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witness: don't warn about matrix inconsistencies without holding the mutex

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Lock order checking is done without the witness mutex held, so multiple
threads that are racing to establish a new lock order may read matrix
entries that are in an inconsistent state. Don't print a warning in this
case, but instead just redo the check after taking the witness lock.

Differential Revision:	https://reviews.freebsd.org/D2713
Reviewed by:	jhb
MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division
This commit is contained in:
Mark Johnston 2015-06-07 18:59:47 +00:00
parent 3438d7decf
commit 8b84d791a0
1 changed files with 13 additions and 4 deletions
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17
sys/kern/subr_witness.c
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@ -1170,19 +1170,25 @@ witness_checkorder(struct lock_object *lock, int flags, const char *file,
/*
* Try to perform most checks without a lock. If this succeeds we
* can skip acquiring the lock and return success.
* can skip acquiring the lock and return success. Otherwise we redo
* the check with the lock held to handle races with concurrent updates.
*/
w1 = plock->li_lock->lo_witness;
if (witness_lock_order_check(w1, w))
return;
mtx_lock_spin(&w_mtx);
if (witness_lock_order_check(w1, w)) {
mtx_unlock_spin(&w_mtx);
return;
}
witness_lock_order_add(w1, w);
/*
* Check for duplicate locks of the same type. Note that we only
* have to check for this on the last lock we just acquired. Any
* other cases will be caught as lock order violations.
*/
mtx_lock_spin(&w_mtx);
witness_lock_order_add(w1, w);
if (w1 == w) {
i = w->w_index;
if (!(lock->lo_flags & LO_DUPOK) && !(flags & LOP_DUPOK) &&
@ -1996,7 +2002,10 @@ _isitmyx(struct witness *w1, struct witness *w2, int rmask, const char *fname)
/* The flags on one better be the inverse of the flags on the other */
if (!((WITNESS_ATOD(r1) == r2 && WITNESS_DTOA(r2) == r1) ||
(WITNESS_DTOA(r1) == r2 && WITNESS_ATOD(r2) == r1))) {
(WITNESS_DTOA(r1) == r2 && WITNESS_ATOD(r2) == r1))) {
/* Don't squawk if we're potentially racing with an update. */
if (!mtx_owned(&w_mtx))
return (0);
printf("%s: rmatrix mismatch between %s (index %d) and %s "
"(index %d): w_rmatrix[%d][%d] == %hhx but "
"w_rmatrix[%d][%d] == %hhx\n",