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dafc4d1603
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Paul E. McKenney dafc4d1603 srcu: Update comment after the index flip 
Because there is not guaranteed to be a full memory barrier between
the ->srcu_unlock_count increment of an srcu_read_unlock() and the
->srcu_lock_count increment of the next srcu_read_lock(), this next
srcu_read_lock() is not guaranteed to see the effect of the index flip
just prior to this comment.  However, this next srcu_read_lock() will
execute a full memory barrier, so the srcu_read_lock() after that is
guaranteed to see that index flip.

This guarantee is illustrated by the following diagram of events and
the litmus test following that.

------------------------------------------------------------------------

READER                  UPDATER
-------------           ----------
                           // idx is initially 0.

                           srcu_flip() {
                              smp_mb();
// RSCS

srcu_read_unlock() {
  smp_mb();
                              idx++;    // P
                              smp_mb(); // QQ
                           }

                           srcu_readers_unlock_idx(0) {
        ,--counted------------ count all unlock[0]; // Q
        |
  unlock[0]++;  // X

}
                               smp_mb();
srcu_read_lock() {
  READ(idx) = 0;         ,---- count all lock[0]; // contributes imbalance of 1.
  lock[0]++;  ----counted              |
  smp_mb(); // PP          }           |
}                                      |
                                       |
// RSCS                             not going to effect above scan
                                       |
srcu_read_unlock() {                   |
  smp_mb();                            |
  unlock[0]++;                         |
}                                      |
                                      /
                                     /
srcu_read_lock() {                  |
  READ(idx);  // Y  -----cannot be counted because of P (has to sample idx as 1)
  lock[1]++;
  ...
}

------------------------------------------------------------------------

This makes it similar to the store buffer pattern. Using X, Y, P and Q
annotated above, we get:

------------------------------------------------------------------------

READER                    UPDATER
X (write)                 P (write)

smp_mb(); //PP            smp_mb(); //QQ

Y (read)                  Q (read)

------------------------------------------------------------------------

ASCII art courtesy of Joel Fernandes.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Reported-by: Boqun Feng <boqun.feng@gmail.com>
Reported-by: Frederic Weisbecker <frederic@kernel.org>
Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2023-01-03 17:49:23 -08:00
..
bpf Including fixes from bpf, netfilter and can. 2022-12-21 08:41:32 -08:00
cgroup MM patches for 6.2-rc1. 2022-12-13 19:29:45 -08:00
configs mm, slob: rename CONFIG_SLOB to CONFIG_SLOB_DEPRECATED 2022-12-01 00:09:20 +01:00
debug kdb: use srcu console list iterator 2022-12-02 11:25:00 +01:00
dma dma-mapping: reject GFP_COMP for noncoherent allocations 2022-12-21 08:45:38 +01:00
entry entry: kmsan: introduce kmsan_unpoison_entry_regs() 2022-10-03 14:03:25 -07:00
events New Feature: 2022-12-17 14:06:53 -06:00
futex futex: Resend potentially swallowed owner death notification 2022-12-02 12:20:24 +01:00
gcov gcov: add support for checksum field 2022-12-21 14:31:52 -08:00
irq genirq/msi: Return MSI_XA_DOMAIN_SIZE as the maximum MSI index when no domain is present 2022-12-16 14:04:04 +00:00
kcsan hardening updates for v6.2-rc1 2022-12-14 12:20:00 -08:00
livepatch modules changes for v6.2-rc1 2022-12-13 14:05:39 -08:00
locking MM patches for 6.2-rc1. 2022-12-13 19:29:45 -08:00
module powerpc updates for 6.2 2022-12-19 07:13:33 -06:00
power PM: sleep: Refine error message in try_to_freeze_tasks() 2022-12-06 12:04:34 +01:00
printk Merge branch 'rework/console-list-lock' into for-linus 2022-12-08 11:46:56 +01:00
rcu srcu: Update comment after the index flip 2023-01-03 17:49:23 -08:00
sched hardening updates for v6.2-rc1 2022-12-14 12:20:00 -08:00
time Random number generator updates for Linux 6.2-rc1. 2022-12-12 16:22:22 -08:00
trace Tracing fix for 6.2: 2022-12-21 19:03:42 -08:00
.gitignore
acct.c acct: fix potential integer overflow in encode_comp_t() 2022-11-30 16:13:18 -08:00
async.c Revert "module, async: async_synchronize_full() on module init iff async is used" 2022-02-03 11:20:34 -08:00
audit_fsnotify.c audit: fix potential double free on error path from fsnotify_add_inode_mark 2022-08-22 18:50:06 -04:00
audit_tree.c audit: use fsnotify group lock helpers 2022-04-25 14:37:28 +02:00
audit_watch.c audit_init_parent(): constify path 2022-09-01 17:39:30 -04:00
audit.c audit: use time_after to compare time 2022-08-29 19:47:03 -04:00
audit.h audit: remove selinux_audit_rule_update() declaration 2022-09-07 11:30:15 -04:00
auditfilter.c audit/stable-5.17 PR 20220110 2022-01-11 13:08:21 -08:00
auditsc.c audit: unify audit_filter_{uring(), inode_name(), syscall()} 2022-10-17 14:24:42 -04:00
backtracetest.c
bounds.c mm: multi-gen LRU: minimal implementation 2022-09-26 19:46:09 -07:00
capability.c caps: use type safe idmapping helpers 2022-10-26 10:02:39 +02:00
cfi.c cfi: Switch to -fsanitize=kcfi 2022-09-26 10:13:13 -07:00
compat.c
configs.c
context_tracking.c MAINTAINERS: Add Paul as context tracking maintainer 2022-07-05 13:33:00 -07:00
cpu_pm.c context_tracking: Take IRQ eqs entrypoints over RCU 2022-07-05 13:32:59 -07:00
cpu.c cpu/hotplug: Do not bail-out in DYING/STARTING sections 2022-12-02 12:43:02 +01:00
crash_core.c vmcoreinfo: warn if we exceed vmcoreinfo data size 2022-11-30 16:13:17 -08:00
crash_dump.c
cred.c cred: Do not default to init_cred in prepare_kernel_cred() 2022-11-01 10:04:52 -07:00
delayacct.c delayacct: support re-entrance detection of thrashing accounting 2022-09-26 19:46:07 -07:00
dma.c
exec_domain.c
exit.c exit: Use READ_ONCE() for all oops/warn limit reads 2022-12-16 12:26:57 -08:00
extable.c context_tracking: Take NMI eqs entrypoints over RCU 2022-07-05 13:32:59 -07:00
fail_function.c fail_function: fix wrong use of fei_attr_remove() 2022-09-11 21:55:11 -07:00
fork.c New Feature: 2022-12-17 14:06:53 -06:00
freezer.c freezer,sched: Rewrite core freezer logic 2022-09-07 21:53:50 +02:00
gen_kheaders.sh kbuild: build init/built-in.a just once 2022-09-29 04:40:15 +09:00
groups.c security: Add LSM hook to setgroups() syscall 2022-07-15 18:21:49 +00:00
hung_task.c sched: Fix more TASK_state comparisons 2022-09-30 16:50:39 +02:00
iomem.c
irq_work.c irq_work: use kasan_record_aux_stack_noalloc() record callstack 2022-04-15 14:49:55 -07:00
jump_label.c jump_label: Prevent key->enabled int overflow 2022-12-01 15:53:05 -08:00
kallsyms_internal.h kallsyms: Reduce the memory occupied by kallsyms_seqs_of_names[] 2022-11-12 18:47:36 -08:00
kallsyms_selftest.c kallsyms: Remove unneeded semicolon 2022-11-18 12:56:40 -08:00
kallsyms_selftest.h kallsyms: Add self-test facility 2022-11-15 00:42:02 -08:00
kallsyms.c kallsyms: Add self-test facility 2022-11-15 00:42:02 -08:00
kcmp.c
Kconfig.freezer
Kconfig.hz
Kconfig.locks
Kconfig.preempt Revert "signal, x86: Delay calling signals in atomic on RT enabled kernels" 2022-03-31 10:36:55 +02:00
kcov.c kcov: kmsan: unpoison area->list in kcov_remote_area_put() 2022-10-03 14:03:23 -07:00
kexec_core.c kexec: remove the unneeded result variable 2022-11-18 13:55:07 -08:00
kexec_elf.c
kexec_file.c kexec: replace crash_mem_range with range 2022-11-18 13:55:07 -08:00
kexec_internal.h panic, kexec: make __crash_kexec() NMI safe 2022-09-11 21:55:06 -07:00
kexec.c panic, kexec: make __crash_kexec() NMI safe 2022-09-11 21:55:06 -07:00
kheaders.c
kmod.c
kprobes.c kprobes: kretprobe events missing on 2-core KVM guest 2022-12-15 08:48:40 +09:00
ksysfs.c kernel/ksysfs.c: export kernel cpu byteorder 2022-11-10 19:07:31 +01:00
kthread.c signal: break out of wait loops on kthread_stop() 2022-10-09 16:01:59 -07:00
latencytop.c latencytop: use the last element of latency_record of system 2022-09-11 21:55:12 -07:00
Makefile kernel hardening fixes for v6.2-rc1 2022-12-23 12:00:24 -08:00
module_signature.c
notifier.c notifier: repair slips in kernel-doc comments 2022-11-30 19:32:30 +01:00
nsproxy.c fs/exec: switch timens when a task gets a new mm 2022-10-25 15:15:52 -07:00
padata.c Kbuild updates for v6.2 2022-12-19 12:33:32 -06:00
panic.c kernel hardening fixes for v6.2-rc1 2022-12-23 12:00:24 -08:00
params.c Driver Core changes for 6.2-rc1 2022-12-16 03:54:54 -08:00
pid_namespace.c kernel: pid_namespace: use NULL instead of using plain integer as pointer 2022-04-29 14:38:00 -07:00
pid.c gfs2: Add glockfd debugfs file 2022-06-29 13:07:16 +02:00
profile.c kernel/profile.c: simplify duplicated code in profile_setup() 2022-09-11 21:55:12 -07:00
ptrace.c freezer,sched: Rewrite core freezer logic 2022-09-07 21:53:50 +02:00
range.c
reboot.c kernel/reboot: Add SYS_OFF_MODE_RESTART_PREPARE mode 2022-10-04 15:59:36 +02:00
regset.c
relay.c relay: fix type mismatch when allocating memory in relay_create_buf() 2022-12-11 19:30:19 -08:00
resource_kunit.c
resource.c Driver Core changes for 6.2-rc1 2022-12-16 03:54:54 -08:00
rseq.c rseq: Use pr_warn_once() when deprecated/unknown ABI flags are encountered 2022-11-14 09:58:32 +01:00
scftorture.c scftorture: Fix distribution of short handler delays 2022-04-11 17:07:29 -07:00
scs.c scs: add support for dynamic shadow call stacks 2022-11-09 18:06:35 +00:00
seccomp.c seccomp: Add wait_killable semantic to seccomp user notifier 2022-05-03 14:11:58 -07:00
signal.c hardening updates for v6.2-rc1 2022-12-14 12:20:00 -08:00
smp.c bitmap patches for v6.1-rc1 2022-10-10 12:49:34 -07:00
smpboot.c smpboot: use atomic_try_cmpxchg in cpu_wait_death and cpu_report_death 2022-09-11 21:55:10 -07:00
smpboot.h
softirq.c context_tracking: Take IRQ eqs entrypoints over RCU 2022-07-05 13:32:59 -07:00
stackleak.c stackleak: add on/off stack variants 2022-05-08 01:33:09 -07:00
stacktrace.c uaccess: remove CONFIG_SET_FS 2022-02-25 09:36:06 +01:00
static_call_inline.c static_call: Add call depth tracking support 2022-10-17 16:41:16 +02:00
static_call.c static_call: Don't make __static_call_return0 static 2022-04-05 09:59:38 +02:00
stop_machine.c Scheduler changes in this cycle were: 2022-05-24 11:11:13 -07:00
sys_ni.c kernel/sys_ni: add compat entry for fadvise64_64 2022-08-20 15:17:45 -07:00
sys.c Random number generator updates for Linux 6.1-rc1. 2022-10-10 10:41:21 -07:00
sysctl-test.c kernel/sysctl-test: use SYSCTL_{ZERO/ONE_HUNDRED} instead of i_{zero/one_hundred} 2022-09-08 16:56:45 -07:00
sysctl.c MM patches for 6.2-rc1. 2022-12-13 19:29:45 -08:00
task_work.c task_work: use try_cmpxchg in task_work_add, task_work_cancel_match and task_work_run 2022-09-11 21:55:10 -07:00
taskstats.c genetlink: start to validate reserved header bytes 2022-08-29 12:47:15 +01:00
torture.c torture: Wake up kthreads after storing task_struct pointer 2022-02-01 17:24:39 -08:00
tracepoint.c tracepoint: Optimize the critical region of mutex_lock in tracepoint_module_coming() 2022-09-26 13:01:18 -04:00
tsacct.c taskstats: version 12 with thread group and exe info 2022-04-29 14:38:03 -07:00
ucount.c ucounts: Split rlimit and ucount values and max values 2022-05-18 18:24:57 -05:00
uid16.c
uid16.h
umh.c freezer,sched: Rewrite core freezer logic 2022-09-07 21:53:50 +02:00
up.c
user_namespace.c ucounts: Split rlimit and ucount values and max values 2022-10-09 16:24:05 -07:00
user-return-notifier.c
user.c kernel/user: Allow user_struct::locked_vm to be usable for iommufd 2022-11-30 20:16:49 -04:00
usermode_driver.c blob_to_mnt(): kern_unmount() is needed to undo kern_mount() 2022-05-19 23:25:47 -04:00
utsname_sysctl.c kernel/utsname_sysctl.c: Fix hostname polling 2022-10-23 12:01:01 -07:00
utsname.c
watch_queue.c This was a moderately busy cycle for documentation, but nothing all that 2022-08-02 19:24:24 -07:00
watchdog_hld.c Revert "printk: add functions to prefer direct printing" 2022-06-23 18:41:40 +02:00
watchdog.c powerpc updates for 6.0 2022-08-06 16:38:17 -07:00
workqueue_internal.h
workqueue.c workqueue: Make queue_rcu_work() use call_rcu_hurry() 2022-11-30 13:17:05 -08:00