locks under printf(9) and WITNESS [Was: new panic in cpu_reset() with WITNESS]

Gleb Smirnoff glebius at FreeBSD.org
Mon Jan 23 13:04:55 UTC 2012

On Sat, Jan 21, 2012 at 07:26:55PM +0200, Andriy Gapon wrote:
A> > BTW, we have a quite strange situation with spin locks in console output path.
A> > cnputs_mtx is marked as MTX_NOWITNESS, supposedly because cnputs (printf) can be
A> > called in any locking context (even during normal operation).  But there are a
A> > number of console-specific locks (scrlock, uart_hwmtx, "syscons video lock")
A> > which are acquired under cnputs_mtx, but which are *not* marked as
A> > MTX_NOWITNESS.  More, they are specified in the witness order_lists as if we
A> > certainly know a correct order for them.
A> > I think that the msgbuf mutex also deserves mentioning in this context.
A> > 
A> > I think that all of these spin locks should be marked as MTX_NOWITNESS (as long
A> > as they stay normal spinlocks), because printf(9) should be usable wherever I
A> > stick it in the code.
A> > 
A> > P.S. The above only matters for WITNESS and !WITNESS_SKIPSPIN and I am not sure
A> > if this combination really matters.
A> > 
A> Here's my take at it:
A> diff --git a/sys/kern/kern_cons.c b/sys/kern/kern_cons.c
A> index 5346bc3..97f0f16 100644


This patch works.

Totus tuus, Glebius.

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