lkml.org 
[lkml]   [2009]   [Apr]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    SubjectRe: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier
    Date
    From
    David Howells <dhowells@redhat.com> wrote:

    > So we can't assume that complete(), wake_up() and co. imply any barriers.
    >
    > All we can assume is that try_to_wake_up() implies a write barrier, but we
    > can't assume that that will be called via __wake_up_common().

    So how about this, then?

    David
    ---
    From: David Howells <dhowells@redhat.com>
    Subject: [PATCH] Document that wake_up(), complete() and co. may not imply a memory barrier

    Add to the memory barriers document to note that wake_up(), complete() and
    co. may not be assumed to imply any sort of memory barrier, with the exception
    of try_to_wake_up() and things derived from that.

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    Documentation/memory-barriers.txt | 34 +++++++++++++++++++++++++++++++++-
    kernel/sched.c | 10 ++++++++++
    2 files changed, 43 insertions(+), 1 deletions(-)


    diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
    index f5b7127..6bd626a 100644
    --- a/Documentation/memory-barriers.txt
    +++ b/Documentation/memory-barriers.txt
    @@ -42,6 +42,7 @@ Contents:

    - Interprocessor interaction.
    - Atomic operations.
    + - Wake up of processes
    - Accessing devices.
    - Interrupts.

    @@ -1224,6 +1225,9 @@ Other functions that imply barriers:

    (*) schedule() and similar imply full memory barriers.

    + (*) try_to_wake_up() and things derived from that imply a write memory
    + barrier.
    +

    =================================
    INTER-CPU LOCKING BARRIER EFFECTS
    @@ -1366,13 +1370,15 @@ WHERE ARE MEMORY BARRIERS NEEDED?

    Under normal operation, memory operation reordering is generally not going to
    be a problem as a single-threaded linear piece of code will still appear to
    -work correctly, even if it's in an SMP kernel. There are, however, three
    +work correctly, even if it's in an SMP kernel. There are, however, five
    circumstances in which reordering definitely _could_ be a problem:

    (*) Interprocessor interaction.

    (*) Atomic operations.

    + (*) Wake up of processes.
    +
    (*) Accessing devices.

    (*) Interrupts.
    @@ -1568,6 +1574,32 @@ and in such cases the special barrier primitives will be no-ops.
    See Documentation/atomic_ops.txt for more information.


    +WAKE UP OF PROCESSES
    +--------------------
    +
    +An unlock, write memory barrier or a full memory barrier may be needed before a
    +call to wake up another processes if the waker sets some state that the sleeper
    +will need to see.
    +
    + complete();
    + wake_up();
    + wake_up_all();
    + wake_up_bit();
    + wake_up_interruptible();
    + wake_up_interruptible_all();
    + wake_up_interruptible_nr();
    + wake_up_interruptible_poll();
    + wake_up_interruptible_sync();
    + wake_up_interruptible_sync_poll();
    + wake_up_locked();
    + wake_up_locked_poll();
    + wake_up_nr();
    + wake_up_poll();
    +
    +The sleeper may then need to interpolate a lock, read or full memory barrier
    +before accessing that state.
    +
    +
    ACCESSING DEVICES
    -----------------

    diff --git a/kernel/sched.c b/kernel/sched.c
    index b902e58..7cbc3de 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -2337,6 +2337,8 @@ static int sched_balance_self(int cpu, int flag)
    * runnable without the overhead of this.
    *
    * returns failure only if the task is already active.
    + *
    + * It may be assumed that this function implies a full memory barrier.
    */
    static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)
    {
    @@ -5241,6 +5243,8 @@ void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
    * @mode: which threads
    * @nr_exclusive: how many wake-one or wake-many threads to wake up
    * @key: is directly passed to the wakeup function
    + *
    + * It may not be assumed that this function implies any sort of memory barrier.
    */
    void __wake_up(wait_queue_head_t *q, unsigned int mode,
    int nr_exclusive, void *key)
    @@ -5279,6 +5283,8 @@ void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key)
    * with each other. This can prevent needless bouncing between CPUs.
    *
    * On UP it can prevent extra preemption.
    + *
    + * It may not be assumed that this function implies any sort of memory barrier.
    */
    void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
    int nr_exclusive, void *key)
    @@ -5315,6 +5321,8 @@ EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */
    * awakened in the same order in which they were queued.
    *
    * See also complete_all(), wait_for_completion() and related routines.
    + *
    + * It may not be assumed that this function implies any sort of memory barrier.
    */
    void complete(struct completion *x)
    {
    @@ -5332,6 +5340,8 @@ EXPORT_SYMBOL(complete);
    * @x: holds the state of this particular completion
    *
    * This will wake up all threads waiting on this particular completion event.
    + *
    + * It may not be assumed that this function implies any sort of memory barrier.
    */
    void complete_all(struct completion *x)
    {

    \
     
     \ /
      Last update: 2009-04-22 18:27    [W:0.032 / U:31.292 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site