On 2/18/23 03:23, Martin Pieuchot wrote:
On 24/01/23(Tue) 04:40, Renato Aguiar wrote:
Hi Martin,
"David Hill" <[email protected]> writes:
Yes, same result as before. This patch does not seem to help.
I could also reproduce it with patched 'current' :(
Here's another possible fix I came up with. The idea is to deliberately
allow readers to starve writers for the VM map lock.
My previous fix didn't work because a writer could already be waiting on
the lock before calling vm_map_busy() and there's currently no way to
tell it to back out and stop preventing readers from making progress.
I don't see any simpler solution without rewriting the vm_map_lock()
primitive.
Could you confirm this work for you? If so I'll document the new
RW_FORCEREAD flag and add a comment to explain why we need it in this
case.
Thanks,
Martin
I cannot reproduce the hang with this diff.
Index: kern/kern_rwlock.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_rwlock.c,v
retrieving revision 1.48
diff -u -p -r1.48 kern_rwlock.c
--- kern/kern_rwlock.c 10 May 2022 16:56:16 -0000 1.48
+++ kern/kern_rwlock.c 18 Feb 2023 07:48:23 -0000
@@ -225,7 +225,7 @@ rw_enter(struct rwlock *rwl, int flags)
{
const struct rwlock_op *op;
struct sleep_state sls;
- unsigned long inc, o;
+ unsigned long inc, o, check;
#ifdef MULTIPROCESSOR
/*
* If process holds the kernel lock, then we want to give up on CPU
@@ -248,10 +248,14 @@ rw_enter(struct rwlock *rwl, int flags)
#endif
op = &rw_ops[(flags & RW_OPMASK) - 1];
+ check = op->check;
+
+ if (flags & RW_FORCEREAD)
+ check &= ~RWLOCK_WRWANT;
inc = op->inc + RW_PROC(curproc) * op->proc_mult;
retry:
- while (__predict_false(((o = rwl->rwl_owner) & op->check) != 0)) {
+ while (__predict_false(((o = rwl->rwl_owner) & check) != 0)) {
unsigned long set = o | op->wait_set;
int do_sleep;
Index: sys/rwlock.h
===================================================================
RCS file: /cvs/src/sys/sys/rwlock.h,v
retrieving revision 1.28
diff -u -p -r1.28 rwlock.h
--- sys/rwlock.h 11 Jan 2021 18:49:38 -0000 1.28
+++ sys/rwlock.h 18 Feb 2023 07:44:13 -0000
@@ -117,6 +117,7 @@ struct rwlock {
#define RW_NOSLEEP 0x0040UL /* don't wait for the lock */
#define RW_RECURSEFAIL 0x0080UL /* Fail on recursion for RRW
locks. */
#define RW_DUPOK 0x0100UL /* Permit duplicate lock */
+#define RW_FORCEREAD 0x0200UL /* Let a reader preempt writers. */
/*
* for rw_status() and rrw_status() only: exclusive lock held by
Index: uvm/uvm_map.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_map.c,v
retrieving revision 1.312
diff -u -p -r1.312 uvm_map.c
--- uvm/uvm_map.c 13 Feb 2023 14:52:55 -0000 1.312
+++ uvm/uvm_map.c 18 Feb 2023 07:46:23 -0000
@@ -5406,7 +5406,7 @@ void
vm_map_lock_read_ln(struct vm_map *map, char *file, int line)
{
if ((map->flags & VM_MAP_INTRSAFE) == 0)
- rw_enter_read(&map->lock);
+ rw_enter(&map->lock, RW_READ|RW_FORCEREAD);
else
mtx_enter(&map->mtx);
LPRINTF(("map lock: %p (at %s %d)\n", map, file, line));
