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author | Paolo Valente <paolo.valente@linaro.org> | 2017-06-05 11:11:15 +0300 |
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committer | Jens Axboe <axboe@fb.com> | 2017-06-08 18:51:10 +0300 |
commit | 8f9bebc33dd718283183582fc4a762e178552fb8 (patch) | |
tree | e3fe5ffbc42b51a54a5a9457e006bae360862a77 /block/bfq-iosched.c | |
parent | 85d0331aedff4646b2a2b14561c8be3678ffcee2 (diff) | |
download | linux-8f9bebc33dd718283183582fc4a762e178552fb8.tar.xz |
block, bfq: access and cache blkg data only when safe
In blk-cgroup, operations on blkg objects are protected with the
request_queue lock. This is no more the lock that protects
I/O-scheduler operations in blk-mq. In fact, the latter are now
protected with a finer-grained per-scheduler-instance lock. As a
consequence, although blkg lookups are also rcu-protected, blk-mq I/O
schedulers may see inconsistent data when they access blkg and
blkg-related objects. BFQ does access these objects, and does incur
this problem, in the following case.
The blkg_lookup performed in bfq_get_queue, being protected (only)
through rcu, may happen to return the address of a copy of the
original blkg. If this is the case, then the blkg_get performed in
bfq_get_queue, to pin down the blkg, is useless: it does not prevent
blk-cgroup code from destroying both the original blkg and all objects
directly or indirectly referred by the copy of the blkg. BFQ accesses
these objects, which typically causes a crash for NULL-pointer
dereference of memory-protection violation.
Some additional protection mechanism should be added to blk-cgroup to
address this issue. In the meantime, this commit provides a quick
temporary fix for BFQ: cache (when safe) blkg data that might
disappear right after a blkg_lookup.
In particular, this commit exploits the following facts to achieve its
goal without introducing further locks. Destroy operations on a blkg
invoke, as a first step, hooks of the scheduler associated with the
blkg. And these hooks are executed with bfqd->lock held for BFQ. As a
consequence, for any blkg associated with the request queue an
instance of BFQ is attached to, we are guaranteed that such a blkg is
not destroyed, and that all the pointers it contains are consistent,
while that instance is holding its bfqd->lock. A blkg_lookup performed
with bfqd->lock held then returns a fully consistent blkg, which
remains consistent until this lock is held. In more detail, this holds
even if the returned blkg is a copy of the original one.
Finally, also the object describing a group inside BFQ needs to be
protected from destruction on the blkg_free of the original blkg
(which invokes bfq_pd_free). This commit adds private refcounting for
this object, to let it disappear only after no bfq_queue refers to it
any longer.
This commit also removes or updates some stale comments on locking
issues related to blk-cgroup operations.
Reported-by: Tomas Konir <tomas.konir@gmail.com>
Reported-by: Lee Tibbert <lee.tibbert@gmail.com>
Reported-by: Marco Piazza <mpiazza@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Tested-by: Tomas Konir <tomas.konir@gmail.com>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Tested-by: Marco Piazza <mpiazza@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'block/bfq-iosched.c')
-rw-r--r-- | block/bfq-iosched.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 08ce45096350..ed93da2462ab 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -3665,7 +3665,7 @@ void bfq_put_queue(struct bfq_queue *bfqq) kmem_cache_free(bfq_pool, bfqq); #ifdef CONFIG_BFQ_GROUP_IOSCHED - bfqg_put(bfqg); + bfqg_and_blkg_put(bfqg); #endif } |