Author: attilio
Date: Sat Jan 23 15:54:21 2010
New Revision: 202889
URL: http://svn.freebsd.org/changeset/base/202889
Log:
- Fix a race in sched_switch() of sched_4bsd.
In the case of the thread being on a sleepqueue or a turnstile, the
sched_lock was acquired (without the aid of the td_lock interface) and
the td_lock was dropped. This was going to break locking rules on other
threads willing to access to the thread (via the td_lock interface) and
modify his flags (allowed as long as the container lock was different
by the one used in sched_switch).
In order to prevent this situation, while sched_lock is acquired there
the td_lock gets blocked. [0]
- Merge the ULE's internal function thread_block_switch() into the global
thread_lock_block() and make the former semantic as the default for
thread_lock_block(). This means that thread_lock_block() will not
disable interrupts when called (and consequently thread_unlock_block()
will not re-enabled them when called). This should be done manually
when necessary.
Note, however, that ULE's thread_unblock_switch() is not reaped
because it does reflect a difference in semantic due in ULE (the
td_lock may not be necessarilly still blocked_lock when calling this).
While asymmetric, it does describe a remarkable difference in semantic
that is good to keep in mind.
[0] Reported by: Kohji Okuno
<okuno dot kohji at jp dot panasonic dot com>
Tested by: Giovanni Trematerra
<giovanni dot trematerra at gmail dot com>
MFC: 2 weeks
Modified:
head/sys/kern/kern_mutex.c
head/sys/kern/sched_4bsd.c
head/sys/kern/sched_ule.c
Modified: head/sys/kern/kern_mutex.c
==============================================================================
--- head/sys/kern/kern_mutex.c Sat Jan 23 15:28:18 2010 (r202888)
+++ head/sys/kern/kern_mutex.c Sat Jan 23 15:54:21 2010 (r202889)
@@ -616,7 +616,6 @@ thread_lock_block(struct thread *td)
{
struct mtx *lock;
- spinlock_enter();
THREAD_LOCK_ASSERT(td, MA_OWNED);
lock = td->td_lock;
td->td_lock = &blocked_lock;
@@ -631,7 +630,6 @@ thread_lock_unblock(struct thread *td, s
mtx_assert(new, MA_OWNED);
MPASS(td->td_lock == &blocked_lock);
atomic_store_rel_ptr((volatile void *)&td->td_lock, (uintptr_t)new);
- spinlock_exit();
}
void
Modified: head/sys/kern/sched_4bsd.c
==============================================================================
--- head/sys/kern/sched_4bsd.c Sat Jan 23 15:28:18 2010 (r202888)
+++ head/sys/kern/sched_4bsd.c Sat Jan 23 15:54:21 2010 (r202889)
@@ -920,9 +920,11 @@ sched_sleep(struct thread *td, int pri)
void
sched_switch(struct thread *td, struct thread *newtd, int flags)
{
+ struct mtx *tmtx;
struct td_sched *ts;
struct proc *p;
+ tmtx = NULL;
ts = td->td_sched;
p = td->td_proc;
@@ -931,10 +933,11 @@ sched_switch(struct thread *td, struct t
/*
* Switch to the sched lock to fix things up and pick
* a new thread.
+ * Block the td_lock in order to avoid breaking the critical path.
*/
if (td->td_lock != &sched_lock) {
mtx_lock_spin(&sched_lock);
- thread_unlock(td);
+ tmtx = thread_lock_block(td);
}
if ((td->td_flags & TDF_NOLOAD) == 0)
@@ -1004,7 +1007,7 @@ sched_switch(struct thread *td, struct t
(*dtrace_vtime_switch_func)(newtd);
#endif
- cpu_switch(td, newtd, td->td_lock);
+ cpu_switch(td, newtd, tmtx != NULL ? tmtx : td->td_lock);
lock_profile_obtain_lock_success(&sched_lock.lock_object,
0, 0, __FILE__, __LINE__);
/*
Modified: head/sys/kern/sched_ule.c
==============================================================================
--- head/sys/kern/sched_ule.c Sat Jan 23 15:28:18 2010 (r202888)
+++ head/sys/kern/sched_ule.c Sat Jan 23 15:54:21 2010 (r202889)
@@ -301,7 +301,6 @@ static int sched_pickcpu(struct thread *
static void sched_balance(void);
static int sched_balance_pair(struct tdq *, struct tdq *);
static inline struct tdq *sched_setcpu(struct thread *, int, int);
-static inline struct mtx *thread_block_switch(struct thread *);
static inline void thread_unblock_switch(struct thread *, struct mtx *);
static struct mtx *sched_switch_migrate(struct tdq *, struct thread *, int);
static int sysctl_kern_sched_topology_spec(SYSCTL_HANDLER_ARGS);
@@ -1106,9 +1105,11 @@ sched_setcpu(struct thread *td, int cpu,
* The hard case, migration, we need to block the thread first to
* prevent order reversals with other cpus locks.
*/
+ spinlock_enter();
thread_lock_block(td);
TDQ_LOCK(tdq);
thread_lock_unblock(td, TDQ_LOCKPTR(tdq));
+ spinlock_exit();
return (tdq);
}
@@ -1715,23 +1716,6 @@ sched_unlend_user_prio(struct thread *td
}
/*
- * Block a thread for switching. Similar to thread_block() but does not
- * bump the spin count.
- */
-static inline struct mtx *
-thread_block_switch(struct thread *td)
-{
- struct mtx *lock;
-
- THREAD_LOCK_ASSERT(td, MA_OWNED);
- lock = td->td_lock;
- td->td_lock = &blocked_lock;
- mtx_unlock_spin(lock);
-
- return (lock);
-}
-
-/*
* Handle migration from sched_switch(). This happens only for
* cpu binding.
*/
@@ -1749,7 +1733,7 @@ sched_switch_migrate(struct tdq *tdq, st
* not holding either run-queue lock.
*/
spinlock_enter();
- thread_block_switch(td); /* This releases the lock on tdq. */
+ thread_lock_block(td); /* This releases the lock on tdq. */
/*
* Acquire both run-queue locks before placing the thread on the new
@@ -1769,7 +1753,8 @@ sched_switch_migrate(struct tdq *tdq, st
}
/*
- * Release a thread that was blocked with thread_block_switch().
+ * Variadic version of thread_lock_unblock() that does not assume td_lock
+ * is blocked.
*/
static inline void
thread_unblock_switch(struct thread *td, struct mtx *mtx)
@@ -1825,7 +1810,7 @@ sched_switch(struct thread *td, struct t
} else {
/* This thread must be going to sleep. */
TDQ_LOCK(tdq);
- mtx = thread_block_switch(td);
+ mtx = thread_lock_block(td);
tdq_load_rem(tdq, td);
}
/*
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