On Tue, Oct 01, 2024 at 09:02:04PM -0400, Mathieu Desnoyers wrote: > This API provides existence guarantees of objects through Hazard > Pointers (HP). > > Each HP domain defines a fixed number of hazard pointer slots (nr_cpus) > across the entire system. > > Its main benefit over RCU is that it allows fast reclaim of > HP-protected pointers without needing to wait for a grace period. > > It also allows the hazard pointer scan to call a user-defined callback > to retire a hazard pointer slot immediately if needed. This callback > may, for instance, issue an IPI to the relevant CPU. > > There are a few possible use-cases for this in the Linux kernel: > > - Improve performance of mm_count by replacing lazy active mm by HP. > - Guarantee object existence on pointer dereference to use refcount: > - replace locking used for that purpose in some drivers, > - replace RCU + inc_not_zero pattern, > - rtmutex: Improve situations where locks need to be taken in > reverse dependency chain order by guaranteeing existence of > first and second locks in traversal order, allowing them to be > locked in the correct order (which is reverse from traversal > order) rather than try-lock+retry on nested lock. > > References: > > [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for > lock-free objects," in IEEE Transactions on Parallel and > Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004 > > Link: https://lore.kernel.org/lkml/j3scdl5iymjlxavomgc6u5ndg3svhab6ga23dr36o4f5mt333w@7xslvq6b6hmv/ > Link: https://lpc.events/event/18/contributions/1731/ > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > Cc: Will Deacon <will@xxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Cc: John Stultz <jstultz@xxxxxxxxxx> > Cc: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > Cc: Uladzislau Rezki <urezki@xxxxxxxxx> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > Cc: Zqiang <qiang.zhang1211@xxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: Waiman Long <longman@xxxxxxxxxx> > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: maged.michael@xxxxxxxxx > Cc: Mateusz Guzik <mjguzik@xxxxxxxxx> > Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx> > Cc: rcu@xxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx > Cc: lkmm@xxxxxxxxxxxxxxx > --- > include/linux/hp.h | 154 +++++++++++++++++++++++++++++++++++++++++++++ > kernel/Makefile | 2 +- > kernel/hp.c | 46 ++++++++++++++ > 3 files changed, 201 insertions(+), 1 deletion(-) > create mode 100644 include/linux/hp.h > create mode 100644 kernel/hp.c > > diff --git a/include/linux/hp.h b/include/linux/hp.h > new file mode 100644 > index 000000000000..929e8685a0fd > --- /dev/null > +++ b/include/linux/hp.h > @@ -0,0 +1,154 @@ > +// SPDX-FileCopyrightText: 2024 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > +// > +// SPDX-License-Identifier: LGPL-2.1-or-later > + > +#ifndef _LINUX_HP_H > +#define _LINUX_HP_H > + > +/* > + * HP: Hazard Pointers > + * > + * This API provides existence guarantees of objects through hazard > + * pointers. > + * > + * It uses a fixed number of hazard pointer slots (nr_cpus) across the > + * entire system for each HP domain. > + * > + * Its main benefit over RCU is that it allows fast reclaim of > + * HP-protected pointers without needing to wait for a grace period. > + * > + * It also allows the hazard pointer scan to call a user-defined callback > + * to retire a hazard pointer slot immediately if needed. This callback > + * may, for instance, issue an IPI to the relevant CPU. > + * > + * References: > + * > + * [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for > + * lock-free objects," in IEEE Transactions on Parallel and > + * Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004 > + */ > + > +#include <linux/rcupdate.h> > + > +/* > + * Hazard pointer slot. > + */ > +struct hp_slot { > + void *addr; > +}; > + > +/* > + * Hazard pointer context, returned by hp_use(). > + */ > +struct hp_ctx { > + struct hp_slot *slot; > + void *addr; > +}; > + > +/* > + * hp_scan: Scan hazard pointer domain for @addr. > + * > + * Scan hazard pointer domain for @addr. > + * If @retire_cb is NULL, wait to observe that each slot contains a value > + * that differs from @addr. > + * If @retire_cb is non-NULL, invoke @callback for each slot containing > + * @addr. > + */ > +void hp_scan(struct hp_slot __percpu *percpu_slots, void *addr, > + void (*retire_cb)(int cpu, struct hp_slot *slot, void *addr)); > + > +/* Get the hazard pointer context address (may be NULL). */ > +static inline > +void *hp_ctx_addr(struct hp_ctx ctx) > +{ > + return ctx.addr; > +} > + > +/* > + * 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, > + /* > + * 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? > + 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? 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; > +} > + [...]
