spin_unlock_wait() has an unintuitive 'feature' in that it doesn't
fully serialize against the spin_unlock() we've waited on.
In particular, spin_unlock_wait() only provides a control dependency,
which is a LOAD->STORE order. This means subsequent loads can creep up
and observe state prior to the waited-for unlock. This means we don't
necessarily observe the full critical section.
We must employ smp_acquire__after_ctrl_dep() to upgrade the
LOAD->STORE to LOAD->{LOAD,STORE} aka. load-AQUIRE and thereby ensure
we observe the full critical section we've waited on.
Many spin_unlock_wait() users were unaware of this issue and need
help.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
drivers/ata/libata-eh.c | 4 +++-
kernel/exit.c | 14 ++++++++++++--
kernel/sched/completion.c | 7 +++++++
kernel/task_work.c | 2 +-
4 files changed, 23 insertions(+), 4 deletions(-)
--- a/drivers/ata/libata-eh.c
+++ b/drivers/ata/libata-eh.c
@@ -703,8 +703,10 @@ void ata_scsi_cmd_error_handler(struct S
/* initialize eh_tries */
ap->eh_tries = ATA_EH_MAX_TRIES;
- } else
+ } else {
spin_unlock_wait(ap->lock);
+ smp_acquire__after_ctrl_dep();
+ }
}
EXPORT_SYMBOL(ata_scsi_cmd_error_handler);
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -776,11 +776,16 @@ void do_exit(long code)
exit_signals(tsk); /* sets PF_EXITING */
/*
- * tsk->flags are checked in the futex code to protect against
- * an exiting task cleaning up the robust pi futexes.
+ * Ensure that all new tsk->pi_lock acquisitions must observe
+ * PF_EXITING. Serializes against futex.c:attach_to_pi_owner().
*/
smp_mb();
raw_spin_unlock_wait(&tsk->pi_lock);
+ /*
+ * Ensure that we must observe the pi_state in exit_mm() ->
+ * mm_release() -> exit_pi_state_list().
+ */
+ smp_acquire__after_ctrl_dep();
if (unlikely(in_atomic())) {
pr_info("note: %s[%d] exited with preempt_count %d\n",
@@ -897,6 +902,11 @@ void do_exit(long code)
*/
smp_mb();
raw_spin_unlock_wait(&tsk->pi_lock);
+ /*
+ * Since there are no following loads the LOAD->LOAD order
+ * provided by smp_acquire__after_ctrl_dep() is not
+ * strictly required.
+ */
/* causes final put_task_struct in finish_task_switch(). */
tsk->state = TASK_DEAD;
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -312,6 +312,13 @@ bool completion_done(struct completion *
*/
smp_rmb();
spin_unlock_wait(&x->wait.lock);
+ /*
+ * Even though we've observed 'done', this doesn't mean we can observe
+ * all stores prior to complete(), as the only RELEASE barrier on that
+ * path is provided by the spin_unlock().
+ */
+ smp_acquire__after_ctrl_dep();
+
return true;
}
EXPORT_SYMBOL(completion_done);
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -108,7 +108,7 @@ void task_work_run(void)
* fail, but it can play with *work and other entries.
*/
raw_spin_unlock_wait(&task->pi_lock);
- smp_mb();
+ smp_acquire__after_ctrl_dep();
do {
next = work->next;
--
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