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    SubjectRe: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled
    On 12/15/11 19:20, Frank Rowand wrote:
    >
    > ARMv6 and later have VIPT caches and the TLBs are tagged with an ASID
    > (application specific ID). The number of ASIDs is limited to 256 and
    > the allocation algorithm requires IPIs when all the ASIDs have been
    > used. The IPIs require interrupts enabled during context switch for
    > deadlock avoidance.
    >
    > The RT patch mm-protect-activate-switch-mm.patch disables irqs around
    > activate_mm() and switch_mm(), which are the portion of the ARMv6
    > context switch that require interrupts enabled.
    >
    > The solution for the ARMv6 processors could be to _not_ disable irqs.
    > A more conservative solution is to provide the same environment that
    > the scheduler provides, that is preempt_disable(). This is more
    > resilient for possible future changes to the ARM context switch code
    > that is not aware of the RT patches.
    >
    > This patch will conflict slightly with Catalin's patch set to remove
    > __ARCH_WANT_INTERRUPTS_ON_CTXSW, when that is accepted:
    >
    > http://lkml.indiana.edu/hypermail/linux/kernel/1111.3/01893.html
    >
    > When Catalin's patch set is accepted, this RT patch will need to reverse
    > the change in patch 6 to arch/arm/include/asm/system.h:
    >
    > -#ifndef CONFIG_CPU_HAS_ASID
    > -#define __ARCH_WANT_INTERRUPTS_ON_CTXSW
    > -#endif
    >
    > Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
    >
    > ---
    > fs/exec.c | 8 8 + 0 - 0 !
    > mm/mmu_context.c | 8 8 + 0 - 0 !
    > 2 files changed, 16 insertions(+)
    >
    > Index: b/fs/exec.c
    > ===================================================================
    > --- a/fs/exec.c
    > +++ b/fs/exec.c
    > @@ -837,12 +837,20 @@ static int exec_mmap(struct mm_struct *m
    > }
    > }
    > task_lock(tsk);
    > +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW

    Self critique... I really, really dislike this ifdef because it will
    fail silently if the includes are changed and as a result the include file that
    defines __ARCH_WANT_INTERRUPTS_ON_CTXSW (arch/arm/include/asm/system.h) is not
    included.

    Does anyone have any brilliant ideas for an alternative approach?

    -Frank



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