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Two changes in this cycle: 

- a micro-optimization in static_key_slow_inc_cpuslocked()
  - fix futex death-notification wakeup bug
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'locking-core-2022-12-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar:
 "Two changes in this cycle:

   - a micro-optimization in static_key_slow_inc_cpuslocked()

   - fix futex death-notification wakeup bug"

* tag 'locking-core-2022-12-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Resend potentially swallowed owner death notification
  jump_label: Use atomic_try_cmpxchg() in static_key_slow_inc_cpuslocked()
This commit is contained in:
Linus Torvalds 2022-12-12 15:14:53 -08:00
parent 2f60f83084 90d7588967
commit 617fe4fa82
2 changed files with 19 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
kernel/futex/core.c
View File

@ -638,6 +638,7 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
bool pi, bool pending_op)
{
u32 uval, nval, mval;
pid_t owner;
int err;
/* Futex address must be 32bit aligned */
@ -659,6 +660,10 @@ retry:
* 2. A woken up waiter is killed before it can acquire the
* futex in user space.
*
* In the second case, the wake up notification could be generated
* by the unlock path in user space after setting the futex value
* to zero or by the kernel after setting the OWNER_DIED bit below.
*
* In both cases the TID validation below prevents a wakeup of
* potential waiters which can cause these waiters to block
* forever.
@ -667,24 +672,27 @@ retry:
*
* 1) task->robust_list->list_op_pending != NULL
* @pending_op == true
* 2) User space futex value == 0
* 2) The owner part of user space futex value == 0
* 3) Regular futex: @pi == false
*
* If these conditions are met, it is safe to attempt waking up a
* potential waiter without touching the user space futex value and
* trying to set the OWNER_DIED bit. The user space futex value is
* uncontended and the rest of the user space mutex state is
* consistent, so a woken waiter will just take over the
* uncontended futex. Setting the OWNER_DIED bit would create
* inconsistent state and malfunction of the user space owner died
* handling.
* trying to set the OWNER_DIED bit. If the futex value is zero,
* the rest of the user space mutex state is consistent, so a woken
* waiter will just take over the uncontended futex. Setting the
* OWNER_DIED bit would create inconsistent state and malfunction
* of the user space owner died handling. Otherwise, the OWNER_DIED
* bit is already set, and the woken waiter is expected to deal with
* this.
*/
if (pending_op && !pi && !uval) {
owner = uval & FUTEX_TID_MASK;
if (pending_op && !pi && !owner) {
futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
return 0;
}
if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
if (owner != task_pid_vnr(curr))
return 0;
/*

8
kernel/jump_label.c
View File

@ -115,8 +115,6 @@ EXPORT_SYMBOL_GPL(static_key_count);
void static_key_slow_inc_cpuslocked(struct static_key *key)
{
int v, v1;
STATIC_KEY_CHECK_USE(key);
lockdep_assert_cpus_held();
@ -132,11 +130,9 @@ void static_key_slow_inc_cpuslocked(struct static_key *key)
* so it counts as "enabled" in jump_label_update(). Note that
* atomic_inc_unless_negative() checks >= 0, so roll our own.
*/
for (v = atomic_read(&key->enabled); v > 0; v = v1) {
v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
if (likely(v1 == v))
for (int v = atomic_read(&key->enabled); v > 0; )
if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)))
return;
}
jump_label_lock();
if (atomic_read(&key->enabled) == 0) {