From owner-freebsd-hackers@FreeBSD.ORG  Fri Mar  9 13:28:53 2007
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Date: Fri, 9 Mar 2007 14:28:51 +0100
From: Divacky Roman <xdivac02@stud.fit.vutbr.cz>
To: hackers@freebsd.org
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Cc: alc@freebsd.org
Subject: possibly missed wakeup in swap_pager_getpages()
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hi

when looking at PR103455 I found this and I have a question..

code:

   swp_pager_strategy(bp);

        /*
         * wait for the page we want to complete.  VPO_SWAPINPROG is always
         * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
         * is set in the meta-data.
         */
        VM_OBJECT_LOCK(object);
        while ((mreq->oflags & VPO_SWAPINPROG) != 0) {
                mreq->oflags |= VPO_WANTED;
                vm_page_lock_queues();
                vm_page_flag_set(mreq, PG_REFERENCED);
                vm_page_unlock_queues();
                cnt.v_intrans++;
                if (msleep(mreq, VM_OBJECT_MTX(object), PSWP, "swread", hz*20)) {

the swp_pager_strategy() initiates IO (which should issue the wakeup() necessary
for the msleep()). the problem in the PR is that the msleep() is never (within the
specified 20 seconds) woken up. I wonder if this is because the wakeup arrives BEFORE
the actual msleep() is issued. the page queue locking can take some time so there
is a space for a missed wakeup.

is my analysis correct? if so, can the race be mitigated by moving the flag setting (hence
also the locking) after the msleep()?

thnx

roman