On 02/16, Oleg Nesterov wrote: > > On 02/15, Andrew Morton wrote: > > > > ug. On about the fourth boot with the current -mm lineup I hit: > > > > : BUG: unable to handle kernel paging request at 0000000000200200 > ^^^^^^^^^^^^^^^^ > == LIST_POISON2 > > > : IP: [<ffffffff802444f5>] free_pid+0x35/0x8e > > most probably == hlist_del_rcu(pid_chain) > > > : Call Trace: > > : [<ffffffff80237727>] ? release_task+0x152/0x2e5 > > : [<ffffffff80237f81>] ? do_wait+0x6c7/0xa1c > > : [<ffffffff8022f4cc>] ? default_wake_function+0x0/0xe > > : [<ffffffff8023e670>] ? sys_rt_sigaction+0x7a/0x98 > > : [<ffffffff80238360>] ? sys_wait4+0x8a/0xa1 > > : [<ffffffff8020be4b>] ? system_call_after_swapgs+0x7b/0x80 > > (Can't understand why there is no detach_pid() in this stack trace, > but it is the only possible caller of free_pid()). > > So, detach_pid()->free_pid() hit an already unhashed pid. But this > is not possible? > > This means we already did detach_pid(), but in that case the previous > detach_pid() has set task->pids[].pid = NULL, and we should OOPS earlier, > somewhere at "if (!hlist_empty(&pid->tasks[tmp]))".
Yes, this is not possible. But what possible is: we have the unbalanced put_pid(pid) which frees the live pid. The next alloc_pid() gets the same memory, and initializes all pid->tasks[] lists. Now, if that pid was used as PIDTYPE_PGID/PIDTYPE_SID, the next detach_pid() from this pgrp/sid sees that the pid is not used (all lists are hlist_empty), frees this pid again, and the bug manifests itself this way. tiocspgrp: put_pid(real_tty->pgrp); // ------ WINDOW ------ real_tty->pgrp = get_pid(pgrp); When bash spawns the command, both parent and child do ioctl(TIOCSPGRP,child), and it is possible that both do put_pid() on the same parent's pid. Damn, when you know what the bug is, the test case is trivial: $ while true; do perl -e0; done The kernel crashes. >From 2.6.25-rc2-mm1.bz2 patch: > > - .ioctl = tty_ioctl, > + .unlocked_ioctl = tty_ioctl, and this is why this didn't happen before, I guess. > I'll try to think more about this, but I doubt very much I'll find the > reason :( Ohhh... 7 (!!!) hours of hacking + some vodka did the trick. (Kamalesh, I think you hit the same bug). Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/