Messages in this thread | | | Subject | Re: [PATCH 1/2] syscalls: avoid time() using __cvdso_gettimeofday in use-level's VDSO | From | Vincenzo Frascino <> | Date | Thu, 26 Nov 2020 11:36:54 +0000 |
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Hi Thomas.
On 11/25/20 11:32 AM, Thomas Gleixner wrote: [...]
>>> Here we propose to use '__NR_time' to invoke syscall directly that makes >>> test all get real seconds via ktime_get_real_second. > > This is a general problem and not really just for this particular test > case. > > Due to the internal implementation of ktime_get_real_seconds(), which is > a 2038 safe replacement for the former get_seconds() function, this > accumulation issue can be observed. (time(2) via syscall and newer > versions of VDSO use the same mechanism). > > clock_gettime(CLOCK_REALTIME, &ts); > sec = time(); > assert(sec >= ts.tv_sec); > > That assert can trigger for two reasons: > > 1) Clock was set between the clock_gettime() and time(). > > 2) The clock has advanced far enough that: > > timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC > > #1 is just a property of clock REALTIME. There is nothing we can do > about that. > > #2 is due to the optimized get_seconds()/time() access which avoids to > read the clock. This can happen on bare metal as well, but is far > more likely to be exposed on virt. > > The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE > > clock_gettime(CLOCK_XXX, &ts); > clock_gettime(CLOCK_XXX_COARSE, &tc); > assert(tc.tv_sec >= ts.tv_sec); > > The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec > pair without reading the clock. Same as #2 above just extended to clock > MONOTONIC. > > There is no way to fix this except giving up on the fast accessors and > make everything take the slow path and read the clock, which might make > a lot of people unhappy. > > For clock REALTIME #1 is anyway an issue, so I think documenting this > proper is the right thing to do. > > Thoughts? >
I completely agree with your analysis and the fact that we should document this information.
My proposal would be to use either the vDSO document present in the kernel [1] or the man pages for time(2) and clock_gettime(2). Probably the second would be more accessible to user space developers.
[1] Documentation/ABI/stable/vdso
> Thanks, > > tglx >
-- Regards, Vincenzo
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