Commit 8e7ff730dd96 for kernel

commit 8e7ff730dd96519a333d1570edf1c3fabb6d3629
Author: Yao Kai <yaokai34@huawei.com>
Date:   Tue Aug 4 20:55:30 2026 +0800

    futex: Fix race in futex_pivot_pending() during private hash resize

    A task performing a custom private hash resize can remain blocked in
    uninterruptible sleep indefinitely.  The hung-task detector reports:

      INFO: task futex-resizer:314 blocked for more than 10 seconds.
      task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311

      Call Trace:
       __schedule+0x521/0xf30
       schedule+0x22/0xa0
       futex_hash_allocate+0x3db/0x490
       __do_sys_prctl+0x6f5/0xbd0
       do_syscall_64+0xf9/0x530
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

      Kernel panic - not syncing: hung_task: blocked tasks

    futex_pivot_pending() allows the resize request to continue when
    either no replacement hash is pending (hash_new == NULL) or the current
    hash reference count has reached zero.

    After the final-reference wake, another futex task can complete the
    pivot between the two observations:

      T1                                  T2

      futex_hash_allocate()
        wait_var_event(mm, ...)
          futex_pivot_pending(mm)
            hash_new != NULL
                                          futex_hash()
                                            futex_ref_get(old) -> false
                                            futex_pivot_hash(mm)
                                              hash_new = NULL
                                              __futex_pivot_hash(mm, new)
                                                rcu_assign_pointer(hash, new)
            fph = rcu_dereference(hash) /* new */
            futex_ref_is_dead(fph) -> false
          schedule()

    The pivot changes the state from hash_new != NULL with a dead current
    hash to hash_new == NULL with a live current hash.  Because
    futex_pivot_pending() reads hash_new and hash without serialization,
    the resize task can observe hash_new in the pre-pivot state and hash in
    the post-pivot state, causing futex_pivot_pending() to return false even
    though the pivot has completed.  The task then goes to sleep after the
    wakeup has already been consumed.

    Serialize state reads in futex_pivot_pending() using futex_mm_phash::lock.
    This guarantees that futex_pivot_pending() observes hash_new and hash
    atomically, eliminating the race condition.

    Fixes: bd54df5ea7ca ("futex: Allow to resize the private local hash")
    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Yao Kai <yaokai34@huawei.com>
    Signed-off-by: Thomas Gleixner <tglx@kernel.org>
    Cc: stable@vger.kernel.org
    Link: https://patch.msgid.link/20260804125530.3933754-1-yaokai34@huawei.com

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 2650d1e52803..128c5752f225 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1783,14 +1783,15 @@ void futex_hash_free(struct mm_struct *mm)

 static bool futex_pivot_pending(struct mm_struct *mm)
 {
+	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;

-	guard(rcu)();
+	guard(mutex)(&mmph->lock);

-	if (!mm->futex.phash.hash_new)
+	if (!mmph->hash_new)
 		return true;

-	fph = rcu_dereference(mm->futex.phash.hash);
+	fph = rcu_dereference_raw(mmph->hash);
 	return futex_ref_is_dead(fph);
 }