Date: Sat, 4 Apr 2009 22:23:03 +0000 (UTC) From: Marcel Moolenaar <marcel@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r190704 - head/sys/powerpc/aim Message-ID: <200904042223.n34MN3RG082677@svn.freebsd.org>
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Author: marcel Date: Sat Apr 4 22:23:03 2009 New Revision: 190704 URL: http://svn.freebsd.org/changeset/base/190704 Log: Perform a dummy stwcx. when we switch contexts. The context being switched out may hold a reservation. The stwcx. will clear the reservation. This is architecturally recommended. The scenario this addresses is as follows: 1. Thread 1 performs a lwarx and as such holds a reservation. 2. Thread 1 gets switched out (before doing the matching stwcx.) and thread 2 is switched in. 3. Thread 2 performs a stwcx. to the same reservation granule. This will succeed because the processor has the reservation even though thread 2 didn't do the lwarx. Note that on some processors the address given the stwcx. is not checked. On these processors the mere condition of having a reservation would cause the stwcx. to succeed, irrespective of whether the addresses are the same. The dummy stwcx. is especially important for those processors. Modified: head/sys/powerpc/aim/swtch.S Modified: head/sys/powerpc/aim/swtch.S ============================================================================== --- head/sys/powerpc/aim/swtch.S Sat Apr 4 22:05:36 2009 (r190703) +++ head/sys/powerpc/aim/swtch.S Sat Apr 4 22:23:03 2009 (r190704) @@ -155,6 +155,12 @@ cpu_switchin: mtsr USER_SR,%r5 isync lwz %r1,PCB_SP(%r3) /* Load the stack pointer */ + /* + * Perform a dummy stwcx. to clear any reservations we may have + * inherited from the previous thread. It doesn't matter if the + * stwcx succeeds or not. pcb_context[0] can be clobbered. + */ + stwcx. %r1, 0, %r3 blr /*
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