On Wed, Jul 11, 2018 at 02:49:08PM -0500, Serge E. Hallyn wrote: > Quoting Tycho Andersen ([email protected]): > > 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, let's lock uport->lock when allocating/deallocating > > state->xmit.buf in addition to the per-port mutex. > > > > v2: switch to locking uport->lock on allocation/deallocation instead of > > locking the per-port mutex in uart_put_char. Note that since > > uport->lock is a spin lock, we have to switch the allocation to > > GFP_ATOMIC. > > v3: move the allocation outside the lock, so we can switch back to > > GFP_KERNEL > > v4: * switch to positive condition of state->xmit.buf in > > uart_port_startup() > > * declare `flags` on its own line > > * use the result of uart_port_lock() in uart_shutdown() to avoid > > uninitialized warning > > * don't use the uart_port_lock/unlock macros in uart_port_startup, > > instead test against uport directly; the compiler can't seem to "see" > > through the macros/ref/unref calls to not warn about uninitialized > > flags. We don't need to do a ref here since we hold the per-port > > mutex anyway. > > > > Signed-off-by: Tycho Andersen <[email protected]> > > One question: would it be worth doing: > > > --- > > drivers/tty/serial/serial_core.c | 19 ++++++++++++++----- > > 1 file changed, 14 insertions(+), 5 deletions(-) > > > > diff --git a/drivers/tty/serial/serial_core.c > > b/drivers/tty/serial/serial_core.c > > index 9c14a453f73c..406e8382d3de 100644 > > --- a/drivers/tty/serial/serial_core.c > > +++ b/drivers/tty/serial/serial_core.c > > @@ -182,6 +182,7 @@ static int uart_port_startup(struct tty_struct *tty, > > struct uart_state *state, > > { > > struct uart_port *uport = uart_port_check(state); > > unsigned long page; > > + unsigned long flags; > > int retval = 0; > > > > if (uport->type == PORT_UNKNOWN) > > @@ -196,15 +197,20 @@ static int uart_port_startup(struct tty_struct *tty, > > struct uart_state *state, > > * Initialise and allocate the transmit and temporary > > * buffer. > > */ > > - if (!state->xmit.buf) { > > - /* This is protected by the per port mutex */ > > - page = get_zeroed_page(GFP_KERNEL); > > - if (!page) > > - return -ENOMEM; > > + page = get_zeroed_page(GFP_KERNEL); > > ignoring the alloc failure here. (leave a comment for worried > future reviewers) Then, > > > + if (!page) > > + return -ENOMEM; > > > > + if (uport) > > + spin_lock_irqsave(&uport->lock, flags); > > + if (state->xmit.buf) { > > + free_page(page); > > + } else { > > here if page is NULL (unlock and) return ENOMEM. > > Since it looks like this fn is called on every device open, that > would seem to minimize getting lots of needless ENOMEMs. > > Another alternative is to just wait to do the alloc until we > get here, and drop the spinlock here if we need to alloc. Then > retake the lock, re-check state->xmit.buf; If that is now not > null then free_page(page), or if it is still NULL and page alloc > failed, then return ENOEMEM). That is uglier code, but is > probably the best behavior. > > Still the original patch fixes the real bug so I'm fine with it > as well.
Sure, I'm happy to implement whichever of these we think is best. Greg? Thanks, Tycho
