signal: Exit RCU read-side critical section on each pass through loop

The kill_pid_info() can potentially loop indefinitely if tasks are created
and deleted sufficiently quickly, and if this happens, this function
will remain in a single RCU read-side critical section indefinitely.
This commit therefore exits the RCU read-side critical section on each
pass through the loop.  Because a race must happen to retry the loop,
this should have no performance impact in the common case.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
signal: Document the RCU protection of ->sighand

__cleanup_sighand() frees sighand without RCU grace period. This is
correct but this looks "obviously buggy" and constantly confuses the
readers, add the comments to explain how this works.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Add sparse check for RCU_INIT_POINTER()

Add a sparse check when RCU_INIT_POINTER() is used to assign a non __rcu
annotated pointer.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Optimize cond_resched_rcu_qs()

The current implementation of cond_resched_rcu_qs() can invoke
rcu_note_voluntary_context_switch() twice in the should_resched()
case, once via the call to __schedule() and once directly.  However, as
noted by Joe Lawrence in a patch to the team subsystem, cond_resched()
returns an indication as to whether or not the call to __schedule()
actually happened.  This commit therefore changes cond_resched_rcu_qs()
so as to invoke rcu_note_voluntary_context_switch() only when __schedule()
was not called.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: More info about potential deadlocks with rcu_read_unlock()

The comment above rcu_read_unlock() explains the potential deadlock
if the caller holds one of the locks taken by rt_mutex_unlock() paths,
but it is not clear from this documentation that any lock which can
be taken from interrupt can lead to deadlock as well and we need to
take rt_mutex_lock() into account too.

The problem is that rt_mutex_lock() takes wait_lock without disabling
irqs, and thus an interrupt taking some LOCK can obviously race with
rcu_read_unlock_special() called with the same LOCK held.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Fix FIXME in rcu_tasks_kthread()

This commit affines rcu_tasks_kthread() to the housekeeping CPUs
in CONFIG_NO_HZ_FULL builds.  This is just a default, so systems
administrators are free to put this kthread somewhere else if they wish.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Unify boost and kthread priorities

Rename CONFIG_RCU_BOOST_PRIO to CONFIG_RCU_KTHREAD_PRIO and use this
value for both the per-CPU kthreads (rcuc/N) and the rcu boosting
threads (rcub/n).

Also, create the module_parameter rcutree.kthread_prio to be used on
the kernel command line at boot to set a new value (rcutree.kthread_prio=N).

Signed-off-by: Clark Williams <clark.williams@gmail.com>
[ paulmck: Ported to rcu/dev, applied Paul Bolle and Peter Zijlstra feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Remove redundant TREE_PREEMPT_RCU config option

PREEMPT_RCU and TREE_PREEMPT_RCU serve the same function after
TINY_PREEMPT_RCU has been removed. This patch removes TREE_PREEMPT_RCU
and uses PREEMPT_RCU config option in its place.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Kick rcuo kthreads after their CPU goes offline

If a no-CBs CPU were to post an RCU callback with interrupts disabled
after it entered the idle loop for the last time, there might be no
deferred wakeup for the corresponding rcuo kthreads.  This commit
therefore adds a set of calls to do_nocb_deferred_wakeup() after the
CPU has gone completely offline.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Fix for rcuo online-time-creation reorganization bug

Commit 35ce7f29a44a (rcu: Create rcuo kthreads only for onlined CPUs)
contains checks for the case where CPUs are brought online out of
order, re-wiring the rcuo leader-follower relationships as needed.
Unfortunately, this rewiring was broken.  This apparently went undetected
due to the tendency of systems to bring CPUs online in order.  This commit
nevertheless fixes the rewiring.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: Remove rcu_dynticks * parameters when they are always this_cpu_ptr(&rcu_dynticks)

For some functions in kernel/rcu/tree* the rdtp parameter is always
this_cpu_ptr(rdtp).  Remove the parameter if constant and calculate the
pointer in function.

This will have the advantage that it is obvious that the address are
all per cpu offsets and thus it will enable the use of this_cpu_ops in
the future.

Signed-off-by: Christoph Lameter <cl@linux.com>
[ paulmck: Forward-ported to rcu/dev, whitespace adjustment. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Use DEFINE_PER_CPU_SHARED_ALIGNED for rcu_data

The rcu_data per-CPU variable has a number of fields that are atomically
manipulated, potentially by any CPU.  This situation can result in false
sharing with per-CPU variables that have the misfortune of being allocated
adjacent to rcu_data in memory.  This commit therefore changes the
DEFINE_PER_CPU() to DEFINE_PER_CPU_SHARED_ALIGNED() in order to avoid
this false sharing.

Reported-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_check_callbacks()

The "cpu" argument was kept around on the off-chance that RCU might
offload scheduler-clock interrupts.  However, this offload approach
has been replaced by NO_HZ_FULL, which offloads -all- RCU processing
from qualifying CPUs.  It is therefore time to remove the "cpu" argument
to rcu_check_callbacks(), which this commit does.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_pending()

Because rcu_pending()'s argument is guaranteed to always be the current
CPU, drop the argument and replace per_cpu_ptr() with this_cpu_ptr().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_preempt_check_callbacks()

Because rcu_preempt_check_callbacks()'s argument is guaranteed to
always be the current CPU, drop the argument and replace per_cpu()
with __this_cpu_read().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_note_context_switch()

The "cpu" argument to rcu_note_context_switch() is always the current
CPU, so drop it.  This in turn allows the "cpu" argument to
rcu_preempt_note_context_switch() to be removed, which allows the sole
use of "cpu" in both functions to be replaced with a this_cpu_ptr().
Again, the anticipated cross-CPU uses of these functions has been
replaced by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_needs_cpu()

The "cpu" argument to rcu_needs_cpu() is always the current CPU, so drop
it.  This in turn allows the "cpu" argument to rcu_cpu_has_callbacks()
to be removed, which allows the uses of "cpu" in both functions to be
replaced with a this_cpu_ptr().  Again, the anticipated cross-CPU uses
of these functions has been replaced by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_prepare_for_idle()

The "cpu" argument to rcu_prepare_for_idle() is always the current
CPU, so drop it.  This in turn allows two of the uses of "cpu" in
this function to be replaced with a this_cpu_ptr() and the third by
smp_processor_id(), replacing that of the call to rcu_prepare_for_idle().
Again, the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
rcu: Remove "cpu" argument to rcu_cleanup_after_idle()

The "cpu" argument to rcu_cleanup_after_idle() is always the current
CPU, so drop it.  This moves the smp_processor_id() from the caller to
rcu_cleanup_after_idle(), saving argument-passing overhead.  Again,
the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
cpu: Avoid puts_pending overflow

A long string of get_online_cpus() with each followed by a
put_online_cpu() that fails to acquire cpu_hotplug.lock can result in
overflow of the cpu_hotplug.puts_pending counter.  Although this is
perhaps improbably, a system with absolutely no CPU-hotplug operations
will have an arbitrarily long time in which this overflow could occur.
This commit therefore adds overflow checks to get_online_cpus() and
try_get_online_cpus().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>