git: be8e84c2e924 - main - runq: runq_not_empty() to support racy lookups again

[Olivier Certner]

The branch main has been updated by olce:

URL: 
https://cgit.FreeBSD.org/src/commit/?id=be8e84c2e9240110ce99bb8d14259073340e4ef6

commit be8e84c2e9240110ce99bb8d14259073340e4ef6
Author:     Olivier Certner <o...@freebsd.org>
AuthorDate: 2025-06-20 02:36:09 +0000
Commit:     Olivier Certner <o...@freebsd.org>
CommitDate: 2025-06-20 02:42:16 +0000

    runq: runq_not_empty() to support racy lookups again
    
    The 4BSD assumes that its sched_runnable(), which then calls
    runq_not_empty(), can be called racily (another CPU may be grabbing
    a thread from the global queue at the same time).
    
    This makes runq_not_empty() trip on an assertion in runq_find()
    detecting a mismatch between some bit in the status word and the state
    of the corresponding queue (empty/not empty).
    
    Re-implement runq_not_empty() in an independent and simpler way as an
    alternative to removing the assertion, which remains useful to detect
    races when removing or adding a thread in some runqueue.
    
    Fixes:          79f68322c892 ("runq: runq_check(): Re-implement on top of 
runq_findq()")
    MFC after:      1 month
    Event:          Kitchener-Waterloo Hackathon 202506
    Sponsored by:   The FreeBSD Foundation
---
 sys/kern/kern_switch.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/sys/kern/kern_switch.c b/sys/kern/kern_switch.c
index 917f73682564..42fee61a99a0 100644
--- a/sys/kern/kern_switch.c
+++ b/sys/kern/kern_switch.c
@@ -493,7 +493,7 @@ int
 runq_findq(struct runq *const rq, const int lvl_min, const int lvl_max,
     runq_pred_t *const pred, void *const pred_data)
 {
-       rqsw_t const (*const rqsw)[RQSW_NB] = &rq->rq_status.rq_sw;
+       const rqsw_t (*const rqsw)[RQSW_NB] = &rq->rq_status.rq_sw;
        rqsw_t w;
        int i, last, idx;
 
@@ -568,20 +568,27 @@ runq_first_thread(struct runq *const rq)
 
 /*
  * Return true if there are some processes of any priority on the run queue,
- * false otherwise.  Has no side effects.
+ * false otherwise.  Has no side effects.  Supports racy lookups (required by
+ * 4BSD).
  */
 bool
 runq_not_empty(struct runq *rq)
 {
-       struct thread *const td = runq_first_thread(rq);
+       const rqsw_t (*const rqsw)[RQSW_NB] = &rq->rq_status.rq_sw;
+       int sw_idx;
 
-       if (td != NULL) {
-               CTR2(KTR_RUNQ, "runq_not_empty: idx=%d, td=%p",
-                   td->td_rqindex, td);
-               return (true);
+       for (sw_idx = 0; sw_idx < RQSW_NB; ++sw_idx) {
+               const rqsw_t w = (*rqsw)[sw_idx];
+
+               if (w != 0) {
+                       CTR3(KTR_RUNQ, "runq_not_empty: not empty; "
+                           "rq=%p, sw_idx=%d, bits=" RQSW_PRI,
+                           rq, sw_idx, w);
+                       return (true);
+               }
        }
 
-       CTR0(KTR_RUNQ, "runq_not_empty: empty");
+       CTR1(KTR_RUNQ, "runq_not_empty: empty; rq=%p", rq);
        return (false);
 }