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author | Juergen Gross <jgross@suse.com> | 2018-10-01 07:57:42 +0200 |
---|---|---|
committer | Juergen Gross <jgross@suse.com> | 2018-10-24 10:17:38 +0200 |
commit | 2ac2a7d4d9ff4e01e36f9c3d116582f6f655ab47 (patch) | |
tree | d28c742b57503c1118c87dc79f66515a2e38f010 | |
parent | xen/balloon: Grammar s/Is it/It is/ (diff) | |
download | linux-2ac2a7d4d9ff4e01e36f9c3d116582f6f655ab47.tar.xz linux-2ac2a7d4d9ff4e01e36f9c3d116582f6f655ab47.zip |
xen: fix race in xen_qlock_wait()
In the following situation a vcpu waiting for a lock might not be
woken up from xen_poll_irq():
CPU 1: CPU 2: CPU 3:
takes a spinlock
tries to get lock
-> xen_qlock_wait()
frees the lock
-> xen_qlock_kick(cpu2)
-> xen_clear_irq_pending()
takes lock again
tries to get lock
-> *lock = _Q_SLOW_VAL
-> *lock == _Q_SLOW_VAL ?
-> xen_poll_irq()
frees the lock
-> xen_qlock_kick(cpu3)
And cpu 2 will sleep forever.
This can be avoided easily by modifying xen_qlock_wait() to call
xen_poll_irq() only if the related irq was not pending and to call
xen_clear_irq_pending() only if it was pending.
Cc: stable@vger.kernel.org
Cc: Waiman.Long@hp.com
Cc: peterz@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
-rw-r--r-- | arch/x86/xen/spinlock.c | 15 |
1 files changed, 5 insertions, 10 deletions
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c index 23f6793af88a..290a69ec7d4d 100644 --- a/arch/x86/xen/spinlock.c +++ b/arch/x86/xen/spinlock.c @@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val) if (irq == -1) return; - /* clear pending */ - xen_clear_irq_pending(irq); - barrier(); + /* If irq pending already clear it and return. */ + if (xen_test_irq_pending(irq)) { + xen_clear_irq_pending(irq); + return; + } - /* - * We check the byte value after clearing pending IRQ to make sure - * that we won't miss a wakeup event because of the clearing. - * - * The sync_clear_bit() call in xen_clear_irq_pending() is atomic. - * So it is effectively a memory barrier for x86. - */ if (READ_ONCE(*byte) != val) return; |