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Date:      Thu, 24 Sep 1998 17:33:33 -0700 (PDT)
From:      Matthew Dillon <dillon@backplane.com>
To:        Luoqi Chen <luoqi@watermarkgroup.com>
Cc:        committers@FreeBSD.ORG
Subject:   Kernel is hitting my debugging printf's
Message-ID:  <199809250033.RAA22359@apollo.backplane.com>
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brelse-A: releasing BP marked B_DELWRI 204202b0
vfs_vmio_release: releasing BP marked B_DELWRI 204202b0
						^^^^
						bp->b_flags

						B_DELWRI
						B_CACHE
						B_BUSY
						D_BONE
						B_CLUSTEROK
						B_RELBUF
						B_VMIO

brelse-A: releasing BP marked B_DELWRI 204202b0
vfs_vmio_release: releasing BP marked B_DELWRI 204202b0
brelse-A: releasing BP marked B_DELWRI 204202b0
vfs_vmio_release: releasing BP marked B_DELWRI 204202b0

    I stuck this printf in kern/vfs_bio.c around line 671, in brelse():
	
		...

                /* MATT HACK */
                if ((bp->b_flags & (B_INVAL|B_RELBUF)) && (bp->b_flags & B_DELWRI))
                        printf("brelse-A: releasing BP marked B_DELWRI %08lx\n", (long)bp->b_flags);

                if (bp->b_flags & (B_INVAL | B_RELBUF))
                        vfs_vmio_release(bp);
        } else if (bp->b_flags & B_VMIO) {
                /* MATT HACK */
                if ((bp->b_flags & (B_INVAL|B_RELBUF)) && (bp->b_flags & B_DELWRI))
                        printf("brelse-B: releasing BP marked B_DELWRI %08lx\n", (long)bp->b_flags);


    If I understand the code correctly, this is illegal.  Here's the sequence
    of events as I understand it:

	* small write is made, page associated with bp and modified.  page is
	  now dirty.

	* bdwrite() is called

	* bp is marked B_DELWRI and page is remarked clean (this turns out to
	  be necessary so we can detect if the page is modified again)

	* page is *required* to remain associated with the bp until it is
	  written out by the VFS system, because if the page is disassociated
	  from the bp the fact that it has been marked clean can cause it to
	  be thrown away, thus we lose the write() we did.

	* The logs above show that the page is being disassociated while 
	  B_DELWRI is still set in the bp.  I believe this to be illegal.
	  From looking at the code it appears that B_DELWRI is cleared when
	  actual I/O is initiated on the bp.  The logs above show that that
	  has not occured (B_DELWRI is still set).

    So, can any experts out there tell me if I'm barking up the right tree or
    not?  I don't want to bug John too much since he has other projects going
    on.  Luoqi?  Anyone else ?

					-Matt

    Matthew Dillon  Engineering, HiWay Technologies, Inc. & BEST Internet 
                    Communications & God knows what else.
    <dillon@backplane.com> (Please include original email in any response)

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