Cobalt Preemption of kernel update_fast_timekeeper can cause deadlocks
jan.kiszka at siemens.com
Wed Dec 19 13:44:56 CET 2018
On 19.12.18 13:09, Philippe Gerum via Xenomai wrote:
> On 12/19/18 11:20 AM, Lange Norbert via Xenomai wrote:
>> There is a deadlock issue that haunted me for several weeks,
>> it is caused by the kernels update of the user-visible
>> timekeeping structures used by the VDSO clock_gettime functions.
>> The kernel regularly updates a Timestamp structure, which is accessible in user-mode,
>> it does so by something akin to a rw-lock in update_fast_timekeeper.
>> If cobalt preempts the core during holding the lock, any thread trying to read the time
>> will continue to spin. (This alone is an issue IMHO).
>> If the cobalt thread itself now call the vDSO function as reader,
>> it will spin on the lock and block the lock from getting released.
>> Either the update_fast_timekeeper funtion should not be preemptible by cobalt,
>> or the spin-lock on reading could fallback to the syscall after a certain amount of retries.
>> The later is probably easier to implement, but then could randomly demote cobalt threads.
>> (on the other hand, this would be always a demotion on platforms without the vdso function)
> update_vsyscall() is locking the write-side. If the analysis is correct, this patch may help at the expense of a some cycles more spent uninterruptible:
> diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c
> index 9fb89b6e88c3..e9baa57e8385 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
> @@ -32,11 +32,14 @@ void update_vsyscall(struct timekeeper *tk)
> int vclock_mode = tk->tkr_mono.clock->archdata.vclock_mode;
> struct vsyscall_gtod_data *vdata = &vsyscall_gtod_data;
> + unsigned long flags;
> /* Mark the new vclock used. */
> BUILD_BUG_ON(VCLOCK_MAX >= 32);
> WRITE_ONCE(vclocks_used, READ_ONCE(vclocks_used) | (1 << vclock_mode));
> + flags = hard_cond_local_irq_save();
> /* copy vsyscall data */
> @@ -77,6 +80,8 @@ void update_vsyscall(struct timekeeper *tk)
> + hard_cond_local_irq_restore(flags);
> if (tk->tkr_mono.clock == &clocksource_tsc)
This should rather be an application bug: An RT (Xenomai) thread is apparently
using Linux gettimeofday & Co. (glibc) from RT context. That was never
supported, we rather have RT services for that (CLOCK_HOST_REALTIME).
We may think about detecting such cases better, though. Norbert, are you using
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