* Question about replacing while_each_thread(). @ 2017-02-01 10:47 Tetsuo Handa 2017-02-01 17:19 ` Oleg Nesterov 0 siblings, 1 reply; 2+ messages in thread From: Tetsuo Handa @ 2017-02-01 10:47 UTC (permalink / raw) To: oleg; +Cc: dserrg, snanda, rientjes, linux-kernel Hello. I have a question about commit 0c740d0afc3bff0a ("introduce for_each_thread() to replace the buggy while_each_thread()"). IOPRIO_WHO_USER case in sys_ioprio_set()/sys_ioprio_get() in block/ioprio.c are using rcu_read_lock(); do_each_thread(g, p) { (...snipped...) } while_each_thread(g, p); rcu_read_unlock(); sequence which is unsafe according to that commit, but I'm not sure what the correct fix is. That commit says The new for_each_thread(g, t) helper is always safe under rcu_read_lock() as long as this task_struct can't go away. but what is the requirement for "can't go away" ? Is rcu_read_lock() sufficient (i.e. rcu_read_lock(); for_each_process_thread(g, p) { (...snipped...) } rcu_read_unlock(); is OK) for "can't go away" ? Is tasklist_lock held for read or write required (i.e. read_lock(&tasklist_lock); for_each_process_thread(g, p) { (...snipped...) } read_unlock(&tasklist_lock); is needed) for "can't go away" ? I hope rcu_read_lock() is sufficient according to usage in show_state_filter() and check_hung_uninterruptible_tasks(). Likewise, IOPRIO_WHO_PGRP case are using rcu_read_lock(); do { if ((pgrp) != NULL) hlist_for_each_entry_rcu((p), &(pgrp)->tasks[PIDTYPE_PGID], pids[PIDTYPE_PGID].node) { { struct task_struct *tg___ = p; do { (...snipped...) } while_each_thread(tg___, p); p = tg___; } if (PIDTYPE_PGID == PIDTYPE_PID) break; } } while (0); rcu_read_unlock(); sequence which I guess it is unsafe as well. In this case updating do_each_pid_thread() to use for_each_thread() and updating while_each_pid_thread() not to use while_each_thread() is the correct fix? ^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Question about replacing while_each_thread(). 2017-02-01 10:47 Question about replacing while_each_thread() Tetsuo Handa @ 2017-02-01 17:19 ` Oleg Nesterov 0 siblings, 0 replies; 2+ messages in thread From: Oleg Nesterov @ 2017-02-01 17:19 UTC (permalink / raw) To: Tetsuo Handa; +Cc: dserrg, snanda, rientjes, linux-kernel Hi Tetsuo, On 02/01, Tetsuo Handa wrote: > > Is rcu_read_lock() sufficient (i.e. > > rcu_read_lock(); > for_each_process_thread(g, p) { > (...snipped...) > } > rcu_read_unlock(); > > is OK) for "can't go away" ? Yes, this should work just fine, > Likewise, IOPRIO_WHO_PGRP case are using > > rcu_read_lock(); > do { > if ((pgrp) != NULL) > hlist_for_each_entry_rcu((p), &(pgrp)->tasks[PIDTYPE_PGID], pids[PIDTYPE_PGID].node) { > { > struct task_struct *tg___ = p; > do { > (...snipped...) > } while_each_thread(tg___, p); > p = tg___; > } > if (PIDTYPE_PGID == PIDTYPE_PID) > break; > } > } while (0); > rcu_read_unlock(); > > sequence which I guess it is unsafe as well. Hmm, indeed, I forgot there is another while_each_thread() hidden in do_each_pid_thread() > In this case updating do_each_pid_thread() to use for_each_thread() and > updating while_each_pid_thread() not to use while_each_thread() is > the correct fix? Yes, I think so, just --- a/include/linux/pid.h +++ b/include/linux/pid.h @@ -191,10 +191,10 @@ pid_t pid_vnr(struct pid *pid); #define do_each_pid_thread(pid, type, task) \ do_each_pid_task(pid, type, task) { \ struct task_struct *tg___ = task; \ - do { + for_each_thread(tg__, task) { #define while_each_pid_thread(pid, type, task) \ - } while_each_thread(tg___, task); \ + } \ task = tg___; \ } while_each_pid_task(pid, type, task) #endif /* _LINUX_PID_H */ but perhaps we can also cleanup it... the usage of 'tg___' doesn't look nice either way, so perhaps #define do_each_pid_thread(pid, type, task) do { \ struct task_struct *tg___; \ do_each_pid_task(pid, type, tg___) { \ for_each_thread(tg__, task) { #define while_each_pid_thread(pid, type, task) \ } \ } while_each_pid_task(pid, type, task); \ } while (0) will look a bit better? up to you. Oleg. ^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2017-02-01 17:19 UTC | newest] Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2017-02-01 10:47 Question about replacing while_each_thread() Tetsuo Handa 2017-02-01 17:19 ` Oleg Nesterov
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