git: a15f2c5cc58f - main - proc: perform P_CONTROLT check on fork without SESS_LOCK

[Mateusz Guzik]

The branch main has been updated by mjg:

URL: 
https://cgit.FreeBSD.org/src/commit/?id=a15f2c5cc58f403407daf797a2e6963adffa6c2c

commit a15f2c5cc58f403407daf797a2e6963adffa6c2c
Author:     Mateusz Guzik <m...@freebsd.org>
AuthorDate: 2025-09-25 11:50:01 +0000
Commit:     Mateusz Guzik <m...@freebsd.org>
CommitDate: 2025-09-25 11:50:55 +0000

    proc: perform P_CONTROLT check on fork without SESS_LOCK
    
    The flag is guarded by proc lock which is already held.
    
    After tracing poudriere for over 4 hours like so:
    dtrace -n 'fbt::do_fork:entry { @[curthread->td_proc->p_flag & 0x2] = 
count(); }':
    
        2         15605151
        0         20074116
    
    Over 56% of cases did not have P_CONTROLT set and could have avoided the
    lock.
    
    This reduces hold time of the proctree lock.
---
 sys/kern/kern_fork.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 2ab9b363f8b5..7f6abae187b3 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -610,10 +610,12 @@ do_fork(struct thread *td, struct fork_req *fr, struct 
proc *p2, struct thread *
        p2->p_flag |= p1->p_flag & P_SUGID;
        td2->td_pflags |= td->td_pflags & (TDP_ALTSTACK | TDP_SIGFASTBLOCK);
        td2->td_pflags2 |= td->td_pflags2 & TDP2_UEXTERR;
-       SESS_LOCK(p1->p_session);
-       if (p1->p_session->s_ttyvp != NULL && p1->p_flag & P_CONTROLT)
-               p2->p_flag |= P_CONTROLT;
-       SESS_UNLOCK(p1->p_session);
+       if (p1->p_flag & P_CONTROLT) {
+               SESS_LOCK(p1->p_session);
+               if (p1->p_session->s_ttyvp != NULL)
+                       p2->p_flag |= P_CONTROLT;
+               SESS_UNLOCK(p1->p_session);
+       }
        if (fr->fr_flags & RFPPWAIT)
                p2->p_flag |= P_PPWAIT;