diff options
| -rw-r--r-- | docs/rcu.txt | 285 | ||||
| -rw-r--r-- | hw/9pfs/virtio-9p-synth.c | 1 | ||||
| -rw-r--r-- | include/qemu/atomic.h | 61 | ||||
| -rw-r--r-- | include/qemu/queue.h | 13 | ||||
| -rw-r--r-- | include/qemu/rcu.h | 112 | ||||
| -rw-r--r-- | include/qemu/thread.h | 3 | ||||
| -rw-r--r-- | util/Makefile.objs | 1 | ||||
| -rw-r--r-- | util/rcu.c | 172 |
8 files changed, 645 insertions, 3 deletions
diff --git a/docs/rcu.txt b/docs/rcu.txt new file mode 100644 index 0000000..9938ad3 --- /dev/null +++ b/docs/rcu.txt @@ -0,0 +1,285 @@ +Using RCU (Read-Copy-Update) for synchronization +================================================ + +Read-copy update (RCU) is a synchronization mechanism that is used to +protect read-mostly data structures. RCU is very efficient and scalable +on the read side (it is wait-free), and thus can make the read paths +extremely fast. + +RCU supports concurrency between a single writer and multiple readers, +thus it is not used alone. Typically, the write-side will use a lock to +serialize multiple updates, but other approaches are possible (e.g., +restricting updates to a single task). In QEMU, when a lock is used, +this will often be the "iothread mutex", also known as the "big QEMU +lock" (BQL). Also, restricting updates to a single task is done in +QEMU using the "bottom half" API. + +RCU is fundamentally a "wait-to-finish" mechanism. The read side marks +sections of code with "critical sections", and the update side will wait +for the execution of all *currently running* critical sections before +proceeding, or before asynchronously executing a callback. + +The key point here is that only the currently running critical sections +are waited for; critical sections that are started _after_ the beginning +of the wait do not extend the wait, despite running concurrently with +the updater. This is the reason why RCU is more scalable than, +for example, reader-writer locks. It is so much more scalable that +the system will have a single instance of the RCU mechanism; a single +mechanism can be used for an arbitrary number of "things", without +having to worry about things such as contention or deadlocks. + +How is this possible? The basic idea is to split updates in two phases, +"removal" and "reclamation". During removal, we ensure that subsequent +readers will not be able to get a reference to the old data. After +removal has completed, a critical section will not be able to access +the old data. Therefore, critical sections that begin after removal +do not matter; as soon as all previous critical sections have finished, +there cannot be any readers who hold references to the data structure, +and these can now be safely reclaimed (e.g., freed or unref'ed). + +Here is a picutre: + + thread 1 thread 2 thread 3 + ------------------- ------------------------ ------------------- + enter RCU crit.sec. + | finish removal phase + | begin wait + | | enter RCU crit.sec. + exit RCU crit.sec | | + complete wait | + begin reclamation phase | + exit RCU crit.sec. + + +Note how thread 3 is still executing its critical section when thread 2 +starts reclaiming data. This is possible, because the old version of the +data structure was not accessible at the time thread 3 began executing +that critical section. + + +RCU API +======= + +The core RCU API is small: + + void rcu_read_lock(void); + + Used by a reader to inform the reclaimer that the reader is + entering an RCU read-side critical section. + + void rcu_read_unlock(void); + + Used by a reader to inform the reclaimer that the reader is + exiting an RCU read-side critical section. Note that RCU + read-side critical sections may be nested and/or overlapping. + + void synchronize_rcu(void); + + Blocks until all pre-existing RCU read-side critical sections + on all threads have completed. This marks the end of the removal + phase and the beginning of reclamation phase. + + Note that it would be valid for another update to come while + synchronize_rcu is running. Because of this, it is better that + the updater releases any locks it may hold before calling + synchronize_rcu. + + typeof(*p) atomic_rcu_read(p); + + atomic_rcu_read() is similar to atomic_mb_read(), but it makes + some assumptions on the code that calls it. This allows a more + optimized implementation. + + atomic_rcu_read assumes that whenever a single RCU critical + section reads multiple shared data, these reads are either + data-dependent or need no ordering. This is almost always the + case when using RCU, because read-side critical sections typically + navigate one or more pointers (the pointers that are changed on + every update) until reaching a data structure of interest, + and then read from there. + + RCU read-side critical sections must use atomic_rcu_read() to + read data, unless concurrent writes are presented by another + synchronization mechanism. + + Furthermore, RCU read-side critical sections should traverse the + data structure in a single direction, opposite to the direction + in which the updater initializes it. + + void atomic_rcu_set(p, typeof(*p) v); + + atomic_rcu_set() is also similar to atomic_mb_set(), and it also + makes assumptions on the code that calls it in order to allow a more + optimized implementation. + + In particular, atomic_rcu_set() suffices for synchronization + with readers, if the updater never mutates a field within a + data item that is already accessible to readers. This is the + case when initializing a new copy of the RCU-protected data + structure; just ensure that initialization of *p is carried out + before atomic_rcu_set() makes the data item visible to readers. + If this rule is observed, writes will happen in the opposite + order as reads in the RCU read-side critical sections (or if + there is just one update), and there will be no need for other + synchronization mechanism to coordinate the accesses. + +The following APIs must be used before RCU is used in a thread: + + void rcu_register_thread(void); + + Mark a thread as taking part in the RCU mechanism. Such a thread + will have to report quiescent points regularly, either manually + or through the QemuCond/QemuSemaphore/QemuEvent APIs. + + void rcu_unregister_thread(void); + + Mark a thread as not taking part anymore in the RCU mechanism. + It is not a problem if such a thread reports quiescent points, + either manually or by using the QemuCond/QemuSemaphore/QemuEvent + APIs. + +Note that these APIs are relatively heavyweight, and should _not_ be +nested. + + +DIFFERENCES WITH LINUX +====================== + +- Waiting on a mutex is possible, though discouraged, within an RCU critical + section. This is because spinlocks are rarely (if ever) used in userspace + programming; not allowing this would prevent upgrading an RCU read-side + critical section to become an updater. + +- atomic_rcu_read and atomic_rcu_set replace rcu_dereference and + rcu_assign_pointer. They take a _pointer_ to the variable being accessed. + + +RCU PATTERNS +============ + +Many patterns using read-writer locks translate directly to RCU, with +the advantages of higher scalability and deadlock immunity. + +In general, RCU can be used whenever it is possible to create a new +"version" of a data structure every time the updater runs. This may +sound like a very strict restriction, however: + +- the updater does not mean "everything that writes to a data structure", + but rather "everything that involves a reclamation step". See the + array example below + +- in some cases, creating a new version of a data structure may actually + be very cheap. For example, modifying the "next" pointer of a singly + linked list is effectively creating a new version of the list. + +Here are some frequently-used RCU idioms that are worth noting. + + +RCU list processing +------------------- + +TBD (not yet used in QEMU) + + +RCU reference counting +---------------------- + +Because grace periods are not allowed to complete while there is an RCU +read-side critical section in progress, the RCU read-side primitives +may be used as a restricted reference-counting mechanism. For example, +consider the following code fragment: + + rcu_read_lock(); + p = atomic_rcu_read(&foo); + /* do something with p. */ + rcu_read_unlock(); + +The RCU read-side critical section ensures that the value of "p" remains +valid until after the rcu_read_unlock(). In some sense, it is acquiring +a reference to p that is later released when the critical section ends. +The write side looks simply like this (with appropriate locking): + + qemu_mutex_lock(&foo_mutex); + old = foo; + atomic_rcu_set(&foo, new); + qemu_mutex_unlock(&foo_mutex); + synchronize_rcu(); + free(old); + +Note that the same idiom would be possible with reader/writer +locks: + + read_lock(&foo_rwlock); write_mutex_lock(&foo_rwlock); + p = foo; p = foo; + /* do something with p. */ foo = new; + read_unlock(&foo_rwlock); free(p); + write_mutex_unlock(&foo_rwlock); + free(p); + + +RCU resizable arrays +-------------------- + +Resizable arrays can be used with RCU. The expensive RCU synchronization +only needs to take place when the array is resized. The two items to +take care of are: + +- ensuring that the old version of the array is available between removal + and reclamation; + +- avoiding mismatches in the read side between the array data and the + array size. + +The first problem is avoided simply by not using realloc. Instead, +each resize will allocate a new array and copy the old data into it. +The second problem would arise if the size and the data pointers were +two members of a larger struct: + + struct mystuff { + ... + int data_size; + int data_alloc; + T *data; + ... + }; + +Instead, we store the size of the array with the array itself: + + struct arr { + int size; + int alloc; + T data[]; + }; + struct arr *global_array; + + read side: + rcu_read_lock(); + struct arr *array = atomic_rcu_read(&global_array); + x = i < array->size ? array->data[i] : -1; + rcu_read_unlock(); + return x; + + write side (running under a lock): + if (global_array->size == global_array->alloc) { + /* Creating a new version. */ + new_array = g_malloc(sizeof(struct arr) + + global_array->alloc * 2 * sizeof(T)); + new_array->size = global_array->size; + new_array->alloc = global_array->alloc * 2; + memcpy(new_array->data, global_array->data, + global_array->alloc * sizeof(T)); + + /* Removal phase. */ + old_array = global_array; + atomic_rcu_set(&new_array->data, new_array); + synchronize_rcu(); + + /* Reclamation phase. */ + free(old_array); + } + + +SOURCES +======= + +* Documentation/RCU/ from the Linux kernel diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c index 71262bc..e75aa87 100644 --- a/hw/9pfs/virtio-9p-synth.c +++ b/hw/9pfs/virtio-9p-synth.c @@ -17,6 +17,7 @@ #include "virtio-9p-xattr.h" #include "fsdev/qemu-fsdev.h" #include "virtio-9p-synth.h" +#include "qemu/rcu.h" #include <sys/stat.h> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h index 93c2ae2..98e05ca 100644 --- a/include/qemu/atomic.h +++ b/include/qemu/atomic.h @@ -129,6 +129,67 @@ #define atomic_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i)) #endif +/** + * atomic_rcu_read - reads a RCU-protected pointer to a local variable + * into a RCU read-side critical section. The pointer can later be safely + * dereferenced within the critical section. + * + * This ensures that the pointer copy is invariant thorough the whole critical + * section. + * + * Inserts memory barriers on architectures that require them (currently only + * Alpha) and documents which pointers are protected by RCU. + * + * Unless the __ATOMIC_CONSUME memory order is available, atomic_rcu_read also + * includes a compiler barrier to ensure that value-speculative optimizations + * (e.g. VSS: Value Speculation Scheduling) does not perform the data read + * before the pointer read by speculating the value of the pointer. On new + * enough compilers, atomic_load takes care of such concern about + * dependency-breaking optimizations. + * + * Should match atomic_rcu_set(), atomic_xchg(), atomic_cmpxchg(). + */ +#ifndef atomic_rcu_read +#ifdef __ATOMIC_CONSUME +#define atomic_rcu_read(ptr) ({ \ + typeof(*ptr) _val; \ + __atomic_load(ptr, &_val, __ATOMIC_CONSUME); \ + _val; \ +}) +#else +#define atomic_rcu_read(ptr) ({ \ + typeof(*ptr) _val = atomic_read(ptr); \ + smp_read_barrier_depends(); \ + _val; \ +}) +#endif +#endif + +/** + * atomic_rcu_set - assigns (publicizes) a pointer to a new data structure + * meant to be read by RCU read-side critical sections. + * + * Documents which pointers will be dereferenced by RCU read-side critical + * sections and adds the required memory barriers on architectures requiring + * them. It also makes sure the compiler does not reorder code initializing the + * data structure before its publication. + * + * Should match atomic_rcu_read(). + */ +#ifndef atomic_rcu_set +#ifdef __ATOMIC_RELEASE +#define atomic_rcu_set(ptr, i) do { \ + typeof(*ptr) _val = (i); \ + __atomic_store(ptr, &_val, __ATOMIC_RELEASE); \ +} while(0) +#else +#define atomic_rcu_set(ptr, i) do { \ + smp_wmb(); \ + atomic_set(ptr, i); \ +} while (0) +#endif +#endif + /* These have the same semantics as Java volatile variables. * See http://gee.cs.oswego.edu/dl/jmm/cookbook.html: * "1. Issue a StoreStore barrier (wmb) before each volatile store." diff --git a/include/qemu/queue.h b/include/qemu/queue.h index a98eb3a..c602797 100644 --- a/include/qemu/queue.h +++ b/include/qemu/queue.h @@ -104,6 +104,19 @@ struct { \ (head)->lh_first = NULL; \ } while (/*CONSTCOND*/0) +#define QLIST_SWAP(dstlist, srclist, field) do { \ + void *tmplist; \ + tmplist = (srclist)->lh_first; \ + (srclist)->lh_first = (dstlist)->lh_first; \ + if ((srclist)->lh_first != NULL) { \ + (srclist)->lh_first->field.le_prev = &(srclist)->lh_first; \ + } \ + (dstlist)->lh_first = tmplist; \ + if ((dstlist)->lh_first != NULL) { \ + (dstlist)->lh_first->field.le_prev = &(dstlist)->lh_first; \ + } \ +} while (/*CONSTCOND*/0) + #define QLIST_INSERT_AFTER(listelm, elm, field) do { \ if (((elm)->field.le_next = (listelm)->field.le_next) != NULL) \ (listelm)->field.le_next->field.le_prev = \ diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h new file mode 100644 index 0000000..cfef36e --- /dev/null +++ b/include/qemu/rcu.h @@ -0,0 +1,112 @@ +#ifndef QEMU_RCU_H +#define QEMU_RCU_H + +/* + * urcu-mb.h + * + * Userspace RCU header with explicit memory barrier. + * + * This library is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2.1 of the License, or (at your option) any later version. + * + * This library is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this library; if not, write to the Free Software + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA + * + * IBM's contributions to this file may be relicensed under LGPLv2 or later. + */ + +#include <stdlib.h> +#include <assert.h> +#include <limits.h> +#include <unistd.h> +#include <stdint.h> +#include <stdbool.h> +#include <glib.h> + +#include "qemu/compiler.h" +#include "qemu/thread.h" +#include "qemu/queue.h" +#include "qemu/atomic.h" + +#ifdef __cplusplus +extern "C" { +#endif + +/* + * Important ! + * + * Each thread containing read-side critical sections must be registered + * with rcu_register_thread() before calling rcu_read_lock(). + * rcu_unregister_thread() should be called before the thread exits. + */ + +#ifdef DEBUG_RCU +#define rcu_assert(args...) assert(args) +#else +#define rcu_assert(args...) +#endif + +/* + * Global quiescent period counter with low-order bits unused. + * Using a int rather than a char to eliminate false register dependencies + * causing stalls on some architectures. + */ +extern unsigned long rcu_gp_ctr; + +extern QemuEvent rcu_gp_event; + +struct rcu_reader_data { + /* Data used by both reader and synchronize_rcu() */ + unsigned long ctr; + bool waiting; + + /* Data used for registry, protected by rcu_gp_lock */ + QLIST_ENTRY(rcu_reader_data) node; +}; + +extern __thread struct rcu_reader_data rcu_reader; + +static inline void rcu_read_lock(void) +{ + struct rcu_reader_data *p_rcu_reader = &rcu_reader; + + unsigned ctr = atomic_read(&rcu_gp_ctr); + atomic_xchg(&p_rcu_reader->ctr, ctr); + if (atomic_read(&p_rcu_reader->waiting)) { + atomic_set(&p_rcu_reader->waiting, false); + qemu_event_set(&rcu_gp_event); + } +} + +static inline void rcu_read_unlock(void) +{ + struct rcu_reader_data *p_rcu_reader = &rcu_reader; + + atomic_xchg(&p_rcu_reader->ctr, 0); + if (atomic_read(&p_rcu_reader->waiting)) { + atomic_set(&p_rcu_reader->waiting, false); + qemu_event_set(&rcu_gp_event); + } +} + +extern void synchronize_rcu(void); + +/* + * Reader thread registration. + */ +extern void rcu_register_thread(void); +extern void rcu_unregister_thread(void); + +#ifdef __cplusplus +} +#endif + +#endif /* QEMU_RCU_H */ diff --git a/include/qemu/thread.h b/include/qemu/thread.h index e89fdc9..5114ec8 100644 --- a/include/qemu/thread.h +++ b/include/qemu/thread.h @@ -25,9 +25,6 @@ void qemu_mutex_lock(QemuMutex *mutex); int qemu_mutex_trylock(QemuMutex *mutex); void qemu_mutex_unlock(QemuMutex *mutex); -#define rcu_read_lock() do { } while (0) -#define rcu_read_unlock() do { } while (0) - void qemu_cond_init(QemuCond *cond); void qemu_cond_destroy(QemuCond *cond); diff --git a/util/Makefile.objs b/util/Makefile.objs index 93007e2..ceaba30 100644 --- a/util/Makefile.objs +++ b/util/Makefile.objs @@ -17,3 +17,4 @@ util-obj-y += throttle.o util-obj-y += getauxval.o util-obj-y += readline.o util-obj-y += rfifolock.o +util-obj-y += rcu.o diff --git a/util/rcu.c b/util/rcu.c new file mode 100644 index 0000000..1f737d5 --- /dev/null +++ b/util/rcu.c @@ -0,0 +1,172 @@ +/* + * urcu-mb.c + * + * Userspace RCU library with explicit memory barriers + * + * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> + * Copyright (c) 2009 Paul E. McKenney, IBM Corporation. + * Copyright 2015 Red Hat, Inc. + * + * Ported to QEMU by Paolo Bonzini <pbonzini@redhat.com> + * + * This library is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2.1 of the License, or (at your option) any later version. + * + * This library is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this library; if not, write to the Free Software + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA + * + * IBM's contributions to this file may be relicensed under LGPLv2 or later. + */ + +#include <stdio.h> +#include <assert.h> +#include <stdlib.h> +#include <stdint.h> +#include <errno.h> +#include "qemu/rcu.h" +#include "qemu/atomic.h" + +/* + * Global grace period counter. Bit 0 is always one in rcu_gp_ctr. + * Bits 1 and above are defined in synchronize_rcu. + */ +#define RCU_GP_LOCKED (1UL << 0) +#define RCU_GP_CTR (1UL << 1) + +unsigned long rcu_gp_ctr = RCU_GP_LOCKED; + +QemuEvent rcu_gp_event; +static QemuMutex rcu_gp_lock; + +/* + * Check whether a quiescent state was crossed between the beginning of + * update_counter_and_wait and now. + */ +static inline int rcu_gp_ongoing(unsigned long *ctr) +{ + unsigned long v; + + v = atomic_read(ctr); + return v && (v != rcu_gp_ctr); +} + +/* Written to only by each individual reader. Read by both the reader and the + * writers. + */ +__thread struct rcu_reader_data rcu_reader; + +/* Protected by rcu_gp_lock. */ +typedef QLIST_HEAD(, rcu_reader_data) ThreadList; +static ThreadList registry = QLIST_HEAD_INITIALIZER(registry); + +/* Wait for previous parity/grace period to be empty of readers. */ +static void wait_for_readers(void) +{ + ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders); + struct rcu_reader_data *index, *tmp; + + for (;;) { + /* We want to be notified of changes made to rcu_gp_ongoing + * while we walk the list. + */ + qemu_event_reset(&rcu_gp_event); + + /* Instead of using atomic_mb_set for index->waiting, and + * atomic_mb_read for index->ctr, memory barriers are placed + * manually since writes to different threads are independent. + * atomic_mb_set has a smp_wmb before... + */ + smp_wmb(); + QLIST_FOREACH(index, ®istry, node) { + atomic_set(&index->waiting, true); + } + + /* ... and a smp_mb after. */ + smp_mb(); + + QLIST_FOREACH_SAFE(index, ®istry, node, tmp) { + if (!rcu_gp_ongoing(&index->ctr)) { + QLIST_REMOVE(index, node); + QLIST_INSERT_HEAD(&qsreaders, index, node); + + /* No need for mb_set here, worst of all we + * get some extra futex wakeups. + */ + atomic_set(&index->waiting, false); + } + } + + /* atomic_mb_read has smp_rmb after. */ + smp_rmb(); + + if (QLIST_EMPTY(®istry)) { + break; + } + + /* Wait for one thread to report a quiescent state and + * try again. + */ + qemu_event_wait(&rcu_gp_event); + } + + /* put back the reader list in the registry */ + QLIST_SWAP(®istry, &qsreaders, node); +} + +void synchronize_rcu(void) +{ + qemu_mutex_lock(&rcu_gp_lock); + + if (!QLIST_EMPTY(®istry)) { + /* In either case, the atomic_mb_set below blocks stores that free + * old RCU-protected pointers. + */ + if (sizeof(rcu_gp_ctr) < 8) { + /* For architectures with 32-bit longs, a two-subphases algorithm + * ensures we do not encounter overflow bugs. + * + * Switch parity: 0 -> 1, 1 -> 0. + */ + atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); + wait_for_readers(); + atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); + } else { + /* Increment current grace period. */ + atomic_mb_set(&rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); + } + + wait_for_readers(); + } + + qemu_mutex_unlock(&rcu_gp_lock); +} + +void rcu_register_thread(void) +{ + assert(rcu_reader.ctr == 0); + qemu_mutex_lock(&rcu_gp_lock); + QLIST_INSERT_HEAD(®istry, &rcu_reader, node); + qemu_mutex_unlock(&rcu_gp_lock); +} + +void rcu_unregister_thread(void) +{ + qemu_mutex_lock(&rcu_gp_lock); + QLIST_REMOVE(&rcu_reader, node); + qemu_mutex_unlock(&rcu_gp_lock); +} + +static void __attribute__((__constructor__)) rcu_init(void) +{ + qemu_mutex_init(&rcu_gp_lock); + qemu_event_init(&rcu_gp_event, true); + rcu_register_thread(); +} |