Author: hselasky
Date: Mon Aug 21 11:51:40 2017
New Revision: 322746
URL: https://svnweb.freebsd.org/changeset/base/322746
Log:
Fix for deadlock situation in the LinuxKPI's RCU synchronize API.
Deadlock condition:
The return value of TDQ_LOCKPTR(td) is the same for two threads.
1) The first thread signals a wakeup while keeping the rcu_read_lock().
This invokes sched_add() which in turn will try to lock TDQ_LOCK().
2) The second thread is calling synchronize_rcu() calling mi_switch() over
and over again trying to yield(). This prevents the first thread from running
and releasing the RCU reader lock.
Solution:
Release the thread lock while yielding to allow other threads to acquire the
lock pointed to by TDQ_LOCKPTR(td).
Found by: KrishnamRaju ErapaRaju <krish...@chelsio.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies
Modified:
head/sys/compat/linuxkpi/common/src/linux_rcu.c
Modified: head/sys/compat/linuxkpi/common/src/linux_rcu.c
==============================================================================
--- head/sys/compat/linuxkpi/common/src/linux_rcu.c Mon Aug 21 10:26:11
2017 (r322745)
+++ head/sys/compat/linuxkpi/common/src/linux_rcu.c Mon Aug 21 11:51:40
2017 (r322746)
@@ -258,6 +258,15 @@ linux_synchronize_rcu_cb(ck_epoch_t *epoch __unused, c
sched_prio(td, prio);
/* task switch */
mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
+
+ /*
+ * Release the thread lock while yielding to
+ * allow other threads to acquire the lock
+ * pointed to by TDQ_LOCKPTR(td). Else a
+ * deadlock like situation might happen.
+ */
+ thread_unlock(td);
+ thread_lock(td);
}
} else {
/*
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