Messages in this thread | | | Date | Wed, 3 Nov 2010 18:17:18 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call |
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On 11/03, Sergey Senozhatsky wrote: > > Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for > find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns. > Add rcu_read_lock/rcu_read_unlock to call find_task_by_vpid. > > Tetsuo Handa wrote: > | Quoting from one of posts in that thead > | http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388 > | > || Usually tasklist gives enough protection, but if copy_process() fails > || it calls free_pid() lockless and does call_rcu(delayed_put_pid(). > || This means, without rcu lock find_pid_ns() can't scan the hash table > || safely. > > Thomas Gleixner wrote: > | We can remove the tasklist_lock while at it. rcu_read_lock is enough. > > Patch also replaces thread_group_leader with has_group_leader_pid > in accordance to comment by Oleg Nesterov: > > | ... thread_group_leader() check is not relaible without > | tasklist. If we race with de_thread() find_task_by_vpid() can find > | the new leader before it updates its ->group_leader. > | > | perhaps it makes sense to change posix_cpu_timer_create() to use > | has_group_leader_pid() instead, just to make this code not look racy > | and avoid adding new problems. > > > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c > index 6842eeb..05bb717 100644 > --- a/kernel/posix-cpu-timers.c > +++ b/kernel/posix-cpu-timers.c > @@ -37,13 +37,13 @@ static int check_clock(const clockid_t which_clock) > if (pid == 0) > return 0; > > - read_lock(&tasklist_lock); > + rcu_read_lock(); > p = find_task_by_vpid(pid); > if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ? > - same_thread_group(p, current) : thread_group_leader(p))) { > + same_thread_group(p, current) : has_group_leader_pid(p))) { > error = -EINVAL; > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > > return error; > } > @@ -390,7 +390,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > INIT_LIST_HEAD(&new_timer->it.cpu.entry); > > - read_lock(&tasklist_lock); > + rcu_read_lock(); > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) { > if (pid == 0) { > p = current; > @@ -404,7 +404,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > p = current->group_leader; > } else { > p = find_task_by_vpid(pid); > - if (p && !thread_group_leader(p)) > + if (p && !has_group_leader_pid(p)) > p = NULL; > } > } > @@ -414,7 +414,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > } else { > ret = -EINVAL; > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > > return ret; > } >
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