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perf: Fix contexted inheritance

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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>
This commit is contained in:
Peter Zijlstra 2011-01-17 13:45:37 +01:00 committed by Ingo Molnar
parent ad7f4e3f7b
commit c5ed514559
1 changed files with 5 additions and 6 deletions
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11
kernel/perf_event.c
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@ -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);