lkml.org 
[lkml]   [2019]   [Dec]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 4.14 195/209] futex: Add mutex around futex exit
    Date
    From: Thomas Gleixner <tglx@linutronix.de>

    commit 3f186d974826847a07bc7964d79ec4eded475ad9 upstream.

    The mutex will be used in subsequent changes to replace the busy looping of
    a waiter when the futex owner is currently executing the exit cleanup to
    prevent a potential live lock.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Ingo Molnar <mingo@kernel.org>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20191106224556.845798895@linutronix.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/futex.h | 1 +
    include/linux/sched.h | 1 +
    kernel/futex.c | 16 ++++++++++++++++
    3 files changed, 18 insertions(+)

    --- a/include/linux/futex.h
    +++ b/include/linux/futex.h
    @@ -68,6 +68,7 @@ static inline void futex_init_task(struc
    INIT_LIST_HEAD(&tsk->pi_state_list);
    tsk->pi_state_cache = NULL;
    tsk->futex_state = FUTEX_STATE_OK;
    + mutex_init(&tsk->futex_exit_mutex);
    }

    void futex_exit_recursive(struct task_struct *tsk);
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -959,6 +959,7 @@ struct task_struct {
    #endif
    struct list_head pi_state_list;
    struct futex_pi_state *pi_state_cache;
    + struct mutex futex_exit_mutex;
    unsigned int futex_state;
    #endif
    #ifdef CONFIG_PERF_EVENTS
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -3721,12 +3721,23 @@ static void futex_cleanup(struct task_st
    */
    void futex_exit_recursive(struct task_struct *tsk)
    {
    + /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
    + if (tsk->futex_state == FUTEX_STATE_EXITING)
    + mutex_unlock(&tsk->futex_exit_mutex);
    tsk->futex_state = FUTEX_STATE_DEAD;
    }

    static void futex_cleanup_begin(struct task_struct *tsk)
    {
    /*
    + * Prevent various race issues against a concurrent incoming waiter
    + * including live locks by forcing the waiter to block on
    + * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
    + * attach_to_pi_owner().
    + */
    + mutex_lock(&tsk->futex_exit_mutex);
    +
    + /*
    * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
    *
    * This ensures that all subsequent checks of tsk->futex_state in
    @@ -3749,6 +3760,11 @@ static void futex_cleanup_end(struct tas
    * take another loop until it becomes visible.
    */
    tsk->futex_state = state;
    + /*
    + * Drop the exit protection. This unblocks waiters which observed
    + * FUTEX_STATE_EXITING to reevaluate the state.
    + */
    + mutex_unlock(&tsk->futex_exit_mutex);
    }

    void futex_exec_release(struct task_struct *tsk)

    \
     
     \ /
      Last update: 2019-12-04 19:19    [W:3.094 / U:0.744 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site