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SubjectRe: linux-next: manual merge of the rcu tree with the tip tree
----- On Aug 1, 2017, at 12:03 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Tue, Aug 01, 2017 at 12:04:05AM +0000, Mathieu Desnoyers wrote:
>> ----- On Jul 31, 2017, at 12:13 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com
>> wrote:
>>
>> > On Mon, Jul 31, 2017 at 01:50:29PM +1000, Stephen Rothwell wrote:
>> >> Hi Paul,
>> >>
>> >> Today's linux-next merge of the rcu tree got a conflict in:
>> >>
>> >> arch/x86/mm/tlb.c
>> >>
>> >> between commit:
>> >>
>> >> 94b1b03b519b ("x86/mm: Rework lazy TLB mode and TLB freshness tracking")
>> >>
>> >> from the tip tree and commit:
>> >>
>> >> d7713e8f8b23 ("membarrier: Expedited private command")
>> >>
>> >> from the rcu tree.
>> >>
>> >> I fixed it up (the former removed the comment and the load_cr3(), so I
>> >> just dropped the commend change in the latter) and can carry the fix as
>> >> necessary. This is now fixed as far as linux-next is concerned, but any
>> >> non trivial conflicts should be mentioned to your upstream maintainer
>> >> when your tree is submitted for merging. You may also want to consider
>> >> cooperating with the maintainer of the conflicting tree to minimise any
>> >> particularly complex conflicts.
>> >
>> > Thank you, Stephen!
>> >
>> > Mathieu, Peter, our commit log reads as if removal of load_cr3() would
>> > simply result in relying on the ordering provided by the atomic ops
>> > in switch_mm() for mm_cpumask(), so that only the commit log and the
>> > comment need changing.
>> >
>> > Please let me know if I am missing something here.
>>
>> I think you are right. Both load_cr3() and mm_cpumask update operations
>> (LOCK prefixed) provide the appropriate barriers on x86. So it's just a
>> matter of adapting the comment to the new reality.
>
> Like this?

The updated comment in the commit message looks good, but I would be
tempted to add a comment in x86 switch_mm_irqs_off() stating the
following requirement just before the line invoking cpumask_set_cpu():

/*
* The full memory barrier implied by mm_cpumask update operations
* is required by the membarrier system call.
*/

Thanks,

Mathieu

>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit fde19879b6bd1abc0c1d4d5f945efed61bf7eb8c
> Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Date: Fri Jul 28 16:40:40 2017 -0400
>
> membarrier: Expedited private command
>
> Implement MEMBARRIER_CMD_PRIVATE_EXPEDITED with IPIs using cpumask built
> from all runqueues for which current thread's mm is the same as the
> thread calling sys_membarrier. It executes faster than the non-expedited
> variant (no blocking). It also works on NOHZ_FULL configurations.
>
> Scheduler-wise, it requires a memory barrier before and after context
> switching between processes (which have different mm). The memory
> barrier before context switch is already present. For the barrier after
> context switch:
>
> * Our TSO archs can do RELEASE without being a full barrier. Look at
> x86 spin_unlock() being a regular STORE for example. But for those
> archs, all atomics imply smp_mb and all of them have atomic ops in
> switch_mm() for mm_cpumask().
>
> * From all weakly ordered machines, only ARM64 and PPC can do RELEASE,
> the rest does indeed do smp_mb(), so there the spin_unlock() is a full
> barrier and we're good.
>
> * ARM64 has a very heavy barrier in switch_to(), which suffices.
>
> * PPC just removed its barrier from switch_to(), but appears to be
> talking about adding something to switch_mm(). So add a
> smp_mb__after_unlock_lock() for now, until this is settled on the PPC
> side.
>
> Changes since v3:
> - Properly document the memory barriers provided by each architecture.
>
> Changes since v2:
> - Address comments from Peter Zijlstra,
> - Add smp_mb__after_unlock_lock() after finish_lock_switch() in
> finish_task_switch() to add the memory barrier we need after storing
> to rq->curr. This is much simpler than the previous approach relying
> on atomic_dec_and_test() in mmdrop(), which actually added a memory
> barrier in the common case of switching between userspace processes.
> - Return -EINVAL when MEMBARRIER_CMD_SHARED is used on a nohz_full
> kernel, rather than having the whole membarrier system call returning
> -ENOSYS. Indeed, CMD_PRIVATE_EXPEDITED is compatible with nohz_full.
> Adapt the CMD_QUERY mask accordingly.
>
> Changes since v1:
> - move membarrier code under kernel/sched/ because it uses the
> scheduler runqueue,
> - only add the barrier when we switch from a kernel thread. The case
> where we switch from a user-space thread is already handled by
> the atomic_dec_and_test() in mmdrop().
> - add a comment to mmdrop() documenting the requirement on the implicit
> memory barrier.
>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Andrew Hunter <ahh@google.com>
> CC: Maged Michael <maged.michael@gmail.com>
> CC: gromer@google.com
> CC: Avi Kivity <avi@scylladb.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Tested-by: Dave Watson <davejwatson@fb.com>
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index f66488dfdbc9..3b035584272f 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8621,7 +8621,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> M: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> L: linux-kernel@vger.kernel.org
> S: Supported
> -F: kernel/membarrier.c
> +F: kernel/sched/membarrier.c
> F: include/uapi/linux/membarrier.h
>
> MEMORY MANAGEMENT
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 659ae8094ed5..c8f7d98d8cb9 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -360,6 +360,8 @@ __notrace_funcgraph struct task_struct *__switch_to(struct
> task_struct *prev,
> /*
> * Complete any pending TLB or cache maintenance on this CPU in case
> * the thread migrates to a different CPU.
> + * This full barrier is also required by the membarrier system
> + * call.
> */
> dsb(ish);
>
> diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
> index e0b108bd2624..6d47b3249d8a 100644
> --- a/include/uapi/linux/membarrier.h
> +++ b/include/uapi/linux/membarrier.h
> @@ -40,14 +40,33 @@
> * (non-running threads are de facto in such a
> * state). This covers threads from all processes
> * running on the system. This command returns 0.
> + * @MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> + * Execute a memory barrier on each running
> + * thread belonging to the same process as the current
> + * thread. Upon return from system call, the
> + * caller thread is ensured that all its running
> + * threads siblings have passed through a state
> + * where all memory accesses to user-space
> + * addresses match program order between entry
> + * to and return from the system call
> + * (non-running threads are de facto in such a
> + * state). This only covers threads from the
> + * same processes as the caller thread. This
> + * command returns 0. The "expedited" commands
> + * complete faster than the non-expedited ones,
> + * they never block, but have the downside of
> + * causing extra overhead.
> *
> * Command to be passed to the membarrier system call. The commands need to
> * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
> * the value 0.
> */
> enum membarrier_cmd {
> - MEMBARRIER_CMD_QUERY = 0,
> - MEMBARRIER_CMD_SHARED = (1 << 0),
> + MEMBARRIER_CMD_QUERY = 0,
> + MEMBARRIER_CMD_SHARED = (1 << 0),
> + /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
> + /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
> + MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
> };
>
> #endif /* _UAPI_LINUX_MEMBARRIER_H */
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 4cb8e8b23c6e..9c323a6daa46 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -108,7 +108,6 @@ obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
> obj-$(CONFIG_JUMP_LABEL) += jump_label.o
> obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
> obj-$(CONFIG_TORTURE_TEST) += torture.o
> -obj-$(CONFIG_MEMBARRIER) += membarrier.o
>
> obj-$(CONFIG_HAS_IOMEM) += memremap.o
>
> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> deleted file mode 100644
> index 9f9284f37f8d..000000000000
> --- a/kernel/membarrier.c
> +++ /dev/null
> @@ -1,70 +0,0 @@
> -/*
> - * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> - *
> - * membarrier system call
> - *
> - * This program is free software; you can redistribute it and/or modify
> - * it under the terms of the GNU General Public License as published by
> - * the Free Software Foundation; either version 2 of the License, or
> - * (at your option) any later version.
> - *
> - * This program 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 General Public License for more details.
> - */
> -
> -#include <linux/syscalls.h>
> -#include <linux/membarrier.h>
> -#include <linux/tick.h>
> -
> -/*
> - * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> - * except MEMBARRIER_CMD_QUERY.
> - */
> -#define MEMBARRIER_CMD_BITMASK (MEMBARRIER_CMD_SHARED)
> -
> -/**
> - * sys_membarrier - issue memory barriers on a set of threads
> - * @cmd: Takes command values defined in enum membarrier_cmd.
> - * @flags: Currently needs to be 0. For future extensions.
> - *
> - * If this system call is not implemented, -ENOSYS is returned. If the
> - * command specified does not exist, or if the command argument is invalid,
> - * this system call returns -EINVAL. For a given command, with flags argument
> - * set to 0, this system call is guaranteed to always return the same value
> - * until reboot.
> - *
> - * All memory accesses performed in program order from each targeted thread
> - * is guaranteed to be ordered with respect to sys_membarrier(). If we use
> - * the semantic "barrier()" to represent a compiler barrier forcing memory
> - * accesses to be performed in program order across the barrier, and
> - * smp_mb() to represent explicit memory barriers forcing full memory
> - * ordering across the barrier, we have the following ordering table for
> - * each pair of barrier(), sys_membarrier() and smp_mb():
> - *
> - * The pair ordering is detailed as (O: ordered, X: not ordered):
> - *
> - * barrier() smp_mb() sys_membarrier()
> - * barrier() X X O
> - * smp_mb() X O O
> - * sys_membarrier() O O O
> - */
> -SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> -{
> - /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
> - if (tick_nohz_full_enabled())
> - return -ENOSYS;
> - if (unlikely(flags))
> - return -EINVAL;
> - switch (cmd) {
> - case MEMBARRIER_CMD_QUERY:
> - return MEMBARRIER_CMD_BITMASK;
> - case MEMBARRIER_CMD_SHARED:
> - if (num_online_cpus() > 1)
> - synchronize_sched();
> - return 0;
> - default:
> - return -EINVAL;
> - }
> -}
> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
> index 53f0164ed362..78f54932ea1d 100644
> --- a/kernel/sched/Makefile
> +++ b/kernel/sched/Makefile
> @@ -25,3 +25,4 @@ obj-$(CONFIG_SCHED_DEBUG) += debug.o
> obj-$(CONFIG_CGROUP_CPUACCT) += cpuacct.o
> obj-$(CONFIG_CPU_FREQ) += cpufreq.o
> obj-$(CONFIG_CPU_FREQ_GOV_SCHEDUTIL) += cpufreq_schedutil.o
> +obj-$(CONFIG_MEMBARRIER) += membarrier.o
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bfee6ea7db49..f77269c6c2f8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2640,6 +2640,16 @@ static struct rq *finish_task_switch(struct task_struct
> *prev)
> prev_state = prev->state;
> vtime_task_switch(prev);
> perf_event_task_sched_in(prev, current);
> + /*
> + * The membarrier system call requires a full memory barrier
> + * after storing to rq->curr, before going back to user-space.
> + *
> + * TODO: This smp_mb__after_unlock_lock can go away if PPC end
> + * up adding a full barrier to switch_mm(), or we should figure
> + * out if a smp_mb__after_unlock_lock is really the proper API
> + * to use.
> + */
> + smp_mb__after_unlock_lock();
> finish_lock_switch(rq, prev);
> finish_arch_post_lock_switch();
>
> @@ -3329,6 +3339,21 @@ static void __sched notrace __schedule(bool preempt)
> if (likely(prev != next)) {
> rq->nr_switches++;
> rq->curr = next;
> + /*
> + * The membarrier system call requires each architecture
> + * to have a full memory barrier after updating
> + * rq->curr, before returning to user-space. For TSO
> + * (e.g. x86), the architecture must provide its own
> + * barrier in switch_mm(). For weakly ordered machines
> + * for which spin_unlock() acts as a full memory
> + * barrier, finish_lock_switch() in common code takes
> + * care of this barrier. For weakly ordered machines for
> + * which spin_unlock() acts as a RELEASE barrier (only
> + * arm64 and PowerPC), arm64 has a full barrier in
> + * switch_to(), and PowerPC has
> + * smp_mb__after_unlock_lock() before
> + * finish_lock_switch().
> + */
> ++*switch_count;
>
> trace_sched_switch(preempt, prev, next);
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> new file mode 100644
> index 000000000000..a92fddc22747
> --- /dev/null
> +++ b/kernel/sched/membarrier.c
> @@ -0,0 +1,152 @@
> +/*
> + * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> + *
> + * membarrier system call
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program 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 General Public License for more details.
> + */
> +
> +#include <linux/syscalls.h>
> +#include <linux/membarrier.h>
> +#include <linux/tick.h>
> +#include <linux/cpumask.h>
> +
> +#include "sched.h" /* for cpu_rq(). */
> +
> +/*
> + * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> + * except MEMBARRIER_CMD_QUERY.
> + */
> +#define MEMBARRIER_CMD_BITMASK \
> + (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
> +
> +static void ipi_mb(void *info)
> +{
> + smp_mb(); /* IPIs should be serializing but paranoid. */
> +}
> +
> +static void membarrier_private_expedited(void)
> +{
> + int cpu;
> + bool fallback = false;
> + cpumask_var_t tmpmask;
> +
> + if (num_online_cpus() == 1)
> + return;
> +
> + /*
> + * Matches memory barriers around rq->curr modification in
> + * scheduler.
> + */
> + smp_mb(); /* system call entry is not a mb. */
> +
> + /*
> + * Expedited membarrier commands guarantee that they won't
> + * block, hence the GFP_NOWAIT allocation flag and fallback
> + * implementation.
> + */
> + if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
> + /* Fallback for OOM. */
> + fallback = true;
> + }
> +
> + cpus_read_lock();
> + for_each_online_cpu(cpu) {
> + struct task_struct *p;
> +
> + /*
> + * Skipping the current CPU is OK even through we can be
> + * migrated at any point. The current CPU, at the point
> + * where we read raw_smp_processor_id(), is ensured to
> + * be in program order with respect to the caller
> + * thread. Therefore, we can skip this CPU from the
> + * iteration.
> + */
> + if (cpu == raw_smp_processor_id())
> + continue;
> + rcu_read_lock();
> + p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> + if (p && p->mm == current->mm) {
> + if (!fallback)
> + __cpumask_set_cpu(cpu, tmpmask);
> + else
> + smp_call_function_single(cpu, ipi_mb, NULL, 1);
> + }
> + rcu_read_unlock();
> + }
> + if (!fallback) {
> + smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> + free_cpumask_var(tmpmask);
> + }
> + cpus_read_unlock();
> +
> + /*
> + * Memory barrier on the caller thread _after_ we finished
> + * waiting for the last IPI. Matches memory barriers around
> + * rq->curr modification in scheduler.
> + */
> + smp_mb(); /* exit from system call is not a mb */
> +}
> +
> +/**
> + * sys_membarrier - issue memory barriers on a set of threads
> + * @cmd: Takes command values defined in enum membarrier_cmd.
> + * @flags: Currently needs to be 0. For future extensions.
> + *
> + * If this system call is not implemented, -ENOSYS is returned. If the
> + * command specified does not exist, not available on the running
> + * kernel, or if the command argument is invalid, this system call
> + * returns -EINVAL. For a given command, with flags argument set to 0,
> + * this system call is guaranteed to always return the same value until
> + * reboot.
> + *
> + * All memory accesses performed in program order from each targeted thread
> + * is guaranteed to be ordered with respect to sys_membarrier(). If we use
> + * the semantic "barrier()" to represent a compiler barrier forcing memory
> + * accesses to be performed in program order across the barrier, and
> + * smp_mb() to represent explicit memory barriers forcing full memory
> + * ordering across the barrier, we have the following ordering table for
> + * each pair of barrier(), sys_membarrier() and smp_mb():
> + *
> + * The pair ordering is detailed as (O: ordered, X: not ordered):
> + *
> + * barrier() smp_mb() sys_membarrier()
> + * barrier() X X O
> + * smp_mb() X O O
> + * sys_membarrier() O O O
> + */
> +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +{
> + if (unlikely(flags))
> + return -EINVAL;
> + switch (cmd) {
> + case MEMBARRIER_CMD_QUERY:
> + {
> + int cmd_mask = MEMBARRIER_CMD_BITMASK;
> +
> + if (tick_nohz_full_enabled())
> + cmd_mask &= ~MEMBARRIER_CMD_SHARED;
> + return cmd_mask;
> + }
> + case MEMBARRIER_CMD_SHARED:
> + /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
> + if (tick_nohz_full_enabled())
> + return -EINVAL;
> + if (num_online_cpus() > 1)
> + synchronize_sched();
> + return 0;
> + case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> + membarrier_private_expedited();
> + return 0;
> + default:
> + return -EINVAL;
> + }
> +}
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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