Messages in this thread Patches in this message | | | Subject | Re: 2.6.25-rc9 -- INFO: possible circular locking dependency detected | From | Peter Zijlstra <> | Date | Mon, 14 Apr 2008 14:42:27 +0200 |
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On Mon, 2008-04-14 at 17:57 +0530, Gautham R Shenoy wrote:
> > Ok, so cpu_hotplug has a few issues imho: > > > > - access to active_writer isn't serialized and thus racey > > - holding the lock over the 'write' section generates the stuff above > > > > So basically we want a reader/writer lock, where get/put_online_cpu is > > the read side and cpu_hotplug_begin/done the write side. > > > > We want: > > - readers to recurse in readers (code excluding cpu-hotplug can > > call code needing the same). > > > > - readers to recurse in the writer (the code changing the state can > > call code needing a stable state) > > > > rwlock_t isn't quite suitable because it doesn't allow reader in writer > > recursion and doesn't allow sleeping. > > > > No lockdep annotation _yet_, because current lockdep recursive reader > > support is: > > - broken (have a patch for that) > > - doesn't support reader in writer recursion (will have to do something > > about that) > > > > Ok, so aside from the obviuos disclaimers, I've only compiled this and > > might have made things way too complicated - but a slightly feverish > > brain does that at times. What do people think? > > You beat me to it! > > I just whipped up a similar patch, with slight differences, and lockdep > annotations :)
lockdep doesn't quite acecpt reader in writer recursion without a little patch like so:
(fold of two patches - one fixing the recursion another adding reader-writer recursion)
cpu_hotplug should use 3.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> --- Index: linux-2.6-2/kernel/lockdep.c =================================================================== --- linux-2.6-2.orig/kernel/lockdep.c +++ linux-2.6-2/kernel/lockdep.c @@ -1281,6 +1281,13 @@ check_deadlock(struct task_struct *curr, */ if ((read == 2) && prev->read) return 2; + /* + * Allow read-after-write recursion of the same + * lock class (i.e. write_lock(lock)+read_lock(lock)): + */ + if (read == 3) + return 2; + return print_deadlock_bug(curr, prev, next); } return 1; @@ -1557,12 +1564,11 @@ static int validate_chain(struct task_st if (!ret) return 0; /* - * Mark recursive read, as we jump over it when - * building dependencies (just like we jump over - * trylock entries): + * If we are the first recursive read, don't jump over our + * dependency. */ - if (ret == 2) - hlock->read = 2; + if (hlock->read >= 2 && ret != 2) + hlock->read = 1; /* * Add dependency only if this lock is not the head * of the chain, and if it's not a secondary read-lock: Index: linux-2.6-2/lib/locking-selftest.c =================================================================== --- linux-2.6-2.orig/lib/locking-selftest.c +++ linux-2.6-2/lib/locking-selftest.c @@ -1135,12 +1135,12 @@ void locking_selftest(void) debug_locks_silent = !debug_locks_verbose; DO_TESTCASE_6R("A-A deadlock", AA); - DO_TESTCASE_6R("A-B-B-A deadlock", ABBA); - DO_TESTCASE_6R("A-B-B-C-C-A deadlock", ABBCCA); - DO_TESTCASE_6R("A-B-C-A-B-C deadlock", ABCABC); - DO_TESTCASE_6R("A-B-B-C-C-D-D-A deadlock", ABBCCDDA); - DO_TESTCASE_6R("A-B-C-D-B-D-D-A deadlock", ABCDBDDA); - DO_TESTCASE_6R("A-B-C-D-B-C-D-A deadlock", ABCDBCDA); + DO_TESTCASE_6("A-B-B-A deadlock", ABBA); + DO_TESTCASE_6("A-B-B-C-C-A deadlock", ABBCCA); + DO_TESTCASE_6("A-B-C-A-B-C deadlock", ABCABC); + DO_TESTCASE_6("A-B-B-C-C-D-D-A deadlock", ABBCCDDA); + DO_TESTCASE_6("A-B-C-D-B-D-D-A deadlock", ABCDBDDA); + DO_TESTCASE_6("A-B-C-D-B-C-D-A deadlock", ABCDBCDA); DO_TESTCASE_6("double unlock", double_unlock); DO_TESTCASE_6("initialize held", init_held); DO_TESTCASE_6_SUCCESS("bad unlock order", bad_unlock_order); Index: linux-2.6-2/include/linux/lockdep.h =================================================================== --- linux-2.6-2.orig/include/linux/lockdep.h +++ linux-2.6-2/include/linux/lockdep.h @@ -291,6 +291,7 @@ extern void lockdep_init_map(struct lock * 0: exclusive (write) acquire * 1: read-acquire (no recursion allowed) * 2: read-acquire with same-instance recursion allowed + * 3: 2 + reader in writer recursion * * Values for check: * > comments below > > > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> > > --- > > diff --git a/kernel/cpu.c b/kernel/cpu.c > > index 2011ad8..119d837 100644 > > --- a/kernel/cpu.c > > +++ b/kernel/cpu.c > > @@ -27,12 +27,13 @@ static int cpu_hotplug_disabled; > > > > static struct { > > struct task_struct *active_writer; > > - struct mutex lock; /* Synchronizes accesses to refcount, */ > > + spinlock_t lock; /* Synchronizes accesses to refcount, */ > > /* > > * Also blocks the new readers during > > * an ongoing cpu hotplug operation. > > */ > > int refcount; > > + wait_queue_head_t reader_queue; > > wait_queue_head_t writer_queue; > > } cpu_hotplug; > > > > @@ -41,8 +42,9 @@ static struct { > > void __init cpu_hotplug_init(void) > > { > > cpu_hotplug.active_writer = NULL; > > - mutex_init(&cpu_hotplug.lock); > > + spin_lock_init(&cpu_hotplug.lock); > > cpu_hotplug.refcount = 0; > > + init_waitqueue_head(&cpu_hotplug.reader_queue); > > init_waitqueue_head(&cpu_hotplug.writer_queue); > > } > > > > @@ -51,27 +53,42 @@ void __init cpu_hotplug_init(void) > > void get_online_cpus(void) > > { > > might_sleep(); > > + > > + spin_lock(&cpu_hotplug.lock); > > if (cpu_hotplug.active_writer == current) > > - return; > We don't need to perform this check holding the spinlock. > The reason being, this check should succeed only for get_online_cpus() > invoked from the CPU-hotplug callback path. and by that time, > the writer thread would have set active_writer to it's task struct > value. > > For every other thread, when it's trying to check if it is the > active_writer, the value is either NULL, or the value of the currently > active writer's task_struct, but never current. > > Am I missing something ?
I guess you're right - inside makes me feel better though :-) And its not like its a fast path or something like that.
> > - mutex_lock(&cpu_hotplug.lock); > > + goto unlock; > > + > > + if (cpu_hotplug.active_writer) { > > + DEFINE_WAIT(wait); > > + > > + for (;;) { > > + prepare_to_wait(&cpu_hotplug.reader_queue, &wait, > > + TASK_UNINTERRUPTIBLE); > > + if (!cpu_hotplug.active_writer) > > + break; > > + spin_unlock(&cpu_hotplug.lock); > > + schedule(); > > + spin_lock(&cpu_hotplug.lock); > > + } > > + finish_wait(&cpu_hotplug.reader_queue, &wait); > > + } > > cpu_hotplug.refcount++; > > - mutex_unlock(&cpu_hotplug.lock); > > - > > + unlock: > > + spin_unlock(&cpu_hotplug.lock); > > } > > EXPORT_SYMBOL_GPL(get_online_cpus); > > > > void put_online_cpus(void) > > { > > + spin_lock(&cpu_hotplug.lock); > > if (cpu_hotplug.active_writer == current) > > - return; > > ditto! > > > - mutex_lock(&cpu_hotplug.lock); > > - cpu_hotplug.refcount--; > > + goto unlock; > > > > - if (unlikely(writer_exists()) && !cpu_hotplug.refcount) > > + cpu_hotplug.refcount--; > > + if (!cpu_hotplug.refcount) > > wake_up(&cpu_hotplug.writer_queue); > hmm.. when there is no active writer, can't we avoid this > additional wake up ?? > > - > > - mutex_unlock(&cpu_hotplug.lock); > > - > > + unlock: > > + spin_unlock(&cpu_hotplug.lock); > > } > > EXPORT_SYMBOL_GPL(put_online_cpus); > > > > @@ -95,45 +112,41 @@ void cpu_maps_update_done(void) > > * This ensures that the hotplug operation can begin only when the > > * refcount goes to zero. > > * > > - * Note that during a cpu-hotplug operation, the new readers, if any, > > - * will be blocked by the cpu_hotplug.lock > > - * > > - * Since cpu_maps_update_begin is always called after invoking > > - * cpu_maps_update_begin, we can be sure that only one writer is active. > > - * > > - * Note that theoretically, there is a possibility of a livelock: > > - * - Refcount goes to zero, last reader wakes up the sleeping > > - * writer. > > - * - Last reader unlocks the cpu_hotplug.lock. > > - * - A new reader arrives at this moment, bumps up the refcount. > > - * - The writer acquires the cpu_hotplug.lock finds the refcount > > - * non zero and goes to sleep again. > > - * > > - * However, this is very difficult to achieve in practice since > > - * get_online_cpus() not an api which is called all that often. > > - * > > + * cpu_hotplug is basically an unfair recursive reader/writer lock that > > + * allows reader in writer recursion. > > */ > > static void cpu_hotplug_begin(void) > > { > > - DECLARE_WAITQUEUE(wait, current); > > - > > - mutex_lock(&cpu_hotplug.lock); > > + might_sleep(); > > > > - cpu_hotplug.active_writer = current; > > - add_wait_queue_exclusive(&cpu_hotplug.writer_queue, &wait); > > - while (cpu_hotplug.refcount) { > > - set_current_state(TASK_UNINTERRUPTIBLE); > > - mutex_unlock(&cpu_hotplug.lock); > > - schedule(); > > - mutex_lock(&cpu_hotplug.lock); > > + spin_lock(&cpu_hotplug.lock); > > + if (cpu_hotplug.refcount || cpu_hotplug.active_writer) { > if we reach this point, there can be only one writer, i.e. > ourselves! > > Because, the other writers are serialized by the > cpu_add_remove_lock in cpu_up()/cpu_down(). > > Hence we can safely assign cpu_hotplug.active_writer to current.
Ah, missed that. Does it make sense to keep it like this, in case this outer lock goes away?
> > + DEFINE_WAIT(wait); > > + > > + for (;;) { > > + prepare_to_wait(&cpu_hotplug.writer_queue, &wait, > > + TASK_UNINTERRUPTIBLE); > > + if (!cpu_hotplug.refcount && !cpu_hotplug.active_writer) > > + break; > > + spin_unlock(&cpu_hotplug.lock); > > + schedule(); > > + spin_lock(&cpu_hotplug.lock); > > + } > > + finish_wait(&cpu_hotplug.writer_queue, &wait); > > } > > - remove_wait_queue_locked(&cpu_hotplug.writer_queue, &wait); > > + cpu_hotplug.active_writer = current; > > > > + spin_unlock(&cpu_hotplug.lock); > > } > > > > static void cpu_hotplug_done(void) > > { > > + spin_lock(&cpu_hotplug.lock); > > cpu_hotplug.active_writer = NULL; > > - mutex_unlock(&cpu_hotplug.lock); > > + if (!list_empty(&cpu_hotplug.writer_queue.task_list)) > > + wake_up(&cpu_hotplug.writer_queue); > > + else > > + wake_up_all(&cpu_hotplug.reader_queue); > > + spin_unlock(&cpu_hotplug.lock); > > } > > /* Need to know about CPUs going up/down? */ > > int __cpuinit register_cpu_notifier(struct notifier_block *nb) > > > > Looks good otherwise!
Thanks..
lockdep annotations:
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> --- Index: linux-2.6-2/include/linux/lockdep.h =================================================================== --- linux-2.6-2.orig/include/linux/lockdep.h +++ linux-2.6-2/include/linux/lockdep.h @@ -458,4 +458,19 @@ static inline void print_irqtrace_events # define rwsem_release(l, n, i) do { } while (0) #endif +#ifdef CONFIG_DEBUG_LOCK_ALLOC +# ifdef CONFIG_PROVE_LOCKING +# define cpu_hotplug_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 2, i) +# define cpu_hotplug_acquire_read(l, s, t, i) lock_acquire(l, s, t, 3, 2, i) +# else +# define cpu_hotplug_acquire(l, s, t, i) lock_acquire(l, s, t, 0, 1, i) +# define cpu_hotplug_acquire_read(l, s, t, i) lock_acquire(l, s, t, 3, 1, i) +# endif +# define cpu_hotplug_release(l, n, i) lock_release(l, n, i) +#else +# define cpu_hotplug_acquire(l, s, t, i) do { } while (0) +# define cpu_hotplug_acquire_read(l, s, t, i) do { } while (0) +# define cpu_hotplug_release(l, n, i) do { } while (0) +#endif + #endif /* __LINUX_LOCKDEP_H */ Index: linux-2.6-2/kernel/cpu.c =================================================================== --- linux-2.6-2.orig/kernel/cpu.c +++ linux-2.6-2/kernel/cpu.c @@ -35,12 +35,20 @@ static struct { int refcount; wait_queue_head_t reader_queue; wait_queue_head_t writer_queue; +#ifdef CONFIG_DEBUG_LOCK_ALLOC + struct lockdep_map dep_map; +#endif } cpu_hotplug; #define writer_exists() (cpu_hotplug.active_writer != NULL) void __init cpu_hotplug_init(void) { +#ifdef CONFIG_DEBUG_LOCK_ALLOC + static struct lockdep_class_key __key; + + lockdep_init_map(&cpu_hotplug.dep_map, "cpu_hotplug", &__key, 0); +#endif cpu_hotplug.active_writer = NULL; spin_lock_init(&cpu_hotplug.lock); cpu_hotplug.refcount = 0; @@ -54,6 +62,8 @@ void get_online_cpus(void) { might_sleep(); + cpu_hotplug_acquire_read(&cpu_hotplug.dep_map, 0, 0, _THIS_IP_); + spin_lock(&cpu_hotplug.lock); if (cpu_hotplug.active_writer == current) goto unlock; @@ -80,6 +90,8 @@ EXPORT_SYMBOL_GPL(get_online_cpus); void put_online_cpus(void) { + cpu_hotplug_release(&cpu_hotplug.lock, 1, _THIS_IP_); + spin_lock(&cpu_hotplug.lock); if (cpu_hotplug.active_writer == current) goto unlock; @@ -119,6 +131,8 @@ static void cpu_hotplug_begin(void) { might_sleep(); + cpu_hotplug_acquire(&cpu_hotplug.dep_map, 0, 0, _THIS_IP_); + spin_lock(&cpu_hotplug.lock); if (cpu_hotplug.refcount || cpu_hotplug.active_writer) { DEFINE_WAIT(wait); @@ -140,6 +154,8 @@ static void cpu_hotplug_begin(void) static void cpu_hotplug_done(void) { + cpu_hotplug_release(&cpu_hotplug.lock, 1, _THIS_IP_); + spin_lock(&cpu_hotplug.lock); cpu_hotplug.active_writer = NULL; if (!list_empty(&cpu_hotplug.writer_queue.task_list))
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