t that perf_counter_init_task had to take that lock
and then take whatever lock/mutex protects the top-level counter's
child_list.  But the counter enable/disable functions need to take
that lock in order to traverse the list, then for each counter take
the lock in that counter's context in order to change the counter's
state safely, which will lead to a deadlock.

To solve this, we now have both a mutex and a spinlock in the context,
and taking either is sufficient to ensure the list of counters can't
change - you have to take both before changing the list.  Now
perf_counter_init_task takes the mutex instead of the lock (which
incidentally means that inherit_counter can use GFP_KERNEL instead of
GFP_ATOMIC) and thus avoids the possible deadlock.  Similarly the new
enable/disable functions can take the mutex while traversing the list
of child counters without incurring a possible deadlock when the
counter manipulation code locks the context for a child counter.

We also had an misfeature that the first counter added to a context
would possibly not go on until the next sched-in, because we were
using ctx->nr_active to detect if the context was running on a CPU.
But nr_active is the number of active counters, and if that was zero
(because the context didn't have any counters yet) it would look like
the context wasn't running on a cpu and so the retry code in
__perf_install_in_context wouldn't retry.  So this adds an 'is_active'
field that is set when the context is on a CPU, even if it has no
counters.  The is_active field is only used for task contexts, not for
per-cpu contexts.

If we enable a subsidiary counter in a group that is active on a CPU,
and the arch code can't enable the counter, then we have to pull the
whole group off the CPU.  We do this with group_sched_out, which gets
moved up in the file so it comes before all its callers.  This also
adds similar logic to __perf_install_in_context so that the "all on,
or none" invariant of groups is preserved when adding a new counter to
a group.

Signed-off-by: Paul Mackerras <paulus@samba.org>
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