CVS commit: src/sys/kern

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

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.