95 lines
3.0 KiB
Diff
95 lines
3.0 KiB
Diff
From aafcd8f8692fb9e389608c1efad2e57c0bbb9362 Mon Sep 17 00:00:00 2001
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From: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Tue, 8 Apr 2014 15:30:07 -0700
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Subject: [PATCH 08/10] futex: avoid race between requeue and wake
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commit 69cd9eba38867a493a043bb13eb9b33cad5f1a9a upstream.
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Jan Stancek reported:
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"pthread_cond_broadcast/4-1.c testcase from openposix testsuite (LTP)
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occasionally fails, because some threads fail to wake up.
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Testcase creates 5 threads, which are all waiting on same condition.
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Main thread then calls pthread_cond_broadcast() without holding mutex,
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which calls:
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futex(uaddr1, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647, uaddr2, ..)
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This immediately wakes up single thread A, which unlocks mutex and
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tries to wake up another thread:
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futex(uaddr2, FUTEX_WAKE_PRIVATE, 1)
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If thread A manages to call futex_wake() before any waiters are
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requeued for uaddr2, no other thread is woken up"
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The ordering constraints for the hash bucket waiter counting are that
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the waiter counts have to be incremented _before_ getting the spinlock
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(because the spinlock acts as part of the memory barrier), but the
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"requeue" operation didn't honor those rules, and nobody had even
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thought about that case.
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This fairly simple patch just increments the waiter count for the target
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hash bucket (hb2) when requeing a futex before taking the locks. It
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then decrements them again after releasing the lock - the code that
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actually moves the futex(es) between hash buckets will do the additional
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required waiter count housekeeping.
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Reported-and-tested-by: Jan Stancek <jstancek@redhat.com>
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Acked-by: Davidlohr Bueso <davidlohr@hp.com>
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Cc: Peter Zijlstra <peterz@infradead.org>
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Cc: Thomas Gleixner <tglx@linutronix.de>
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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---
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kernel/futex.c | 5 +++++
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1 file changed, 5 insertions(+)
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diff --git a/kernel/futex.c b/kernel/futex.c
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index 08ec814..16b1f2c 100644
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--- a/kernel/futex.c
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+++ b/kernel/futex.c
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@@ -1450,6 +1450,7 @@ retry:
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hb2 = hash_futex(&key2);
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retry_private:
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+ hb_waiters_inc(hb2);
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double_lock_hb(hb1, hb2);
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if (likely(cmpval != NULL)) {
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@@ -1459,6 +1460,7 @@ retry_private:
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if (unlikely(ret)) {
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double_unlock_hb(hb1, hb2);
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+ hb_waiters_dec(hb2);
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ret = get_user(curval, uaddr1);
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if (ret)
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@@ -1508,6 +1510,7 @@ retry_private:
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break;
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case -EFAULT:
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double_unlock_hb(hb1, hb2);
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+ hb_waiters_dec(hb2);
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put_futex_key(&key2);
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put_futex_key(&key1);
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ret = fault_in_user_writeable(uaddr2);
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@@ -1517,6 +1520,7 @@ retry_private:
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case -EAGAIN:
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/* The owner was exiting, try again. */
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double_unlock_hb(hb1, hb2);
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+ hb_waiters_dec(hb2);
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put_futex_key(&key2);
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put_futex_key(&key1);
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cond_resched();
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@@ -1592,6 +1596,7 @@ retry_private:
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out_unlock:
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double_unlock_hb(hb1, hb2);
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+ hb_waiters_dec(hb2);
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/*
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* drop_futex_key_refs() must be called outside the spinlocks. During
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--
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1.9.2
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