PR/52603 CVS commit: src/sys/kern

["Manuel Bouyer" <bouyer%netbsd.org@localhost>]

The following reply was made to PR port-arm/52603; it has been noted by GNATS.

From: "Manuel Bouyer" <bouyer%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/52603 CVS commit: src/sys/kern
Date: Mon, 16 Oct 2017 15:03:57 +0000

 Module Name:	src
 Committed By:	bouyer
 Date:		Mon Oct 16 15:03:57 UTC 2017
 
 Modified Files:
 	src/sys/kern: subr_pcu.c
 
 Log Message:
 PR port-arm/52603:
 There is a race here, as seen on arm with FPU:
 LWP L is running but not on CPU, has its FPU state on CPU2 which
 has not been released yet, so fpexc still has VFP_FPEXC_EN set in the PCB copy.
 
 LWP L is scheduled on CPU1, CPU1 calls cpu_switchto() for L in mi_switch().
 cpu_switchto() will set VFP_FPEXC_EN in the FPU's fpexc register per the
 PCB fpexc copy.
 
 Before CPU1 calls pcu_switchpoint() for L, CPU2 calls
 pcu_do_op(PCU_CMD_SAVE | PCU_CMD_RELEASE) for L because it still holds its
 FPU state and wants to load another lwp. This cause VFP_FPEXC_EN to
 be cleared in the PCB copy, but not in CPU1's register. L's l_pcu_cpu is
 set to NULL.
 
 When CPU1 calls pcu_switchpoint() for L it see l_pcu_cpu is NULL, and doesn't
 call the release callback.
 
 Now CPU1 has its FPU enabled but with the wrong FPU state.
 
 Fix by releasing the PCU even if l_pcu_cpu is NULL.
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.20 -r1.21 src/sys/kern/subr_pcu.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.