Re: [PATCH] srcu: Implement more-efficient reader counts

[Lance Roy]

On Mon, 23 Jan 2017 16:42:52 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Jan 23, 2017 at 01:35:18PM -0800, Lance Roy wrote:
> > SRCU uses two per-cpu counters: a nesting counter to count the number of
> > active critical sections, and a sequence counter to ensure that the nesting
> > counters don't change while they are being added together in
> > srcu_readers_active_idx_check().
> > 
> > This patch instead uses per-cpu lock and unlock counters. Because both
> > counters only increase and srcu_readers_active_idx_check() reads the unlock
> > counter before the lock counter, this achieves the same end without having
> > to increment two different counters in srcu_read_lock(). This also saves a
> > smp_mb() in srcu_readers_active_idx_check().
> > 
> > Possible bug: There is no guarantee that the lock counter won't overflow
> > during srcu_readers_active_idx_check(), as there are no memory barriers
> > around srcu_flip() (see comment in srcu_readers_active_idx_check() for
> > details). However, this problem was already present before this patch.
> > 
> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Signed-off-by: Lance Roy <ldr709@xxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>  
> 
> OK, this only has differences only in the comments, so I can reasonably
> substitute it, even this near the next merge window.
> 
> But let's talk about the potential overflow.  If I understand correctly,
> for this to happen, there needs to be ULONG_MAX/2 or thereabouts
> srcu_read_lock() calls without matching srcu_read_unlock() calls.
> Let's focus on 32-bit systems for the moment.
> 
> Lockdep allows at most 48 locks held at a given time by any single task,
> and RCU does not pass in a non-NULL nest_lock -- if you acquire more than
> that, a lockdep kernel will splat.  Each task has at least one 4K page of
> kernel stack, so there can be at most 1,048,576 tasks (actually quite a
> bit fewer due to the size of the task_struct and so on).  This allows
> at most 50,331,648 unmatched srcu_read_lock() calls in the system,
> which is not sufficient to cause overflow.
> 
> Or am I missing something here?
> 
> 								Thanx, Paul

It isn't the nest that is the problem. Here is a previous email I wrote
describing the problem:


The trouble is that disabling preemption is not enough to ensure that there
is at most one srcu_read_lock() call per CPU that missed the srcu_flip().

Define a reader to be an SRCU lock+unlock pair. A reader is called active if it
has incremented ->lock_count[] but hasn't incremented ->unlock_count[] yet, and
completed if it has incremented ->unlock_count[]. I think that we only want to
limit the number of active readers and the number of CPUs. In particular, I
don't think there should be a limit on the rate of completion of read side
critical section.

The goal of srcu_readers_active_idx_check() is to verify that there were zero
active readers on the inactive index at some time during its execution. To do
this, it totals the unlock counts, executes a memory barrier, totals the lock
counts, and takes the difference. This difference counts the readers that are
active when srcu_readers_lock_idx() gets to their CPU, plus the readers that
completed after srcu_readers_unlock_idx() and before srcu_readers_lock_idx().
If the true (infinite precision) value of the difference is zero, then there
were no active readers at some point while srcu_readers_lock_idx() is running.
However, the difference is only stored in a long, so there is a potential for
overflow if too many readers complete during srcu_readers_active_idx_check().

For example, let's say there are three threads, each running on their own CPU:

int data, flag;
struct srcu_struct *sp = /* ... */;

Thread 0:
	data = 1;
	synchronize_srcu(sp);
	flag = 1;

Thread 1:
	int data_r, flag_r;
	int idx = srcu_read_lock(sp);
	data_r = data;
	flag_r = flag;
	srcu_read_unlock(sp, idx);
	BUG_ON(flag_r == 1 && data_r == 0);

Thread 2:
	while (true) {
		int idx = srcu_read_lock(sp);
		srcu_read_unlock(sp, idx);
	}

Let's take the following execution order. Thread 1 increments
the CPU 1 version of sp->lock_count[0], sets idx to zero, and loads data (0)
into data_r. Thread 0 then sets data to be 1, verifies that there are no
readers on index 1, and increments sp->completed, but the CPU actually doesn't
preform the last operation, putting it off until the next memory barrier. Thread
0 then calls srcu_readers_active_idx_check() on index 0, which runs
srcu_readers_unlock_idx() (returning 0). Right after srcu_readers_unlock_idx()
completes, thread 2 starts running. Since Thread 0 hasn't actually incremented
sp->completed in any way that is visible to thread 2, srcu_read_lock() will
still return 0. Thread 2 can then run for ULONG_MAX iterations, setting
the CPU 2 version of sp->unlock_count[0] to ULONG_MAX. CPU 0 then finally gets
around to incrementing sp->completed, runs its memory barrier, and then reads
the lock counts: 1, 0, ULONG_MAX. The total of ULONG_MAX+1 will overflow to 0
and compare equal with earlier unlock count. Thread 0 will then think that the
grace period is over and set flag to one. Thread 1 can then read flag (1) into
flag_r and run srcu_read_unlock(). The BUG_ON statement will then fail.

Although ULONG_MAX readers completed during srcu_readers_active_idx_check(),
there were at most 2 active readers at any time, so this program doesn't run
into any limit.

I hope that was clear enough.

Thanks,
Lance