From owner-freebsd-alpha  Sat Nov  9 13:10:23 2002
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Date: Sat, 09 Nov 2002 15:10:04 -0600
From: "Alan L. Cox" <alc@imimic.com>
Organization: iMimic Networking, Inc.
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To: Jeff Roberson <jroberson@chesapeake.net>,
	Matthew Jacob <mjacob@feral.com>,
	Andrew Gallatin <gallatin@cs.duke.edu>, alpha@FreeBSD.ORG,
	alc@FreeBSD.ORG, John Baldwin <jhb@FreeBSD.ORG>
Subject: Re: alpha: top of tree kernel blooie
References: <20021109150359.B97372-100000@mail.chesapeake.net> <3DCD7244.DEEA7387@imimic.com>
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"Alan L. Cox" wrote:
> ...
> 2. Don't we compile by default for the older Alphas that lack byte
> manipulation instructions?  Thus, a byte store is implemented by a
> read-modify-write sequence of instructions.  Thus, two simultaneous
> uma_dbg_alloc()s on adjacent locations in us_freelist could cause
> corruption.
> 

Specifically, here's the code sequence for the assignment of 255 to
us_freelist[] in uma_dbg_alloc() when compiling for ev5:

 47c:   0b 00 43 2c     ldq_u   t1,11(t2)
 480:   2b 00 7b 23     lda     t12,43(t12)
 484:   42 00 5b 48     mskbl   t1,t12,t1
 488:   ff ff 3f 20     lda     t0,-1
 48c:   61 01 3b 48     insbl   t0,t12,t0
 490:   01 04 22 44     or      t0,t1,t0
 494:   0b 00 23 3c     stq_u   t0,11(t2)

Regards,
Alan

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