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perf: Fix find_get_context() vs perf_event_exit_task() race

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find_get_context() must not install the new perf_event_context
if the task has already passed perf_event_exit_task().

If nothing else, this means the memory leak. Initially
ctx->refcount == 2, it is supposed that
perf_event_exit_task_context() should participate and do the
necessary put_ctx().

find_lively_task_by_vpid() checks PF_EXITING but this buys
nothing, by the time we call find_get_context() this task can be
already dead. To the point, cmpxchg() can succeed when the task
has already done the last schedule().

Change find_get_context() to populate task->perf_event_ctxp[]
under task->perf_event_mutex, this way we can trust PF_EXITING
because perf_event_exit_task() takes the same mutex.

Also, change perf_event_exit_task_context() to use
rcu_dereference(). Probably this is not strictly needed, but
with or without this change find_get_context() can race with
setup_new_exec()->perf_event_exit_task(), rcu_dereference()
looks better.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
LKML-Reference: <20110119182207.GB12183@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2011-01-19 19:22:07 +01:00 committed by Ingo Molnar
parent c56eb8fb6d
commit dbe08d82ce
1 changed files with 20 additions and 14 deletions
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34
kernel/perf_event.c
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@ -2201,13 +2201,6 @@ find_lively_task_by_vpid(pid_t vpid)
if (!task)
return ERR_PTR(-ESRCH);
/*
* Can't attach events to a dying task.
*/
err = -ESRCH;
if (task->flags & PF_EXITING)
goto errout;
/* Reuse ptrace permission checks for now. */
err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
@ -2268,14 +2261,27 @@ find_get_context(struct pmu *pmu, struct task_struct *task, int cpu)
get_ctx(ctx);
if (cmpxchg(&task->perf_event_ctxp[ctxn], NULL, ctx)) {
/*
* We raced with some other task; use
* the context they set.
*/
err = 0;
mutex_lock(&task->perf_event_mutex);
/*
* If it has already passed perf_event_exit_task().
* we must see PF_EXITING, it takes this mutex too.
*/
if (task->flags & PF_EXITING)
err = -ESRCH;
else if (task->perf_event_ctxp[ctxn])
err = -EAGAIN;
else
rcu_assign_pointer(task->perf_event_ctxp[ctxn], ctx);
mutex_unlock(&task->perf_event_mutex);
if (unlikely(err)) {
put_task_struct(task);
kfree(ctx);
goto retry;
if (err == -EAGAIN)
goto retry;
goto errout;
}
}
@ -6127,7 +6133,7 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
* scheduled, so we are now safe from rescheduling changing
* our context.
*/
child_ctx = child->perf_event_ctxp[ctxn];
child_ctx = rcu_dereference(child->perf_event_ctxp[ctxn]);
task_ctx_sched_out(child_ctx, EVENT_ALL);
/*