git: c9e023541aef - main - pbuf_ctor(): lock the buffer with LK_NOWAIT

The branch main has been updated by rew:

URL: https://cgit.FreeBSD.org/src/commit/?id=c9e023541aefdabf0b602e1fd62f5e21a6f8ffa2

commit c9e023541aefdabf0b602e1fd62f5e21a6f8ffa2
Author:     Robert Wing <rew@FreeBSD.org>
AuthorDate: 2022-02-07 19:05:20 +0000
Commit:     Robert Wing <rew@FreeBSD.org>
CommitDate: 2022-02-07 19:05:20 +0000

    pbuf_ctor(): lock the buffer with LK_NOWAIT
    
    This LOR happens when reading from a file backed MD device:
    
    lock order reversal:
     1st 0xfffffe00431eaac0 pbufwait (pbufwait, lockmgr) @ /cobra/src/sys/vm/vm_pager.c:471
     2nd 0xfffff80003f17930 ufs (ufs, lockmgr) @ /cobra/src/sys/dev/md/md.c:977
    lock order pbufwait -> ufs attempted at:
        #0 0xffffffff80c78ead at witness_checkorder+0xbdd
        #1 0xffffffff80bd6a52 at lockmgr_lock_flags+0x182
        #2 0xffffffff80f52d5c at ffs_lock+0x6c
        #3 0xffffffff80d0f3f4 at _vn_lock+0x54
        #4 0xffffffff80708629 at mdstart_vnode+0x499
        #5 0xffffffff807060ec at md_kthread+0x20c
        #6 0xffffffff80bbfcd0 at fork_exit+0x80
        #7 0xffffffff810b809e at fork_trampoline+0xe
    
    This LOR was previously blessed by witness before commit 531f8cfea06b
    ("Use dedicated lock name for pbufs").
    
    Instead of blessing ufs and pbufwait, use LK_NOWAIT to prevent recording
    the lock order. LK_NOWAIT will be a nop here as the lock is dropped in
    pbuf_dtor(). The takes the same approach as 5875b94c7493 ("buf_alloc():
    lock the buffer with LK_NOWAIT").
    
    Reviewed by:    kib
    Differential Revision:  https://reviews.freebsd.org/D34183
---
 sys/vm/vm_pager.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sys/vm/vm_pager.c b/sys/vm/vm_pager.c
index 7dba1c45143d..9b66964b8d8f 100644
--- a/sys/vm/vm_pager.c
+++ b/sys/vm/vm_pager.c
@@ -467,7 +467,7 @@ pbuf_ctor(void *mem, int size, void *arg, int flags)
 	bp->b_ioflags = 0;
 	bp->b_iodone = NULL;
 	bp->b_error = 0;
-	BUF_LOCK(bp, LK_EXCLUSIVE, NULL);
+	BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL);
 
 	return (0);
 }