP1A== Received: from [172.16.0.134] (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XKCKf3WScz5Zd; Thu, 3 Oct 2024 09:32:54 -0400 (EDT) Message-ID: Date: Thu, 3 Oct 2024 09:30:53 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 3/4] hp: Implement Hazard Pointers To: Boqun Feng Cc: Linus Torvalds , Andrew Morton , Peter Zijlstra , linux-kernel@vger.kernel.org, Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Alan Stern , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev References: <20241002010205.1341915-1-mathieu.desnoyers@efficios.com> <20241002010205.1341915-4-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2024-10-03 02:24, Boqun Feng wrote: > On Tue, Oct 01, 2024 at 09:02:04PM -0400, Mathieu Desnoyers wrote: [...] >> +/* >> + * hp_allocate: Allocate a hazard pointer. >> + * >> + * Allocate a hazard pointer slot for @addr. The object existence should >> + * be guaranteed by the caller. >> + * >> + * Returns a hazard pointer context. >> + */ >> +static inline >> +struct hp_ctx hp_allocate(struct hp_slot __percpu *percpu_slots, void *addr) >> +{ >> + struct hp_slot *slot; >> + struct hp_ctx ctx; >> + >> + if (!addr) >> + goto fail; >> + slot = this_cpu_ptr(percpu_slots); > > Are you assuming this is called with preemption disabled? Otherwise, > there could two threads picking up the same hazard pointer slot on one > CPU, Indeed, this minimalist implementation only covers the preempt-off use-case, where there is a single use of HP per CPU at any given time (e.g. for the lazy mm use-case). It expects to be called from preempt-off context. I will update the comment accordingly. > >> + /* >> + * A single hazard pointer slot per CPU is available currently. >> + * Other hazard pointer domains can eventually have a different >> + * configuration. >> + */ >> + if (READ_ONCE(slot->addr)) >> + goto fail; > > .. and they could both read an empty slot, and both think they > successfully protect the objects, which could be different objects. > > Or am I missing something subtle here? You are correct, I should document this. > >> + WRITE_ONCE(slot->addr, addr); /* Store B */ >> + ctx.slot = slot; >> + ctx.addr = addr; >> + return ctx; >> + >> +fail: >> + ctx.slot = NULL; >> + ctx.addr = NULL; >> + return ctx; >> +} >> + >> +/* >> + * hp_dereference_allocate: Dereference and allocate a hazard pointer. >> + * >> + * Returns a hazard pointer context. >> + */ >> +static inline >> +struct hp_ctx hp_dereference_allocate(struct hp_slot __percpu *percpu_slots, void * const * addr_p) >> +{ >> + struct hp_slot *slot; >> + void *addr, *addr2; >> + struct hp_ctx ctx; >> + >> + addr = READ_ONCE(*addr_p); >> +retry: >> + ctx = hp_allocate(percpu_slots, addr); >> + if (!hp_ctx_addr(ctx)) >> + goto fail; >> + /* Memory ordering: Store B before Load A. */ >> + smp_mb(); >> + /* >> + * Use RCU dereference without lockdep checks, because >> + * lockdep is not aware of HP guarantees. >> + */ >> + addr2 = rcu_access_pointer(*addr_p); /* Load A */ > > Why rcu_access_pointer() instead of READ_ONCE()? Because you want to > mark the head of address dependency? Yes, the intent here is to mark the address dependency and provide a publication guarantee similar to RCU pairing rcu_assign_pointer and rcu_dereference. Do you see any reason why READ_ONCE() would suffice here ? Thanks, Mathieu > > Regards, > Boqun > >> + /* >> + * If @addr_p content has changed since the first load, >> + * clear the hazard pointer and try again. >> + */ >> + if (!ptr_eq(addr2, addr)) { >> + WRITE_ONCE(slot->addr, NULL); >> + if (!addr2) >> + goto fail; >> + addr = addr2; >> + goto retry; >> + } >> + ctx.slot = slot; >> + ctx.addr = addr2; >> + return ctx; >> + >> +fail: >> + ctx.slot = NULL; >> + ctx.addr = NULL; >> + return ctx; >> +} >> + > [...] -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com