perf: Fix perf_lock_task_context() vs RCU
Jiri managed to trigger this warning: [] ====================================================== [] [ INFO: possible circular locking dependency detected ] [] 3.10.0+ #228 Tainted: G W [] ------------------------------------------------------- [] p/6613 is trying to acquire lock: [] (rcu_node_0){..-...}, at: [<ffffffff810ca797>] rcu_read_unlock_special+0xa7/0x250 [] [] but task is already holding lock: [] (&ctx->lock){-.-...}, at: [<ffffffff810f2879>] perf_lock_task_context+0xd9/0x2c0 [] [] which lock already depends on the new lock. [] [] the existing dependency chain (in reverse order) is: [] [] -> #4 (&ctx->lock){-.-...}: [] -> #3 (&rq->lock){-.-.-.}: [] -> #2 (&p->pi_lock){-.-.-.}: [] -> #1 (&rnp->nocb_gp_wq[1]){......}: [] -> #0 (rcu_node_0){..-...}: Paul was quick to explain that due to preemptible RCU we cannot call rcu_read_unlock() while holding scheduler (or nested) locks when part of the read side critical section was preemptible. Therefore solve it by making the entire RCU read side non-preemptible. Also pull out the retry from under the non-preempt to play nice with RT. Reported-by: Jiri Olsa <jolsa@redhat.com> Helped-out-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: <stable@kernel.org> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -947,8 +947,18 @@ perf_lock_task_context(struct task_struct *task, int ctxn, unsigned long *flags)
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{
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struct perf_event_context *ctx;
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rcu_read_lock();
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retry:
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/*
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* One of the few rules of preemptible RCU is that one cannot do
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* rcu_read_unlock() while holding a scheduler (or nested) lock when
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* part of the read side critical section was preemptible -- see
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* rcu_read_unlock_special().
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*
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* Since ctx->lock nests under rq->lock we must ensure the entire read
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* side critical section is non-preemptible.
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*/
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preempt_disable();
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rcu_read_lock();
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ctx = rcu_dereference(task->perf_event_ctxp[ctxn]);
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if (ctx) {
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/*
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@ -964,6 +974,8 @@ retry:
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raw_spin_lock_irqsave(&ctx->lock, *flags);
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if (ctx != rcu_dereference(task->perf_event_ctxp[ctxn])) {
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raw_spin_unlock_irqrestore(&ctx->lock, *flags);
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rcu_read_unlock();
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preempt_enable();
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goto retry;
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}
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@ -973,6 +985,7 @@ retry:
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}
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}
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rcu_read_unlock();
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preempt_enable();
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return ctx;
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}
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