Do not skip two elements of the tid_buffer when reusing the buffer

slot. This eventually results in exhaustion of the tid space, causing
new threads get tid -1 as identifier.

The bad effect of having the thread id equal to -1 is that
UMTX_OP_UMUTEX_WAIT returns EFAULT for a lock owned by such thread,
because casuword cannot distinguish between literal value -1 read from
the address and -1 returned as an indication of faulted
access. _thr_umutex_lock() helper from libthr does not check for
errors from _umtx_op_err(2), causing an infinite loop in
mutex_lock_sleep().

We observed the JVM processes hanging and consuming enormous amount of
system time on machines with approximately 100 days uptime.

Reported by:	Mykola Dzham <freebsd levsha org ua>
MFC after:	1 week

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
index 7c21644..9d92447 100644
--- a/sys/kern/kern_thread.c
+++ b/sys/kern/kern_thread.c
@@ -116,7 +116,7 @@ tid_free(lwpid_t tid)
 	mtx_lock(&tid_lock);
 	if ((tid_tail + 1) % TID_BUFFER_SIZE == tid_head) {
 		tmp_tid = tid_buffer[tid_head++];
-		tid_head = (tid_head + 1) % TID_BUFFER_SIZE;
+		tid_head %= TID_BUFFER_SIZE;
 	}
 	tid_buffer[tid_tail++] = tid;
 	tid_tail %= TID_BUFFER_SIZE;