[tip:perf/urgent] perf: Fix contexted inheritance

tip-bot for Peter Zijlstra
2011-01-18T19:07:32Z

Commit-ID: c5ed5145591774bd9a2960ba4ca45a02fc70aad1
Gitweb: http://git.kernel.org/tip/c5ed5145591774bd9a2960ba4ca45a02fc70aad1
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Mon, 17 Jan 2011 13:45:37 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 18 Jan 2011 15:10:35 +0100

perf: Fix contexted inheritance

Linus reported that the RCU lockdep annotation bits triggered for this
rcu_dereference() because we're not holding rcu_read_lock().

Going over the code I cannot convince myself its correct:

- holding a ref on the parent_ctx, doesn't avoid it being uncloned
concurrently (as the comment says), so we can race with a free.

- holding parent_ctx->mutex doesn't avoid the above free from taking
place either, it would at best avoid parent_ctx from being freed.

I.e. the warning is correct. To fix the bug, serialize against the
unclone_ctx() call by extending the reach of the parent_ctx->lock.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/perf_event.c | 11 +++++------
1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index b782b7a..76be4c7 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -6494,7 +6494,6 @@ int perf_event_init_context(struct task_struct *child, int ctxn)

raw_spin_lock_irqsave(&parent_ctx->lock, flags);
parent_ctx->rotate_disable = 0;
- raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);

child_ctx = child->perf_event_ctxp[ctxn];

@@ -6502,12 +6501,11 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
/*
* Mark the child context as a clone of the parent
* context, or of whatever the parent is a clone of.
- * Note that if the parent is a clone, it could get
- * uncloned at any point, but that doesn't matter
- * because the list of events and the generation
- * count can't have changed since we took the mutex.
+ *
+ * Note that if the parent is a clone, the holding of
+ * parent_ctx->lock avoids it from being uncloned.
*/
- cloned_ctx = rcu_dereference(parent_ctx->parent_ctx);
+ cloned_ctx = parent_ctx->parent_ctx;
if (cloned_ctx) {
child_ctx->parent_ctx = cloned_ctx;
child_ctx->parent_gen = parent_ctx->parent_gen;
@@ -6518,6 +6516,7 @@ int perf_event_init_context(struct task_struct *child, int ctxn)
get_ctx(child_ctx->parent_ctx);
}

+ raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
mutex_unlock(&parent_ctx->mutex);

perf_unpin_context(parent_ctx);