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    SubjectRe: [PATCH 1/2] kernel, timekeeping, add trylock option to ktime_get_with_offset()
    On Wed, Jan 06, 2016 at 08:00:33AM -0500, Prarit Bhargava wrote:
    > -ktime_t ktime_get_with_offset(enum tk_offsets offs)
    > +ktime_t ktime_get_with_offset(enum tk_offsets offs, int trylock)
    > {
    > struct timekeeper *tk = &tk_core.timekeeper;
    > unsigned int seq;
    > ktime_t base, *offset = offsets[offs];
    > s64 nsecs;
    > + unsigned long flags = 0;
    > +
    > + if (unlikely(!timekeeping_initialized))
    > + return ktime_set(0, 0);
    >
    > WARN_ON(timekeeping_suspended);
    >
    > + if (trylock && !raw_spin_trylock_irqsave(&timekeeper_lock, flags))
    > + return ktime_set(KTIME_MAX, 0);
    > +

    Are you trying to avoid a deadlock caused by calling printk() with
    timekeeper_lock locked?

    I believe this is already unsafe, as explained in the commit log
    of 6d9bcb62 (timekeeping: use printk_deferred when holding
    timekeeping seqlock).

    So directly calling ktime_get() from printk would just turn a
    rare deadlock into a certain one - perhaps a good thing?


    --
    Jiri Bohac <jbohac@suse.cz>
    SUSE Labs, SUSE CZ



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