On 01/06/2016 11:04 AM, Jiri Bohac wrote:
> 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?
Not exactly. When I initially sent this as a RFE to jstultz he pointed out that
if CPU A had acquired the timekeeper_lock (and therefore incremented tk_core.seq
for a write), and CPU B panicked, no output would occur because the reads of
tk_core.seq would spin indefinitely.
>
> I believe this is already unsafe, as explained in the commit log
> of 6d9bcb62 (timekeeping: use printk_deferred when holding
> timekeeping seqlock).
Hmm ... John Stultz, any suggestions here?
P.
>
> So directly calling ktime_get() from printk would just turn a
> rare deadlock into a certain one - perhaps a good thing?
>
>
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