[patch] IPMI KCS can drop the lock while servicing a request

Adrian Chadd adrian at freebsd.org
Tue Apr 2 22:43:26 UTC 2013

On 23 March 2013 20:11, Eric van Gyzen <eric at vangyzen.net> wrote:
> At work, we discovered that our application's IPMI thread would often use a
> lot of CPU time.  The KCS thread uses DELAY to wait for the BMC, so it can
> run without sleeping for a "long" time with a slow BMC.  It also holds the
> ipmi_softc.ipmi_lock during this time.  When using adaptive mutexes, an
> application thread that wants to operate on the ipmi_pending_requests list
> will also spin during this same time.
> We see no reason that the KCS thread needs to hold the lock while servicing
> a request.  We've been running with the attached patch for a few months,
> with no ill effects.

Typically holding a lock is to serialise both program state and hardware state.

The whole "unlock; do blocking work or sleep; lock" thing needs to be
very carefully thought out.


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