Revert "sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem"
authorTejun Heo <tj@kernel.org>
Wed, 16 Sep 2015 15:51:12 +0000 (11:51 -0400)
committerTejun Heo <tj@kernel.org>
Wed, 16 Sep 2015 15:51:12 +0000 (11:51 -0400)
This reverts commit d59cfc09c32a2ae31f1c3bc2983a0cd79afb3f14.

d59cfc09c32a ("sched, cgroup: replace signal_struct->group_rwsem with
a global percpu_rwsem") and b5ba75b5fc0e ("cgroup: simplify
threadgroup locking") changed how cgroup synchronizes against task
fork and exits so that it uses global percpu_rwsem instead of
per-process rwsem; unfortunately, the write [un]lock paths of
percpu_rwsem always involve synchronize_rcu_expedited() which turned
out to be too expensive.

Improvements for percpu_rwsem are scheduled to be merged in the coming
v4.4-rc1 merge window which alleviates this issue.  For now, revert
the two commits to restore per-process rwsem.  They will be re-applied
for the v4.4-rc1 merge window.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org # v4.2+
include/linux/cgroup-defs.h
include/linux/init_task.h
include/linux/sched.h
kernel/cgroup.c
kernel/fork.c

index 4d8fcf2187dcaa822b8f629655e83d667a9913ac..8492721b39be8f0fffa793c5ef39f33d32b5c5fb 100644 (file)
@@ -473,31 +473,8 @@ struct cgroup_subsys {
        unsigned int depends_on;
 };
 
-extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
-
-/**
- * cgroup_threadgroup_change_begin - threadgroup exclusion for cgroups
- * @tsk: target task
- *
- * Called from threadgroup_change_begin() and allows cgroup operations to
- * synchronize against threadgroup changes using a percpu_rw_semaphore.
- */
-static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
-{
-       percpu_down_read(&cgroup_threadgroup_rwsem);
-}
-
-/**
- * cgroup_threadgroup_change_end - threadgroup exclusion for cgroups
- * @tsk: target task
- *
- * Called from threadgroup_change_end().  Counterpart of
- * cgroup_threadcgroup_change_begin().
- */
-static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
-{
-       percpu_up_read(&cgroup_threadgroup_rwsem);
-}
+void cgroup_threadgroup_change_begin(struct task_struct *tsk);
+void cgroup_threadgroup_change_end(struct task_struct *tsk);
 
 #else  /* CONFIG_CGROUPS */
 
index d0b380ee7d67abbd421bf69fdd63ff10b2aa88b1..e38681f4912d02f9c571dd7d232a4b4bb4911ea4 100644 (file)
 extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 
+#ifdef CONFIG_CGROUPS
+#define INIT_GROUP_RWSEM(sig)                                          \
+       .group_rwsem = __RWSEM_INITIALIZER(sig.group_rwsem),
+#else
+#define INIT_GROUP_RWSEM(sig)
+#endif
+
 #ifdef CONFIG_CPUSETS
 #define INIT_CPUSET_SEQ(tsk)                                                   \
        .mems_allowed_seq = SEQCNT_ZERO(tsk.mems_allowed_seq),
@@ -57,6 +64,7 @@ extern struct fs_struct init_fs;
        INIT_PREV_CPUTIME(sig)                                          \
        .cred_guard_mutex =                                             \
                 __MUTEX_INITIALIZER(sig.cred_guard_mutex),             \
+       INIT_GROUP_RWSEM(sig)                                           \
 }
 
 extern struct nsproxy init_nsproxy;
index a4ab9daa387c0bbcaca1923620ceb2ed74bfd84e..b7b9501b41af4eab6a096601c3baf85c85854807 100644 (file)
@@ -762,6 +762,18 @@ struct signal_struct {
        unsigned audit_tty_log_passwd;
        struct tty_audit_buf *tty_audit_buf;
 #endif
+#ifdef CONFIG_CGROUPS
+       /*
+        * group_rwsem prevents new tasks from entering the threadgroup and
+        * member tasks from exiting,a more specifically, setting of
+        * PF_EXITING.  fork and exit paths are protected with this rwsem
+        * using threadgroup_change_begin/end().  Users which require
+        * threadgroup to remain stable should use threadgroup_[un]lock()
+        * which also takes care of exec path.  Currently, cgroup is the
+        * only user.
+        */
+       struct rw_semaphore group_rwsem;
+#endif
 
        oom_flags_t oom_flags;
        short oom_score_adj;            /* OOM kill score adjustment */
index 115091efa889b13cc227ae61dbdb1725b2df1275..2c9eae6ad9704d3278557f6c692d2ef0027b587b 100644 (file)
@@ -46,7 +46,6 @@
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/rwsem.h>
-#include <linux/percpu-rwsem.h>
 #include <linux/string.h>
 #include <linux/sort.h>
 #include <linux/kmod.h>
@@ -104,8 +103,6 @@ static DEFINE_SPINLOCK(cgroup_idr_lock);
  */
 static DEFINE_SPINLOCK(release_agent_path_lock);
 
-struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
-
 #define cgroup_assert_mutex_or_rcu_locked()                            \
        RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&                       \
                           !lockdep_is_held(&cgroup_mutex),             \
@@ -874,6 +871,48 @@ static struct css_set *find_css_set(struct css_set *old_cset,
        return cset;
 }
 
+void cgroup_threadgroup_change_begin(struct task_struct *tsk)
+{
+       down_read(&tsk->signal->group_rwsem);
+}
+
+void cgroup_threadgroup_change_end(struct task_struct *tsk)
+{
+       up_read(&tsk->signal->group_rwsem);
+}
+
+/**
+ * threadgroup_lock - lock threadgroup
+ * @tsk: member task of the threadgroup to lock
+ *
+ * Lock the threadgroup @tsk belongs to.  No new task is allowed to enter
+ * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
+ * change ->group_leader/pid.  This is useful for cases where the threadgroup
+ * needs to stay stable across blockable operations.
+ *
+ * fork and exit explicitly call threadgroup_change_{begin|end}() for
+ * synchronization.  While held, no new task will be added to threadgroup
+ * and no existing live task will have its PF_EXITING set.
+ *
+ * de_thread() does threadgroup_change_{begin|end}() when a non-leader
+ * sub-thread becomes a new leader.
+ */
+static void threadgroup_lock(struct task_struct *tsk)
+{
+       down_write(&tsk->signal->group_rwsem);
+}
+
+/**
+ * threadgroup_unlock - unlock threadgroup
+ * @tsk: member task of the threadgroup to unlock
+ *
+ * Reverse threadgroup_lock().
+ */
+static inline void threadgroup_unlock(struct task_struct *tsk)
+{
+       up_write(&tsk->signal->group_rwsem);
+}
+
 static struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
 {
        struct cgroup *root_cgrp = kf_root->kn->priv;
@@ -2074,9 +2113,9 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
        lockdep_assert_held(&css_set_rwsem);
 
        /*
-        * We are synchronized through cgroup_threadgroup_rwsem against
-        * PF_EXITING setting such that we can't race against cgroup_exit()
-        * changing the css_set to init_css_set and dropping the old one.
+        * We are synchronized through threadgroup_lock() against PF_EXITING
+        * setting such that we can't race against cgroup_exit() changing the
+        * css_set to init_css_set and dropping the old one.
         */
        WARN_ON_ONCE(tsk->flags & PF_EXITING);
        old_cset = task_css_set(tsk);
@@ -2133,11 +2172,10 @@ static void cgroup_migrate_finish(struct list_head *preloaded_csets)
  * @src_cset and add it to @preloaded_csets, which should later be cleaned
  * up by cgroup_migrate_finish().
  *
- * This function may be called without holding cgroup_threadgroup_rwsem
- * even if the target is a process.  Threads may be created and destroyed
- * but as long as cgroup_mutex is not dropped, no new css_set can be put
- * into play and the preloaded css_sets are guaranteed to cover all
- * migrations.
+ * This function may be called without holding threadgroup_lock even if the
+ * target is a process.  Threads may be created and destroyed but as long
+ * as cgroup_mutex is not dropped, no new css_set can be put into play and
+ * the preloaded css_sets are guaranteed to cover all migrations.
  */
 static void cgroup_migrate_add_src(struct css_set *src_cset,
                                   struct cgroup *dst_cgrp,
@@ -2240,7 +2278,7 @@ err:
  * @threadgroup: whether @leader points to the whole process or a single task
  *
  * Migrate a process or task denoted by @leader to @cgrp.  If migrating a
- * process, the caller must be holding cgroup_threadgroup_rwsem.  The
+ * process, the caller must be holding threadgroup_lock of @leader.  The
  * caller is also responsible for invoking cgroup_migrate_add_src() and
  * cgroup_migrate_prepare_dst() on the targets before invoking this
  * function and following up with cgroup_migrate_finish().
@@ -2368,7 +2406,7 @@ out_release_tset:
  * @leader: the task or the leader of the threadgroup to be attached
  * @threadgroup: attach the whole threadgroup?
  *
- * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
+ * Call holding cgroup_mutex and threadgroup_lock of @leader.
  */
 static int cgroup_attach_task(struct cgroup *dst_cgrp,
                              struct task_struct *leader, bool threadgroup)
@@ -2490,7 +2528,7 @@ retry_find_task:
        get_task_struct(tsk);
        rcu_read_unlock();
 
-       percpu_down_write(&cgroup_threadgroup_rwsem);
+       threadgroup_lock(tsk);
        if (threadgroup) {
                if (!thread_group_leader(tsk)) {
                        /*
@@ -2500,7 +2538,7 @@ retry_find_task:
                         * try again; this is
                         * "double-double-toil-and-trouble-check locking".
                         */
-                       percpu_up_write(&cgroup_threadgroup_rwsem);
+                       threadgroup_unlock(tsk);
                        put_task_struct(tsk);
                        goto retry_find_task;
                }
@@ -2510,7 +2548,7 @@ retry_find_task:
        if (!ret)
                ret = cgroup_attach_task(cgrp, tsk, threadgroup);
 
-       percpu_up_write(&cgroup_threadgroup_rwsem);
+       threadgroup_unlock(tsk);
 
        put_task_struct(tsk);
 out_unlock_cgroup:
@@ -2713,17 +2751,17 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
                                goto out_finish;
                        last_task = task;
 
-                       percpu_down_write(&cgroup_threadgroup_rwsem);
+                       threadgroup_lock(task);
                        /* raced against de_thread() from another thread? */
                        if (!thread_group_leader(task)) {
-                               percpu_up_write(&cgroup_threadgroup_rwsem);
+                               threadgroup_unlock(task);
                                put_task_struct(task);
                                continue;
                        }
 
                        ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);
 
-                       percpu_up_write(&cgroup_threadgroup_rwsem);
+                       threadgroup_unlock(task);
                        put_task_struct(task);
 
                        if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
@@ -5045,7 +5083,6 @@ int __init cgroup_init(void)
        unsigned long key;
        int ssid, err;
 
-       BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));
 
index 7d5f0f118a6348f81f08f10dd7dbb499f89dd243..2845623fb58264eec28a8b99b48d4511856e718d 100644 (file)
@@ -1149,6 +1149,10 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
        tty_audit_fork(sig);
        sched_autogroup_fork(sig);
 
+#ifdef CONFIG_CGROUPS
+       init_rwsem(&sig->group_rwsem);
+#endif
+
        sig->oom_score_adj = current->signal->oom_score_adj;
        sig->oom_score_adj_min = current->signal->oom_score_adj_min;