The following patch fixes double free manifesting itself as crash in __rcu_process_callbasks(): http://marc.info/?l=linux-kernel&m=117518764517017&w=2 https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=229112
The problem is with check_dead_utrace() conditionally scheduling "struct utrace" for freeing but not cleaning struct task_struct::utrace pointer leaving it reachable: tsk->utrace_flags = flags; if (flags) spin_unlock(&utrace->lock); else rcu_utrace_free(utrace); OTOH, utrace_release_task() first clears ->utrace pointer, then frees struct utrace itself: Roland inserted some debugging into 2.6.21-rc6-mm1 so that aforementined double free couldn't be reproduced without seeing BUG at kernel/utrace.c:176 first. It triggers if one struct utrace were passed to rcu_utrace_free() second time. With patch applied I no longer see¹ BUG message and double frees on 2-way P3, 8-way ia64, Core 2 Duo boxes. Testcase is at the first link. I _think_ it adds leak if utrace_reap() takes branch without freeing but, well, I hope Roland will give me some clue on how to fix it too. Signed-off-by: Alexey Dobriyan <[EMAIL PROTECTED]> --- kernel/utrace.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-) ¹ But I see whole can of other bugs! I think they were already lurking but weren't easily reproducable without hitting double-free first. FWIW, it's BUG_ON(!list_empty(&tsk->ptracees)); oops at the beginning of remove_engine() NULL ->report_quiesce call which is absent in ptrace utrace ops BUG_ON(tracehook_check_released(p)); --- a/kernel/utrace.c +++ b/kernel/utrace.c @@ -205,7 +205,6 @@ utrace_clear_tsk(struct task_struct *tsk if (utrace->u.live.signal == NULL) { task_lock(tsk); if (likely(tsk->utrace != NULL)) { - rcu_assign_pointer(tsk->utrace, NULL); tsk->utrace_flags &= UTRACE_ACTION_NOREAP; } task_unlock(tsk); @@ -305,10 +304,7 @@ check_dead_utrace(struct task_struct *ts } tsk->utrace_flags = flags; - if (flags) - spin_unlock(&utrace->lock); - else - rcu_utrace_free(utrace); + spin_unlock(&utrace->lock); /* * Now we're finished updating the utrace state. - 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/