[PATCH] tapset: add a tapset function for reading epoch time in ARM

[Josh Stone]

On 06/12/2014 03:38 AM, Frank Ch. Eigler wrote:
> Hi -
> 
>> gettimeofday_ns() for ARM is implemented to return a fake value because
>> it depends on get_cycles() which returns 0 in ARM. This patch adds
>> a tapset function which uses getnstimeofday() for collecting epoch time.
>> [...]
> 
> (Another way could be to wrap getnstimeofday into a get_cycles()
> emulation function that the normal runtime/time.c code could use.)
> 
> Please note that the reason that the other architectures don't just
> call getnstimeofday() or equivalent functions is because these are (or
> have been) unsafe to call from arbitrary probe contexts, such as
> interrupt-related tracepoints, or kprobes triggered from the vicinity
> of the clock functions.  It was found to be better to give an
> approximate time from all contexts rather than letting the kernel
> crash from some of them.  Please analyze whether this is still an
> issue for you. 

Right, see https://sourceware.org/bugzilla/show_bug.cgi?id=2525

The problem was with xtime_lock, now called timekeeper_seq.  If a probe
occurs while that write lock is held, then calls getnstimeofday(), it
would loop forever trying to get the read lock.  I don't see anything to
make me think this has changed on modern kernels.