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    Subject[tip: sched/rt] preempt: Provide preempt_[dis|en]able_nested()
    The following commit has been merged into the sched/rt branch of tip:

    Commit-ID: 555bb4ccd1dd78d0263eae31629fe1fdd65c1fb5
    Gitweb: https://git.kernel.org/tip/555bb4ccd1dd78d0263eae31629fe1fdd65c1fb5
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Thu, 25 Aug 2022 18:41:24 +02:00
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitterDate: Mon, 19 Sep 2022 14:35:07 +02:00

    preempt: Provide preempt_[dis|en]able_nested()

    On PREEMPT_RT enabled kernels, spinlocks and rwlocks are neither disabling
    preemption nor interrupts. Though there are a few places which depend on
    the implicit preemption/interrupt disable of those locks, e.g. seqcount
    write sections, per CPU statistics updates etc.

    To avoid sprinkling CONFIG_PREEMPT_RT conditionals all over the place, add
    preempt_disable_nested() and preempt_enable_nested() which should be
    descriptive enough.

    Add a lockdep assertion for the !PREEMPT_RT case to catch callers which
    do not have preemption disabled.

    Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lore.kernel.org/r/20220825164131.402717-2-bigeasy@linutronix.de

    ---
    include/linux/preempt.h | 42 ++++++++++++++++++++++++++++++++++++++++-
    1 file changed, 42 insertions(+)

    diff --git a/include/linux/preempt.h b/include/linux/preempt.h
    index b4381f2..0df425b 100644
    --- a/include/linux/preempt.h
    +++ b/include/linux/preempt.h
    @@ -421,4 +421,46 @@ static inline void migrate_enable(void) { }

    #endif /* CONFIG_SMP */

    +/**
    + * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
    + *
    + * Use for code which requires preemption protection inside a critical
    + * section which has preemption disabled implicitly on non-PREEMPT_RT
    + * enabled kernels, by e.g.:
    + * - holding a spinlock/rwlock
    + * - soft interrupt context
    + * - regular interrupt handlers
    + *
    + * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
    + * interrupt context and regular interrupt handlers are preemptible and
    + * only prevent migration. preempt_disable_nested() ensures that preemption
    + * is disabled for cases which require CPU local serialization even on
    + * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
    + *
    + * The use cases are code sequences which are not serialized by a
    + * particular lock instance, e.g.:
    + * - seqcount write side critical sections where the seqcount is not
    + * associated to a particular lock and therefore the automatic
    + * protection mechanism does not work. This prevents a live lock
    + * against a preempting high priority reader.
    + * - RMW per CPU variable updates like vmstat.
    + */
    +/* Macro to avoid header recursion hell vs. lockdep */
    +#define preempt_disable_nested() \
    +do { \
    + if (IS_ENABLED(CONFIG_PREEMPT_RT)) \
    + preempt_disable(); \
    + else \
    + lockdep_assert_preemption_disabled(); \
    +} while (0)
    +
    +/**
    + * preempt_enable_nested - Undo the effect of preempt_disable_nested()
    + */
    +static __always_inline void preempt_enable_nested(void)
    +{
    + if (IS_ENABLED(CONFIG_PREEMPT_RT))
    + preempt_enable();
    +}
    +
    #endif /* __LINUX_PREEMPT_H */
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