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    Subject[PATCH V4] scsi: avoid potential deadlock in iscsi_if_rx func
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    In iscsi_if_rx func, after receiving one request through iscsi_if_recv_msg func,
    iscsi_if_send_reply will be called to try to reply the request in do-loop.
    If the return of iscsi_if_send_reply func return -EAGAIN all the time, one deadlock will occur.

    For example, a client only send msg without calling recvmsg func, then it will result in the watchdog soft lockup.
    The details are given as follows,

    Details of the special case which can cause deadlock:

    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ISCSI);
    retval = bind(sock_fd, (struct sock addr*) & src_addr, sizeof(src_addr);
    while (1) {
    state_msg = sendmsg(sock_fd, &msg, 0);
    //Note: recvmsg(sock_fd, &msg, 0) is not processed here.
    }
    close(sock_fd);

    watchdog: BUG: soft lockup - CPU#7 stuck for 22s! [netlink_test:253305] Sample time: 4000897528 ns(HZ: 250) Sample stat:
    curr: user: 675503481560, nice: 321724050, sys: 448689506750, idle: 4654054240530, iowait: 40885550700, irq: 14161174020, softirq: 8104324140, st: 0
    deta: user: 0, nice: 0, sys: 3998210100, idle: 0, iowait: 0, irq: 1547170, softirq: 242870, st: 0 Sample softirq:
    TIMER: 992
    SCHED: 8
    Sample irqstat:
    irq 2: delta 1003, curr: 3103802, arch_timer
    CPU: 7 PID: 253305 Comm: netlink_test Kdump: loaded Tainted: G OE
    Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
    pstate: 40400005 (nZcv daif +PAN -UAO)
    pc : __alloc_skb+0x104/0x1b0
    lr : __alloc_skb+0x9c/0x1b0
    sp : ffff000033603a30
    x29: ffff000033603a30 x28: 00000000000002dd
    x27: ffff800b34ced810 x26: ffff800ba7569f00
    x25: 00000000ffffffff x24: 0000000000000000
    x23: ffff800f7c43f600 x22: 0000000000480020
    x21: ffff0000091d9000 x20: ffff800b34eff200
    x19: ffff800ba7569f00 x18: 0000000000000000
    x17: 0000000000000000 x16: 0000000000000000
    x15: 0000000000000000 x14: 0001000101000100
    x13: 0000000101010000 x12: 0101000001010100
    x11: 0001010101010001 x10: 00000000000002dd
    x9 : ffff000033603d58 x8 : ffff800b34eff400
    x7 : ffff800ba7569200 x6 : ffff800b34eff400
    x5 : 0000000000000000 x4 : 00000000ffffffff
    x3 : 0000000000000000 x2 : 0000000000000001
    x1 : ffff800b34eff2c0 x0 : 0000000000000300 Call trace:
    __alloc_skb+0x104/0x1b0
    iscsi_if_rx+0x144/0x12bc [scsi_transport_iscsi]
    netlink_unicast+0x1e0/0x258
    netlink_sendmsg+0x310/0x378
    sock_sendmsg+0x4c/0x70
    sock_write_iter+0x90/0xf0
    __vfs_write+0x11c/0x190
    vfs_write+0xac/0x1c0
    ksys_write+0x6c/0xd8
    __arm64_sys_write+0x24/0x30
    el0_svc_common+0x78/0x130
    el0_svc_handler+0x38/0x78
    el0_svc+0x8/0xc

    Here, we add one limit of retry times in do-loop to avoid the deadlock.

    V4:
    - modify the patch subject, no code change.

    V3:
    - replace the error with warning as suggested by Ulrich

    V2:
    - add some debug kernel message as suggested by Lee Duncan

    Signed-off-by: Bo Wu <wubo40@huawei.com>
    Reviewed-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
    Reviewed-by: Lee Duncan <LDuncan@suse.com>
    ---
    drivers/scsi/scsi_transport_iscsi.c | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/drivers/scsi/scsi_transport_iscsi.c b/drivers/scsi/scsi_transport_iscsi.c
    index 417b868d8735..ed8d9709b9b9 100644
    --- a/drivers/scsi/scsi_transport_iscsi.c
    +++ b/drivers/scsi/scsi_transport_iscsi.c
    @@ -24,6 +24,8 @@

    #define ISCSI_TRANSPORT_VERSION "2.0-870"

    +#define ISCSI_SEND_MAX_ALLOWED 10
    +
    #define CREATE_TRACE_POINTS
    #include <trace/events/iscsi.h>

    @@ -3682,6 +3684,7 @@ iscsi_if_rx(struct sk_buff *skb)
    struct nlmsghdr *nlh;
    struct iscsi_uevent *ev;
    uint32_t group;
    + int retries = ISCSI_SEND_MAX_ALLOWED;

    nlh = nlmsg_hdr(skb);
    if (nlh->nlmsg_len < sizeof(*nlh) + sizeof(*ev) ||
    @@ -3712,6 +3715,10 @@ iscsi_if_rx(struct sk_buff *skb)
    break;
    err = iscsi_if_send_reply(portid, nlh->nlmsg_type,
    ev, sizeof(*ev));
    + if (err == -EAGAIN && --retries < 0) {
    + printk(KERN_WARNING "Send reply failed, error %d\n", err);
    + break;
    + }
    } while (err < 0 && err != -ECONNREFUSED && err != -ESRCH);
    skb_pull(skb, rlen);
    }
    --
    1.8.3.1
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