Re: Cobalt Preemption of kernel update_fast_timekeeper can cause deadlocks

[Philippe Gerum <rpm@xenomai.org>]

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();
+
 	gtod_write_begin(vdata);
 
 	/* copy vsyscall data */
@@ -77,6 +80,8 @@ void update_vsyscall(struct timekeeper *tk)
 
 	gtod_write_end(vdata);
 
+	hard_cond_local_irq_restore(flags);
+
 	if (tk->tkr_mono.clock == &clocksource_tsc)
 		ipipe_update_hostrt(tk);
 }

-- 
Philippe.