Caution - possible system instability on attempted fix for "WRITE_ERROR" problem (see enclosed)

[Karl Denninger]

Hi folks;

This may be the wrong place, given what I did, but I wanted to give a "heads
up" here given the impending release of 5.4-RELEASE

This refers to http://www.freebsd.org/cgi/query-pr.cgi?pr=77643

In an attempt to mitigate this, I saw the following commit in the CVS logs:


mdodd       2005-03-02 04:01:37 UTC 
    
  FreeBSD src repository
    
  Modified files:
    sys/dev/ata          ata-queue.c
  Log:
  When resubmitting a timed out request, reset donecount.
  
  Submitted by:     Nate Lawson <nate AT root.org>
  
  Revision  Changes    Path
  1.42      +1 -0      src/sys/dev/ata/ata-queue.c

Is this change supposed to be "safe" against a 5.4-PRERELEASE kernel from
today (CVSupped about 1700 CST)?

If it is supposed to be, its NOT!

It DOES fix the failure to requeue timed out requests, but it also provokes
radical destabilization of the interrupt system in the kernel (e.g. receive
serial interrupts "disappear", etc) leading evenutally to a panic.

BTW, it <DOES> appear to fix the requeue problem with disks, and wth this in
a disk that takes a timeout (but is actually working) does not disconnect
from a GEOM mirror; the retried write succeeds.

However, for obvious reasons the kernel instability that results from the
retried write is not acceptable :)

Don't know if this is germane to what is about to show up in 5.4-RELEASE,
but if it is, this urgently needs to be looked at.

Needless to say I've backed this one out.

Will also put this against the PR to dissuade others from trying the same
thing...

--
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