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Filename Latest commit message Latest commit date
Paolo Bonzini cdf7abc461 srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context 
Linu Cherian reported a WARN in cleanup_srcu_struct() when shutting
down a guest running iperf on a VFIO assigned device.  This happens
because irqfd_wakeup() calls srcu_read_lock(&kvm->irq_srcu) in interrupt
context, while a worker thread does the same inside kvm_set_irq().  If the
interrupt happens while the worker thread is executing __srcu_read_lock(),
updates to the Classic SRCU ->lock_count[] field or the Tree SRCU
->srcu_lock_count[] field can be lost.

The docs say you are not supposed to call srcu_read_lock() and
srcu_read_unlock() from irq context, but KVM interrupt injection happens
from (host) interrupt context and it would be nice if SRCU supported the
use case.  KVM is using SRCU here not really for the "sleepable" part,
but rather due to its IPI-free fast detection of grace periods.  It is
therefore not desirable to switch back to RCU, which would effectively
revert commit 719d93cd5f ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING",
2014-01-16).

However, the docs are overly conservative.  You can have an SRCU instance
only has users in irq context, and you can mix process and irq context
as long as process context users disable interrupts.  In addition,
__srcu_read_unlock() actually uses this_cpu_dec() on both Tree SRCU and
Classic SRCU.  For those two implementations, only srcu_read_lock()
is unsafe.

When Classic SRCU's __srcu_read_unlock() was changed to use this_cpu_dec(),
in commit 5a41344a3d ("srcu: Simplify __srcu_read_unlock() via
this_cpu_dec()", 2012-11-29), __srcu_read_lock() did two increments.
Therefore it kept __this_cpu_inc(), with preempt_disable/enable in
the caller.  Tree SRCU however only does one increment, so on most
architectures it is more efficient for __srcu_read_lock() to use
this_cpu_inc(), and any performance differences appear to be down in
the noise.

Unlike Classic and Tree SRCU, Tiny SRCU does increments and decrements on
a single variable.  Therefore, as Peter Zijlstra pointed out, Tiny SRCU's
implementation already supports mixed-context use of srcu_read_lock()
and srcu_read_unlock(), at least as long as uses of srcu_read_lock()
and srcu_read_unlock() in each handler are nested and paired properly.
In other words, it is still illegal to (say) invoke srcu_read_lock()
in an interrupt handler and to invoke the matching srcu_read_unlock()
in a softirq handler.  Therefore, the only change required for Tiny SRCU
is to its comments.

Fixes: 719d93cd5f ("kvm/irqchip: Speed up KVM_SET_GSI_ROUTING")
Reported-by: Linu Cherian <linuc.decode@gmail.com>
Suggested-by: Linu Cherian <linuc.decode@gmail.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paolo Bonzini <pbonzini@redhat.com>
2017-06-08 08:24:26 -07:00
..
bpf bpf: fix wrong exposure of map_flags into fdinfo for lpm 2017-05-25 13:44:28 -04:00
cgroup Merge branch 'for-4.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup 2017-05-01 13:52:24 -07:00
configs config: android-base: enable hardened usercopy and kernel ASLR 2017-02-27 18:43:46 -08:00
debug sched/headers: Prepare for new header dependencies before moving code to <linux/sched/debug.h> 2017-03-02 08:42:34 +01:00
events perf/callchain: Force USER_DS when invoking perf_callchain_user() 2017-05-10 07:54:00 +02:00
gcov gcov: support GCC 7.1 2017-05-12 15:57:15 -07:00
irq Merge branch 'irq-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-05-21 11:45:26 -07:00
livepatch livepatch: Make livepatch dependent on !TRIM_UNUSED_KSYMS 2017-05-27 00:27:37 +02:00
locking futex,rt_mutex: Fix rt_mutex_cleanup_proxy_lock() 2017-05-22 21:57:18 +02:00
power Merge branches 'pm-sleep' and 'powercap' 2017-05-22 20:32:05 +02:00
printk TTY/Serial patches for 4.12-rc1 2017-05-08 18:49:23 -07:00
rcu srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context 2017-06-08 08:24:26 -07:00
sched Merge branches 'intel_pstate', 'pm-cpufreq' and 'pm-cpufreq-sched' 2017-05-22 20:28:22 +02:00
time posix-timers: Make signal printks conditional 2017-05-23 23:39:57 +02:00
trace ftrace: Fix memory leak in ftrace_graph_release() 2017-05-26 22:35:48 -04:00
.gitignore
acct.c sched/headers: Prepare to move cputime functionality from <linux/sched.h> into <linux/sched/cputime.h> 2017-03-02 08:42:39 +01:00
async.c
audit.c Merge branch 'stable-4.12' of git://git.infradead.org/users/pcmoore/audit 2017-05-03 09:21:59 -07:00
audit.h audit: Use timespec64 to represent audit timestamps 2017-05-02 10:16:05 -04:00
audit_fsnotify.c Merge branch 'fsnotify' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs 2017-05-03 11:05:15 -07:00
audit_tree.c Merge branch 'fsnotify' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs 2017-05-03 11:05:15 -07:00
audit_watch.c Merge branch 'fsnotify' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs 2017-05-03 11:05:15 -07:00
auditfilter.c audit: kernel generated netlink traffic should have a portid of 0 2017-05-02 10:16:05 -04:00
auditsc.c Merge branch 'fsnotify' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs 2017-05-03 11:05:15 -07:00
backtracetest.c
bounds.c
capability.c capability: export has_capability 2017-01-12 07:01:56 -07:00
compat.c time: Change k_clock nsleep() to use timespec64 2017-04-14 21:49:56 +02:00
configs.c Replace <asm/uaccess.h> with <linux/uaccess.h> globally 2016-12-24 11:46:01 -08:00
context_tracking.c
cpu.c Merge branch 'linus' into locking/core, to pick up fixes 2017-04-14 10:29:40 +02:00
cpu_pm.c
crash_core.c ia64: reuse append_elf_note() and final_note() functions 2017-05-08 17:15:11 -07:00
crash_dump.c
cred.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/coredump.h> 2017-03-02 08:42:28 +01:00
delayacct.c sched/headers: Prepare to move cputime functionality from <linux/sched.h> into <linux/sched/cputime.h> 2017-03-02 08:42:39 +01:00
dma.c
elfcore.c
exec_domain.c
exit.c userfaultfd: non-cooperative: rollback userfaultfd_exit 2017-03-09 17:01:09 -08:00
extable.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2017-02-22 10:15:09 -08:00
fork.c Merge branch 'core-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-05-27 08:52:27 -07:00
freezer.c
futex.c futex: Clarify mark_wake_futex memory barrier usage 2017-04-15 16:03:46 +02:00
futex_compat.c Replace <asm/uaccess.h> with <linux/uaccess.h> globally 2016-12-24 11:46:01 -08:00
groups.c mm, vmalloc: use __GFP_HIGHMEM implicitly 2017-05-08 17:15:13 -07:00
hung_task.c kernel/hung_task.c: defer showing held locks 2017-05-08 17:15:10 -07:00
irq_work.c
jump_label.c This release has no new tracing features, just clean ups, minor fixes 2017-02-27 13:26:17 -08:00
kallsyms.c bpf: make jited programs visible in traces 2017-02-17 13:40:05 -05:00
kcmp.c
Kconfig.freezer
Kconfig.hz
Kconfig.locks
Kconfig.preempt
kcov.c kcov: simplify interrupt check 2017-05-08 17:15:12 -07:00
kexec.c
kexec_core.c ia64: reuse append_elf_note() and final_note() functions 2017-05-08 17:15:11 -07:00
kexec_file.c kexec, x86/purgatory: Unbreak it and clean it up 2017-03-10 20:55:09 +01:00
kexec_internal.h kexec, x86/purgatory: Unbreak it and clean it up 2017-03-10 20:55:09 +01:00
kmod.c sched/headers, vfs/execve: Prepare to move the do_execve*() prototypes from <linux/sched.h> to <linux/binfmts.h> 2017-03-02 08:42:39 +01:00
kprobes.c kprobes/x86: Fix to set RWX bits correctly before releasing trampoline 2017-05-26 22:37:00 -04:00
ksysfs.c crash: move crashkernel parsing and vmcore related code under CONFIG_CRASH_CORE 2017-05-08 17:15:11 -07:00
kthread.c cgroup, kthread: close race window where new kthreads can be migrated to non-root cgroups 2017-03-17 10:18:47 -04:00
latencytop.c sched/headers: Prepare to move sched_info_on() and force_schedstat_enabled() from <linux/sched.h> to <linux/sched/stat.h> 2017-03-02 08:42:39 +01:00
Makefile crash: move crashkernel parsing and vmcore related code under CONFIG_CRASH_CORE 2017-05-08 17:15:11 -07:00
membarrier.c Fix: Disable sys_membarrier when nohz_full is enabled 2017-01-23 11:32:16 -08:00
memremap.c mm, zone_device: Replace {get, put}_zone_device_page() with a single reference to fix pmem crash 2017-05-01 09:15:53 +02:00
module-internal.h
module.c kernel/module.c: use set_memory.h header 2017-05-08 17:15:14 -07:00
module_signing.c
notifier.c kernel/notifier.c: simplify expression 2017-02-24 17:46:56 -08:00
nsproxy.c perf: Add PERF_RECORD_NAMESPACES to include namespaces related info 2017-03-13 15:57:41 -03:00
padata.c padata: get_next is never NULL 2017-04-21 20:30:46 +08:00
panic.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/debug.h> 2017-03-02 08:42:34 +01:00
params.c boot/param: Move next_arg() function to lib/cmdline.c for later reuse 2017-04-18 10:37:13 +02:00
pid.c pidns: disable pid allocation if pid_ns_prepare_proc() is failed in alloc_pid() 2017-05-08 17:15:12 -07:00
pid_namespace.c pid_ns: Sleep in TASK_INTERRUPTIBLE in zap_pid_ns_processes 2017-05-13 17:26:01 -05:00
profile.c sched/headers: Prepare to move sched_info_on() and force_schedstat_enabled() from <linux/sched.h> to <linux/sched/stat.h> 2017-03-02 08:42:39 +01:00
ptrace.c ptrace: Properly initialize ptracer_cred on fork 2017-05-23 07:40:44 -05:00
range.c
reboot.c
relay.c Merge branch 'work.splice' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-05-02 11:38:06 -07:00
resource.c
seccomp.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/task_stack.h> 2017-03-02 08:42:36 +01:00
signal.c Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-05-10 10:30:46 -07:00
smp.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/idle.h> 2017-03-02 08:42:26 +01:00
smpboot.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/task.h> 2017-03-02 08:42:35 +01:00
smpboot.h
softirq.c sched/core: Remove 'task' parameter and rename tsk_restore_flags() to current_restore_flags() 2017-04-11 09:06:32 +02:00
stacktrace.c stacktrace/x86: add function for detecting reliable stack traces 2017-03-08 09:18:02 +01:00
stop_machine.c
sys.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2017-05-05 11:08:43 -07:00
sys_ni.c move aio compat to fs/aio.c 2016-12-22 22:58:37 -05:00
sysctl.c proc/sysctl: fix the int overflow for jiffies conversion 2017-05-08 17:15:10 -07:00
sysctl_binary.c net: Remove NET_CORE_BUDGET_USECS from sysctl binary interface. 2017-04-21 15:59:52 -04:00
task_work.c
taskstats.c taskstats: add e/u/stime for TGID command 2017-05-08 17:15:12 -07:00
test_kprobes.c
torture.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/clock.h> 2017-03-02 08:42:27 +01:00
tracepoint.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/task.h> 2017-03-02 08:42:35 +01:00
tsacct.c sched/headers: Prepare to move cputime functionality from <linux/sched.h> into <linux/sched/cputime.h> 2017-03-02 08:42:39 +01:00
ucount.c ucount: Remove the atomicity from ucount->count 2017-03-06 15:26:37 -06:00
uid16.c sched/headers: Prepare to remove <linux/cred.h> inclusion from <linux/sched.h> 2017-03-02 08:42:31 +01:00
up.c
user-return-notifier.c
user.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/user.h> 2017-03-02 08:42:29 +01:00
user_namespace.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/signal.h> 2017-03-02 08:42:29 +01:00
utsname.c sched/headers: Prepare to move the task_lock()/unlock() APIs to <linux/sched/task.h> 2017-03-02 08:42:38 +01:00
utsname_sysctl.c sched/headers: Remove <linux/rwsem.h> from <linux/sched.h> 2017-03-03 01:45:36 +01:00
watchdog.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/debug.h> 2017-03-02 08:42:34 +01:00
watchdog_hld.c sched/headers: Prepare for new header dependencies before moving code to <linux/sched/debug.h> 2017-03-02 08:42:34 +01:00
workqueue.c Merge branch 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-05-01 19:12:53 -07:00
workqueue_internal.h