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    Subject[PATCH] ioprio: grab rcu_read_lock in sys_ioprio_{set,get}()
    Date
    Using:
    - CONFIG_LOCKUP_DETECTOR=y
    - CONFIG_PREEMPT=y
    - CONFIG_LOCKDEP=y
    - CONFIG_PROVE_LOCKING=y
    - CONFIG_PROVE_RCU=y
    found a missing rcu lock during boot on a 512 MiB x86_64 ubuntu vm:
    ===================================================
    [ INFO: suspicious rcu_dereference_check() usage. ]
    ---------------------------------------------------
    kernel/pid.c:419 invoked rcu_dereference_check() without protection!

    other info that might help us debug this:

    rcu_scheduler_active = 1, debug_locks = 0
    1 lock held by ureadahead/1355:
    #0: (tasklist_lock){.+.+..}, at: [<ffffffff8115bc09>] sys_ioprio_set+0x7f/0x29e

    stack backtrace:
    Pid: 1355, comm: ureadahead Not tainted 2.6.37-dbg-DEV #1
    Call Trace:
    [<ffffffff8109c10c>] lockdep_rcu_dereference+0xaa/0xb3
    [<ffffffff81088cbf>] find_task_by_pid_ns+0x44/0x5d
    [<ffffffff81088cfa>] find_task_by_vpid+0x22/0x24
    [<ffffffff8115bc3e>] sys_ioprio_set+0xb4/0x29e
    [<ffffffff8147cf21>] ? trace_hardirqs_off_thunk+0x3a/0x3c
    [<ffffffff8105c409>] sysenter_dispatch+0x7/0x2c
    [<ffffffff8147cee2>] ? trace_hardirqs_on_thunk+0x3a/0x3f

    The fix is to:
    a) grab rcu lock in sys_ioprio_{set,get}() and
    b) avoid grabbing tasklist_lock.
    Discussion in: http://marc.info/?l=linux-kernel&m=128951324702889

    Signed-off-by: Greg Thelen <gthelen@google.com>
    ---
    fs/ioprio.c | 13 ++++---------
    1 files changed, 4 insertions(+), 9 deletions(-)

    diff --git a/fs/ioprio.c b/fs/ioprio.c
    index 748cfb9..7da2a06 100644
    --- a/fs/ioprio.c
    +++ b/fs/ioprio.c
    @@ -103,12 +103,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
    }

    ret = -ESRCH;
    - /*
    - * We want IOPRIO_WHO_PGRP/IOPRIO_WHO_USER to be "atomic",
    - * so we can't use rcu_read_lock(). See re-copy of ->ioprio
    - * in copy_process().
    - */
    - read_lock(&tasklist_lock);
    + rcu_read_lock();
    switch (which) {
    case IOPRIO_WHO_PROCESS:
    if (!who)
    @@ -153,7 +148,7 @@ free_uid:
    ret = -EINVAL;
    }

    - read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    return ret;
    }

    @@ -197,7 +192,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
    int ret = -ESRCH;
    int tmpio;

    - read_lock(&tasklist_lock);
    + rcu_read_lock();
    switch (which) {
    case IOPRIO_WHO_PROCESS:
    if (!who)
    @@ -250,6 +245,6 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
    ret = -EINVAL;
    }

    - read_unlock(&tasklist_lock);
    + rcu_read_unlock();
    return ret;
    }
    --
    1.7.3.1


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