shithub: riscv

libc: fix wunlock() libthread deadlock when wunlock() was used by threads running within the same proc, the wunlock() can deadlock as it keeps holding the RWLock.lock spinlock while indirectly calling _threadrendezvous(). when _threadrendezvous() switches to another thread in the same proc, then that thread can hang at rlock()/wlock()/runlock() again waiting for wunlock() to release the spinlock which will never happen as lock() does not schedule threads. wunlock() is changed to release the spinlock during rendezvous wakeup of readers. note that this is a bit dangerous as more readers might queue concurrently now which means that if we cannot keep up with the wakeups, we might keep on waking readers forever. that will be another patch for the future.

--- a/sys/src/libc/9sys/qlock.c

+++ b/sys/src/libc/9sys/qlock.c

@@ -58,7 +58,6 @@

 		return;

 	/* chain into waiting list */

 	mp = getqlp();

 	p = q->tail;

@@ -259,17 +258,19 @@

 	if(p->state != QueuingR)

 		abort();

-	/* wake waiting readers */

-	while(q->head != nil && q->head->state == QueuingR){

-		p = q->head;

+	q->writer = 0;

+	do {

+		/* wake waiting readers */

 		q->head = p->next;

+		if(q->head == nil)

+			q->tail = nil;

 		q->readers++;

+		unlock(&q->lock);

 		while((*_rendezvousp)(p, 0) == (void*)~0)

-	}

-	if(q->head == nil)

-		q->tail = nil;

-	q->writer = 0;

+		lock(&q->lock);

+		p = q->head;

+	} while(p != nil && p->state == QueuingR && q->writer == 0);

 	unlock(&q->lock);