On 08/26, Sasha Levin wrote: > > On 08/21/2014 01:17 PM, Oleg Nesterov wrote: > >> Is there a race between kill() and exit() brought on by the kill path only > >> > using the RCU read lock? This doesn't prevent ->real_cred from being > >> > modified, but it looks like this should, in combination with > >> > delayed_put_task_struct(), prevent it from being cleared. > > Yes, rcu should protect us from both delayed_put_pid() and > > delayed_put_task(). > > Everything looks correct... And there are a lot of other similar users of > > find_vpid/find_task_by_vpid/pid_task/etc under rcu, I can't recall any bug > > in this area. > > > > I am puzzled. Note also that ->signal == NULL. Will try to think more, > > but so far I have no any idea. > > I've hit something similar earlier today, and it might be related:
Thanks. rsi == ¤t->signal->shared_pending == 0x2a0, so current->signal == 544. Given that this task is current, we can rule out RCU/find_pind/etc problems, this task_struct and its ->signal must be stable. Looks like, something was broken recently. I do not remember the bug reports like this. Say, an unbalanced put_task_struct()... Hmm. > [ 973.452840] BUG: unable to handle kernel NULL pointer dereference at > 00000000000002b0 > [ 973.455347] IP: flush_sigqueue_mask (include/linux/signal.h:118 > kernel/signal.c:715) > [ 973.457526] PGD 4dfdc7067 PUD 5f77d9067 PMD 0 > [ 973.459216] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC > [ 973.460086] Dumping ftrace buffer: > [ 973.460086] (ftrace buffer empty) > [ 973.460086] Modules linked in: > [ 973.460086] CPU: 4 PID: 13145 Comm: trinity-c767 Not tainted > 3.17.0-rc2-next-20140826-sasha-00031-gc48c9ac-dirty #1079 > [ 973.460086] task: ffff880604800000 ti: ffff880586648000 task.ti: > ffff880586648000 > [ 973.460086] RIP: flush_sigqueue_mask (include/linux/signal.h:118 > kernel/signal.c:715) > [ 973.460086] RSP: 0018:ffff88058664bec8 EFLAGS: 00010046 > [ 973.460086] RAX: 0000000000000000 RBX: fffffffffffff730 RCX: > 0000000000010000 > [ 973.460086] RDX: 0000000000000000 RSI: 00000000000002a0 RDI: > ffff88058664bed8 > [ 973.460086] RBP: ffff88058664bf10 R08: 0000000000000001 R09: > 0000000000000001 > [ 973.460086] R10: 000000000002d201 R11: 0000000000000254 R12: > 0000000000000000 > [ 973.460086] R13: ffff88058664bf40 R14: ffff880604800000 R15: > 0000000000000010 > [ 973.460086] FS: 00007fe3a3045700(0000) GS:ffff880277c00000(0000) > knlGS:0000000000000000 > [ 973.460086] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b > [ 973.460086] CR2: 00000000000002b0 CR3: 00000004e23d5000 CR4: > 00000000000006a0 > [ 973.460086] Stack: > [ 973.460086] ffffffffac183690 01017fffb3247180 0000000000010000 > 0000000000000000 > [ 973.460086] 00007fffb3247180 00007fffb3247220 0000000000000011 > 0000000000000000 > [ 973.460086] 0000000000000000 ffff88058664bf78 ffffffffac183ef5 > 0000000000000000 > [ 973.460086] Call Trace: > [ 973.460086] ? do_sigaction (kernel/signal.c:3124 (discriminator 17)) > [ 973.460086] SyS_rt_sigaction (kernel/signal.c:3360 kernel/signal.c:3341) > [ 973.460086] tracesys (arch/x86/kernel/entry_64.S:542) > [ 973.460086] Code: b7 49 09 d5 4d 89 6e 10 48 83 c4 08 5b 41 5c 41 5d 41 5e > 41 5f 5d c3 66 2e 0f 1f 84 00 00 00 00 00 66 66 66 66 90 48 8b 0f 31 c0 <48> > 8b 56 10 48 85 ca 74 7b 55 48 f7 d1 48 89 e5 41 56 48 21 ca > All code > ======== > 0: b7 49 mov $0x49,%bh > 2: 09 d5 or %edx,%ebp > 4: 4d 89 6e 10 mov %r13,0x10(%r14) > 8: 48 83 c4 08 add $0x8,%rsp > c: 5b pop %rbx > d: 41 5c pop %r12 > f: 41 5d pop %r13 > 11: 41 5e pop %r14 > 13: 41 5f pop %r15 > 15: 5d pop %rbp > 16: c3 retq > 17: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) > 1e: 00 00 00 > 21: 66 66 66 66 90 data32 data32 data32 xchg %ax,%ax > 26: 48 8b 0f mov (%rdi),%rcx > 29: 31 c0 xor %eax,%eax > 2b:* 48 8b 56 10 mov 0x10(%rsi),%rdx <-- > trapping instruction > 2f: 48 85 ca test %rcx,%rdx > 32: 74 7b je 0xaf > 34: 55 push %rbp > 35: 48 f7 d1 not %rcx > 38: 48 89 e5 mov %rsp,%rbp > 3b: 41 56 push %r14 > 3d: 48 21 ca and %rcx,%rdx > ... > > Code starting with the faulting instruction > =========================================== > 0: 48 8b 56 10 mov 0x10(%rsi),%rdx > 4: 48 85 ca test %rcx,%rdx > 7: 74 7b je 0x84 > 9: 55 push %rbp > a: 48 f7 d1 not %rcx > d: 48 89 e5 mov %rsp,%rbp > 10: 41 56 push %r14 > 12: 48 21 ca and %rcx,%rdx > ... > [ 973.460086] RIP flush_sigqueue_mask (include/linux/signal.h:118 > kernel/signal.c:715) > [ 973.460086] RSP <ffff88058664bec8> > [ 973.460086] CR2: 00000000000002b0 > > > Thanks, > Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
