From dc121a264dda44e9a0551374ae3baf8fa08ec0e4 Mon Sep 17 00:00:00 2001 From: Andriy Gapon Date: Wed, 24 Jun 2015 14:49:49 +0000 Subject: [PATCH] 5981 Deadlock in dmu_objset_find_dp illumos/illumos-gate@1d3f896f5469c69c1339890ec3d68e9feddb0343 https://www.illumos.org/issues/5981 When dmu_objset_find_dp gets called with a read lock held, it fans out the work to the task queue. Each task in turn acquires its own read lock before calling the callback. If during this process anyone tries to a acquire a write lock, it will stall all read lock requests.Thus the tasks will never finish, the read lock of the caller will never get freed and the write lock never acquired. deadlock. Reviewed by: Matthew Ahrens Reviewed by: Dan McDonald Approved by: Robert Mustacchi Author: Arne Jansen --- uts/common/fs/zfs/dmu_objset.c | 10 +++++++++- uts/common/fs/zfs/dsl_pool.c | 7 +++++++ uts/common/fs/zfs/rrwlock.c | 25 ++++++++++++++++++++++--- uts/common/fs/zfs/sys/dsl_pool.h | 1 + uts/common/fs/zfs/sys/rrwlock.h | 1 + 5 files changed, 40 insertions(+), 4 deletions(-) diff --git a/uts/common/fs/zfs/dmu_objset.c b/uts/common/fs/zfs/dmu_objset.c index cfe9e90f1b2f..ce3d163b1e18 100644 --- a/uts/common/fs/zfs/dmu_objset.c +++ b/uts/common/fs/zfs/dmu_objset.c @@ -1745,7 +1745,15 @@ dmu_objset_find_dp_cb(void *arg) dmu_objset_find_ctx_t *dcp = arg; dsl_pool_t *dp = dcp->dc_dp; - dsl_pool_config_enter(dp, FTAG); + /* + * We need to get a pool_config_lock here, as there are several + * asssert(pool_config_held) down the stack. Getting a lock via + * dsl_pool_config_enter is risky, as it might be stalled by a + * pending writer. This would deadlock, as the write lock can + * only be granted when our parent thread gives up the lock. + * The _prio interface gives us priority over a pending writer. + */ + dsl_pool_config_enter_prio(dp, FTAG); dmu_objset_find_dp_impl(dcp); diff --git a/uts/common/fs/zfs/dsl_pool.c b/uts/common/fs/zfs/dsl_pool.c index 8c5d820edea2..ddd8e7344773 100644 --- a/uts/common/fs/zfs/dsl_pool.c +++ b/uts/common/fs/zfs/dsl_pool.c @@ -1045,6 +1045,13 @@ dsl_pool_config_enter(dsl_pool_t *dp, void *tag) rrw_enter(&dp->dp_config_rwlock, RW_READER, tag); } +void +dsl_pool_config_enter_prio(dsl_pool_t *dp, void *tag) +{ + ASSERT(!rrw_held(&dp->dp_config_rwlock, RW_READER)); + rrw_enter_read_prio(&dp->dp_config_rwlock, tag); +} + void dsl_pool_config_exit(dsl_pool_t *dp, void *tag) { diff --git a/uts/common/fs/zfs/rrwlock.c b/uts/common/fs/zfs/rrwlock.c index e3aa01a31c77..51394c01c431 100644 --- a/uts/common/fs/zfs/rrwlock.c +++ b/uts/common/fs/zfs/rrwlock.c @@ -159,8 +159,8 @@ rrw_destroy(rrwlock_t *rrl) refcount_destroy(&rrl->rr_linked_rcount); } -void -rrw_enter_read(rrwlock_t *rrl, void *tag) +static void +rrw_enter_read_impl(rrwlock_t *rrl, boolean_t prio, void *tag) { mutex_enter(&rrl->rr_lock); #if !defined(DEBUG) && defined(_KERNEL) @@ -176,7 +176,7 @@ rrw_enter_read(rrwlock_t *rrl, void *tag) ASSERT(refcount_count(&rrl->rr_anon_rcount) >= 0); while (rrl->rr_writer != NULL || (rrl->rr_writer_wanted && - refcount_is_zero(&rrl->rr_anon_rcount) && + refcount_is_zero(&rrl->rr_anon_rcount) && !prio && rrn_find(rrl) == NULL)) cv_wait(&rrl->rr_cv, &rrl->rr_lock); @@ -191,6 +191,25 @@ rrw_enter_read(rrwlock_t *rrl, void *tag) mutex_exit(&rrl->rr_lock); } +void +rrw_enter_read(rrwlock_t *rrl, void *tag) +{ + rrw_enter_read_impl(rrl, B_FALSE, tag); +} + +/* + * take a read lock even if there are pending write lock requests. if we want + * to take a lock reentrantly, but from different threads (that have a + * relationship to each other), the normal detection mechanism to overrule + * the pending writer does not work, so we have to give an explicit hint here. + */ +void +rrw_enter_read_prio(rrwlock_t *rrl, void *tag) +{ + rrw_enter_read_impl(rrl, B_TRUE, tag); +} + + void rrw_enter_write(rrwlock_t *rrl) { diff --git a/uts/common/fs/zfs/sys/dsl_pool.h b/uts/common/fs/zfs/sys/dsl_pool.h index 0f6d17514e45..0e27a538208c 100644 --- a/uts/common/fs/zfs/sys/dsl_pool.h +++ b/uts/common/fs/zfs/sys/dsl_pool.h @@ -152,6 +152,7 @@ void dsl_pool_upgrade_dir_clones(dsl_pool_t *dp, dmu_tx_t *tx); void dsl_pool_mos_diduse_space(dsl_pool_t *dp, int64_t used, int64_t comp, int64_t uncomp); void dsl_pool_config_enter(dsl_pool_t *dp, void *tag); +void dsl_pool_config_enter_prio(dsl_pool_t *dp, void *tag); void dsl_pool_config_exit(dsl_pool_t *dp, void *tag); boolean_t dsl_pool_config_held(dsl_pool_t *dp); boolean_t dsl_pool_config_held_writer(dsl_pool_t *dp); diff --git a/uts/common/fs/zfs/sys/rrwlock.h b/uts/common/fs/zfs/sys/rrwlock.h index bddf0768ae07..947b51f8728f 100644 --- a/uts/common/fs/zfs/sys/rrwlock.h +++ b/uts/common/fs/zfs/sys/rrwlock.h @@ -70,6 +70,7 @@ void rrw_init(rrwlock_t *rrl, boolean_t track_all); void rrw_destroy(rrwlock_t *rrl); void rrw_enter(rrwlock_t *rrl, krw_t rw, void *tag); void rrw_enter_read(rrwlock_t *rrl, void *tag); +void rrw_enter_read_prio(rrwlock_t *rrl, void *tag); void rrw_enter_write(rrwlock_t *rrl); void rrw_exit(rrwlock_t *rrl, void *tag); boolean_t rrw_held(rrwlock_t *rrl, krw_t rw);