On Mon, Jun 04, 2018 at 06:01:27PM -0600, Tycho Andersen wrote: > We have reports of the following crash: > > PID: 7 TASK: ffff88085c6d61c0 CPU: 1 COMMAND: "kworker/u25:0" > #0 [ffff88085c6db710] machine_kexec at ffffffff81046239 > #1 [ffff88085c6db760] crash_kexec at ffffffff810fc248 > #2 [ffff88085c6db830] oops_end at ffffffff81008ae7 > #3 [ffff88085c6db860] no_context at ffffffff81050b8f > #4 [ffff88085c6db8b0] __bad_area_nosemaphore at ffffffff81050d75 > #5 [ffff88085c6db900] bad_area_nosemaphore at ffffffff81050e83 > #6 [ffff88085c6db910] __do_page_fault at ffffffff8105132e > #7 [ffff88085c6db9b0] do_page_fault at ffffffff8105152c > #8 [ffff88085c6db9c0] page_fault at ffffffff81a3f122 > [exception RIP: uart_put_char+149] > RIP: ffffffff814b67b5 RSP: ffff88085c6dba78 RFLAGS: 00010006 > RAX: 0000000000000292 RBX: ffffffff827c5120 RCX: 0000000000000081 > RDX: 0000000000000000 RSI: 000000000000005f RDI: ffffffff827c5120 > RBP: ffff88085c6dba98 R8: 000000000000012c R9: ffffffff822ea320 > R10: ffff88085fe4db04 R11: 0000000000000001 R12: ffff881059f9c000 > R13: 0000000000000001 R14: 000000000000005f R15: 0000000000000fba > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018 > #9 [ffff88085c6dbaa0] tty_put_char at ffffffff81497544 > #10 [ffff88085c6dbac0] do_output_char at ffffffff8149c91c > #11 [ffff88085c6dbae0] __process_echoes at ffffffff8149cb8b > #12 [ffff88085c6dbb30] commit_echoes at ffffffff8149cdc2 > #13 [ffff88085c6dbb60] n_tty_receive_buf_fast at ffffffff8149e49b > #14 [ffff88085c6dbbc0] __receive_buf at ffffffff8149ef5a > #15 [ffff88085c6dbc20] n_tty_receive_buf_common at ffffffff8149f016 > #16 [ffff88085c6dbca0] n_tty_receive_buf2 at ffffffff8149f194 > #17 [ffff88085c6dbcb0] flush_to_ldisc at ffffffff814a238a > #18 [ffff88085c6dbd50] process_one_work at ffffffff81090be2 > #19 [ffff88085c6dbe20] worker_thread at ffffffff81091b4d > #20 [ffff88085c6dbeb0] kthread at ffffffff81096384 > #21 [ffff88085c6dbf50] ret_from_fork at ffffffff81a3d69f > > after slogging through some dissasembly: > > ffffffff814b6720 <uart_put_char>: > ffffffff814b6720: 55 push %rbp > ffffffff814b6721: 48 89 e5 mov %rsp,%rbp > ffffffff814b6724: 48 83 ec 20 sub $0x20,%rsp > ffffffff814b6728: 48 89 1c 24 mov %rbx,(%rsp) > ffffffff814b672c: 4c 89 64 24 08 mov %r12,0x8(%rsp) > ffffffff814b6731: 4c 89 6c 24 10 mov %r13,0x10(%rsp) > ffffffff814b6736: 4c 89 74 24 18 mov %r14,0x18(%rsp) > ffffffff814b673b: e8 b0 8e 58 00 callq ffffffff81a3f5f0 <mcount> > ffffffff814b6740: 4c 8b a7 88 02 00 00 mov 0x288(%rdi),%r12 > ffffffff814b6747: 45 31 ed xor %r13d,%r13d > ffffffff814b674a: 41 89 f6 mov %esi,%r14d > ffffffff814b674d: 49 83 bc 24 70 01 00 cmpq $0x0,0x170(%r12) > ffffffff814b6754: 00 00 > ffffffff814b6756: 49 8b 9c 24 80 01 00 mov 0x180(%r12),%rbx > ffffffff814b675d: 00 > ffffffff814b675e: 74 2f je ffffffff814b678f > <uart_put_char+0x6f> > ffffffff814b6760: 48 89 df mov %rbx,%rdi > ffffffff814b6763: e8 a8 67 58 00 callq ffffffff81a3cf10 > <_raw_spin_lock_irqsave> > ffffffff814b6768: 41 8b 8c 24 78 01 00 mov 0x178(%r12),%ecx > ffffffff814b676f: 00 > ffffffff814b6770: 89 ca mov %ecx,%edx > ffffffff814b6772: f7 d2 not %edx > ffffffff814b6774: 41 03 94 24 7c 01 00 add 0x17c(%r12),%edx > ffffffff814b677b: 00 > ffffffff814b677c: 81 e2 ff 0f 00 00 and $0xfff,%edx > ffffffff814b6782: 75 23 jne ffffffff814b67a7 > <uart_put_char+0x87> > ffffffff814b6784: 48 89 c6 mov %rax,%rsi > ffffffff814b6787: 48 89 df mov %rbx,%rdi > ffffffff814b678a: e8 e1 64 58 00 callq ffffffff81a3cc70 > <_raw_spin_unlock_irqrestore> > ffffffff814b678f: 44 89 e8 mov %r13d,%eax > ffffffff814b6792: 48 8b 1c 24 mov (%rsp),%rbx > ffffffff814b6796: 4c 8b 64 24 08 mov 0x8(%rsp),%r12 > ffffffff814b679b: 4c 8b 6c 24 10 mov 0x10(%rsp),%r13 > ffffffff814b67a0: 4c 8b 74 24 18 mov 0x18(%rsp),%r14 > ffffffff814b67a5: c9 leaveq > ffffffff814b67a6: c3 retq > ffffffff814b67a7: 49 8b 94 24 70 01 00 mov 0x170(%r12),%rdx > ffffffff814b67ae: 00 > ffffffff814b67af: 48 63 c9 movslq %ecx,%rcx > ffffffff814b67b2: 41 b5 01 mov $0x1,%r13b > ffffffff814b67b5: 44 88 34 0a mov %r14b,(%rdx,%rcx,1) > ffffffff814b67b9: 41 8b 94 24 78 01 00 mov 0x178(%r12),%edx > ffffffff814b67c0: 00 > ffffffff814b67c1: 83 c2 01 add $0x1,%edx > ffffffff814b67c4: 81 e2 ff 0f 00 00 and $0xfff,%edx > ffffffff814b67ca: 41 89 94 24 78 01 00 mov %edx,0x178(%r12) > ffffffff814b67d1: 00 > ffffffff814b67d2: eb b0 jmp ffffffff814b6784 > <uart_put_char+0x64> > ffffffff814b67d4: 66 66 66 2e 0f 1f 84 data32 data32 nopw > %cs:0x0(%rax,%rax,1) > ffffffff814b67db: 00 00 00 00 00 > > for our build, this is crashing at: > > circ->buf[circ->head] = c; > > Looking in uart_port_startup(), it seems that circ->buf (state->xmit.buf) > protected by the "per-port mutex", which based on uart_port_check() is > state->port.mutex. Indeed, the lock acquired in uart_put_char() is > uport->lock, i.e. not the same lock. > > Anyway, since the lock is not acquired, if uart_shutdown() is called, the > last chunk of that function may release state->xmit.buf before its assigned > to null, and cause the race above. > > To fix it, we simply also acquire state->port.mutex.
Ick. Can't we just grab the same lock in the other place? Grabbing 2 locks for every individual character seems _really_ heavy, don't you think? thanks, greg k-h
