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c0fe5dcb91
condition variable (waiters_pending) was set before being added to the poll queue via poll_wait(). This allowed for a small race window to happen where an event could come in, check the condition variable see it set to true, clear it, and then wake all the waiters. But because the waiter set the variable before adding itself to the queue, the waker could have cleared the variable after it was set and then miss waking it up as it wasn't added to the queue yet. Discussing this bug, we realized that a memory barrier needed to be added too, for the rare case that something polls for a single trace event to happen (and just one, no more to come in), and miss the wakeup due to memory ordering. Ideally, a memory barrier needs to be added on the writer side too, but as that will kill tracing performance and this is for a situation that tracing wasn't even designed for (who traces one instance of an event, use a printk instead!), this isn't worth adding the barrier. But we can in the future add the barrier for when the buffer goes from empty to the first event, as that would cover this case. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAABAgAGBQJT/IgUAAoJEKQekfcNnQGumJgIALG63LVR4ZS+bjHGTxyovCiL EE6Aic7/hHB8ajn/QZJAaVbaxf0woIyPaU6NP5p17rgF44gTtaEzg3hDqOXkBXvh aMLTEz2Xm6nu1VQ5vk//9qqplE+WMXWS6YjjnxErRja90cBZblFy9h9LzwwexLkm oXmhjVF1ke5AKFiXQ+Dj9LJse80MvSEEFk1eeUR7oNqK/4rwzKmkefkUnk2NbST4 cFkOAbTfZnMXlhUhB2EY2Ptprty3scrA7bpe00ClzFmoQ9MxDVlLJBN9aEjaTnxM zKiXsxy/eJ+0IPSOSEajh3IJb96sbqZnt++28vDhck3e6k3G4CQwbuktPdQXUo8= =jful -----END PGP SIGNATURE----- Merge tag 'trace-fixes-v3.17-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace Pull trace buffer epoll hang fix from Steven Rostedt: "Josef Bacik found a bug in the ring_buffer_poll_wait() where the condition variable (waiters_pending) was set before being added to the poll queue via poll_wait(). This allowed for a small race window to happen where an event could come in, check the condition variable see it set to true, clear it, and then wake all the waiters. But because the waiter set the variable before adding itself to the queue, the waker could have cleared the variable after it was set and then miss waking it up as it wasn't added to the queue yet. Discussing this bug, we realized that a memory barrier needed to be added too, for the rare case that something polls for a single trace event to happen (and just one, no more to come in), and miss the wakeup due to memory ordering. Ideally, a memory barrier needs to be added on the writer side too, but as that will kill tracing performance and this is for a situation that tracing wasn't even designed for (who traces one instance of an event, use a printk instead!), this isn't worth adding the barrier. But we can in the future add the barrier for when the buffer goes from empty to the first event, as that would cover this case" * tag 'trace-fixes-v3.17-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace: trace: Fix epoll hang when we race with new entries |
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| bpf | ||
| debug | ||
| events | ||
| gcov | ||
| irq | ||
| locking | ||
| power | ||
| printk | ||
| rcu | ||
| sched | ||
| time | ||
| trace | ||
| .gitignore | ||
| acct.c | ||
| async.c | ||
| audit_tree.c | ||
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| compat.c | ||
| configs.c | ||
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| cred.c | ||
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| irq_work.c | ||
| jump_label.c | ||
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| kcmp.c | ||
| Kconfig.freezer | ||
| Kconfig.hz | ||
| Kconfig.locks | ||
| Kconfig.preempt | ||
| kexec.c | ||
| kmod.c | ||
| kprobes.c | ||
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| Makefile | ||
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| ptrace.c | ||
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| stacktrace.c | ||
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| system_certificates.S | ||
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| task_work.c | ||
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| tracepoint.c | ||
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