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Check the lock lists to see if they are empty directly rather than

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assigning a pointer to the list and then dereferencing the pointer as a
second step.  When the first spin lock is acquired, curthread is not in
a critical section so it may be preempted and would end up using another
CPUs lock list instead of its own.

When this code was in witness_lock() this sequence was safe as curthread
was in a critical section already since witness_lock() is called after the
lock is acquired.

Tested by:	Daniel Lang dl at leo.org
This commit is contained in:
John Baldwin 2004-07-09 17:46:27 +00:00
parent 6421d1dbcb
commit 776b99ee1a
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=131884
1 changed files with 22 additions and 10 deletions
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32
sys/kern/subr_witness.c
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@ -690,22 +690,34 @@ witness_checkorder(struct lock_object *lock, int flags, const char *file,
if (class->lc_flags & LC_SLEEPLOCK) {
/*
* Since spin locks include a critical section, this check
* impliclty enforces a lock order of all sleep locks before
* implicitly enforces a lock order of all sleep locks before
* all spin locks.
*/
if (td->td_critnest != 0)
panic("blockable sleep lock (%s) %s @ %s:%d",
class->lc_name, lock->lo_name, file, line);
lock_list = &td->td_sleeplocks;
} else
lock_list = PCPU_PTR(spinlocks);
/*
* Is this the first lock acquired? If so, then no order checking
* is needed.
*/
if (*lock_list == NULL)
return;
/*
* If this is the first lock acquired then just return as
* no order checking is needed.
*/
if (td->td_sleeplocks == NULL)
return;
lock_list = &td->td_sleeplocks;
} else {
/*
* If this is the first lock, just return as no order
* checking is needed. We check this in both if clauses
* here as unifying the check would require us to use a
* critical section to ensure we don't migrate while doing
* the check. Note that if this is not the first lock, we
* are already in a critical section and are safe for the
* rest of the check.
*/
if (PCPU_GET(spinlocks) == NULL)
return;
lock_list = PCPU_PTR(spinlocks);
}
/*
* Check to see if we are recursing on a lock we already own. If