lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Thu, 02 Jul 2020 19:39:16 +0100 From: Valentin Schneider <valentin.schneider@....com> To: Peter Zijlstra <peterz@...radead.org> Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, vincent.guittot@...aro.org, mgorman@...e.de, Oleg Nesterov <oleg@...hat.com>, david@...morbit.com Subject: Re: [RFC][PATCH] sched: Better document ttwu() Hi, On 02/07/20 13:52, Peter Zijlstra wrote: > Dave hit the problem fixed by commit: > > b6e13e85829f ("sched/core: Fix ttwu() race") > > and failed to understand much of the code involved. Per his request a > few comments to (hopefully) clarify things. > All of the below is already tremendously helpful! I remember it took me quite some time to figure out e.g. how we could observe p->on_cpu && !p->on_rq and why we cared about it in ttwu(). On the bright side, I'm happy that my notes aren't completely off - there may be hope for me yet. Have some small comments below. > Requested-by: Dave Chinner <david@...morbit.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org> > --- > include/linux/sched.h | 12 ++-- > kernel/sched/core.c | 195 +++++++++++++++++++++++++++++++++++++++++++------- > kernel/sched/sched.h | 11 +++ > 3 files changed, 187 insertions(+), 31 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9bd073a10224..ad36f70bef24 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h [...] > + * Special state: > + * > + * System-calls and anything external will use task_rq_lock() which acquires > + * both p->lock and rq->lock. As a consequence the state they change is stable > + * while holding either lock: > + * > + * - sched_setaffinity(): p->cpus_ptr > + * - set_user_nice(): p->se.load, p->static_prio > + * - __sched_setscheduler(): p->sched_class, p->policy, p->*prio, p->se.load, > + * p->dl.dl_{runtime, deadline, period, flags, bw, density} Only extra thing that comes to mind is p->uclamp*; dunno how exhaustive you want this list to be. > + * - sched_setnuma(): p->numa_preferred_nid > + * - sched_move_task()/ > + * cpu_cgroup_fork(): p->sched_task_group > + * > + * p->state <- TASK_*: > + * > + * is changed locklessly using set_current_state(), __set_current_state() or > + * set_special_state(), see their respective comments, or by > + * try_to_wake_up(). This latter uses p->pi_lock to serialize against > + * concurrent self. > + * > + * p->on_rq <- { 0, 1 = TASK_ON_RQ_QUEUED, 2 = TASK_ON_RQ_MIGRATING }: > + * > + * is set by activate_task() and cleared by deactivate_task(), under > + * rq->lock. Non-zero indicates the task is runnable, the special > + * ON_RQ_MIGRATING state is used for migration without holding both > + * rq->locks. It indicates task_cpu() is not stable, see task_rq_lock(). > + * > + * p->on_cpu <- { 0, 1 }: > + * > + * is set by prepare_task() and cleared by finish_task() such that it will be > + * set before p is scheduled-in and cleared after p is scheduled-out, both > + * under rq->lock. Non-zero indicates the task is running on it's CPU. > + * > + * [ The astute reader will observe that it is possible for two tasks on one > + * CPU to have ->on_cpu = 1 at the same time. ] > + * > + * task_cpu(p): is changed by set_task_cpu(), the rules are: > + * > + * - Don't call set_task_cpu() on a blocked task: > + * > + * We don't care what CPU we're not running on, this simplifies hotplug, > + * the CPU assignment of blocked tasks isn't required to be valid. > + * That's more of a good practice rather than a hard rule, right? We do that with proxy execution (the whole migrate to owner's rq thing), at least in its current shape. > + * - for try_to_wake_up(), called under p->pi_lock: > + * > + * This allows try_to_wake_up() to only take one rq->lock, see its comment. > + * > + * - for migration called under rq->lock: > + * [ see task_on_rq_migrating() in task_rq_lock() ] > + * > + * o move_queued_task() > + * o __migrate_swap_task() Isn't that one under double_rq_lock()? > + * o detach_task() > + * > + * - for migration called under double_rq_lock(): > + * > + * o push_rt_task() / pull_rt_task() > + * o push_dl_task() / pull_dl_task() > + * o dl_task_offline_migration() > + * > + */ > + > /* > * __task_rq_lock - lock the rq @p resides on. > */ [...] > } > > /* > - * Called in case the task @p isn't fully descheduled from its runqueue, > - * in this case we must do a remote wakeup. Its a 'light' wakeup though, > - * since all we need to do is flip p->state to TASK_RUNNING, since > - * the task is still ->on_rq. > + * Consider @p being inside a wait loop: > + * > + * for (;;) { > + * set_current_state(TASK_UNINTERRUPTIBLE); > + * > + * if (CONDITION) > + * break; For some reason the alignment is off in my mail view, but looks okay once applied. > + * > + * schedule(); > + * } > + * __set_current_state(TASK_RUNNING); > + * > + * between set_current_state() and schedule(). In this case @p is still > + * runnable, so all that needs doing is change p->state back to TASK_RUNNING in > + * an atomic manner. > + * Sorry if I'm being dense; don't you mean "running" here? If it stops being current inbetween set_current_state() and schedule(), __schedule() will deactivate() it, so AFAICT it can only be either running or deactivated. > + * By taking task_rq(p)->lock we serialize against schedule(), if @p->on_rq > + * then schedule() must still happen and p->state can be changed to > + * TASK_RUNNING. Otherwise we lost the race, schedule() has happened, and we > + * need to do a full wakeup with enqueue. > + * > + * Returns: %true when the wakeup is done, > + * %false otherwise. > */ > -static int ttwu_remote(struct task_struct *p, int wake_flags) > +static int ttwu_runnable(struct task_struct *p, int wake_flags) > { > struct rq_flags rf; > struct rq *rq; [...] > @@ -2494,15 +2608,41 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) > * @state: the mask of task states that can be woken > * @wake_flags: wake modifier flags (WF_*) > * > - * If (@state & @p->state) @p->state = TASK_RUNNING. > + * Conceptually does: > + * > + * If (@state & @p->state) @p->state = TASK_RUNNING. > * > * If the task was not queued/runnable, also place it back on a runqueue. > * > - * Atomic against schedule() which would dequeue a task, also see > - * set_current_state(). > + * This function: > + * - is atomic against schedule() which would dequeue the task; > + * - issues a full memory barrier before accessing @p->state. > + * See the comment with set_current_state(). > + * > + * Uses p->pi_lock to serialize against concurrent wake-ups. > + * > + * Relies on p->pi_lock stabilizing: > + * - p->sched_class > + * - p->cpus_ptr > + * - p->sched_task_group > + * in order to do migration, see its use of select_task_rq()/set_task_cpu(). > * > - * This function executes a full memory barrier before accessing the task > - * state; see set_current_state(). > + * Tries really hard to only take one task_rq(p)->lock for performance. > + * Takes rq->lock in: > + * - ttwu_runnable() -- old rq, unavoidable, see comment there; > + * - ttwu_queue() -- new rq, for enqueue of the task; > + * - psi_ttwu_dequeue() -- much sadness :-( accounting will kill us. > + * > + * As a concequence we race really badly with just about everything. See the s/concequence/consequence/ > + * many memory barriers and their comments for details. The basic order of > + * reading things is: > + * > + * LOAD p->state > + * RMB > + * LOAD p->on_rq > + * RMB > + * LOAD-ACQUIRE p->on_cpu > + * LOAD task_cpu() > * > * Return: %true if @p->state changes (an actual wakeup was done), > * %false otherwise. > @@ -2518,7 +2658,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > /* > * We're waking current, this means 'p->on_rq' and 'task_cpu(p) > * == smp_processor_id()'. Together this means we can special > - * case the whole 'p->on_rq && ttwu_remote()' case below > + * case the whole 'p->on_rq && ttwu_runnable()' case below > * without taking any locks. > * > * In particular: > @@ -2539,8 +2679,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > /* > * If we are going to wake up a thread waiting for CONDITION we > * need to ensure that CONDITION=1 done by the caller can not be > - * reordered with p->state check below. This pairs with mb() in > - * set_current_state() the waiting thread does. > + * reordered with p->state check below. This pairs with smp_store_mb() > + * in set_current_state() that the waiting thread does. > */ > raw_spin_lock_irqsave(&p->pi_lock, flags); > smp_mb__after_spinlock(); > @@ -2575,7 +2715,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > * A similar smb_rmb() lives in try_invoke_on_locked_down_task(). > */ > smp_rmb(); > - if (p->on_rq && ttwu_remote(p, wake_flags)) > + if (p->on_rq && ttwu_runnable(p, wake_flags)) > goto unlock; > > if (p->in_iowait) { > @@ -3134,8 +3274,12 @@ static inline void prepare_task(struct task_struct *next) > /* > * Claim the task as running, we do this before switching to it > * such that any running task will have this set. > + * > + * __schedule()'s rq->lock and smp_mb__after_spin_lock() orders this > + * store against prior state change of @next, also see > + * try_to_wake_up(), specifically smp_load_acquire(&p->on_cpu). smp_*cond*_load_acquire(&p->on_cpu, <blah>) > */ > - next->on_cpu = 1; > + WRITE_ONCE(next->on_cpu, 1); > #endif > } > > @@ -3143,8 +3287,9 @@ static inline void finish_task(struct task_struct *prev) > { > #ifdef CONFIG_SMP > /* > - * After ->on_cpu is cleared, the task can be moved to a different CPU. > - * We must ensure this doesn't happen until the switch is completely > + * This must be the very last reference to @prev from this CPU. After > + * p->on_cpu is cleared, the task can be moved to a different CPU. We > + * must ensure this doesn't happen until the switch is completely > * finished. > * > * In particular, the load of prev->state in finish_task_switch() must > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 5aa6661ecaf1..73c0c5d0034b 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1197,6 +1197,17 @@ struct rq_flags { > #endif > }; > > +/* > + * Lockdep annotation that avoid accidental unlock; any > + * raw_spin_unlock(&rq->lock) without preceding rq_unpin_lock() with the > + * correct cookie will result in a WARN. > + * ISTR that being described (by yourself?) as a "sticky/continuous lockdep_assert_held()", which I think gets the point across. > + * This avoids code that has access to 'struct rq *rq' (basically everything in > + * the scheduler) from accidentally unlocking the rq if they do not also have a > + * copy of the (on-stack) 'struct rq_flags rf'. > + * > + * Also see Documentation/locking/lockdep-design.rst. > + */ > static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf) > { > rf->cookie = lockdep_pin_lock(&rq->lock);
Powered by blists - more mailing lists