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| author | Jens Axboe <axboe@kernel.dk> | 2017-10-04 19:22:24 +0200 |
|---|---|---|
| committer | Jens Axboe <axboe@kernel.dk> | 2017-10-04 19:22:24 +0200 |
| commit | fc13457f74dcf054b0d17efb7b94b46fdf17f412 (patch) | |
| tree | b7a6a14246cf9e9c9967c7072d96a0010d117164 | |
| parent | blk-mq: attempt to fix atomic flag memory ordering (diff) | |
| download | linux-fc13457f74dcf054b0d17efb7b94b46fdf17f412.tar.xz linux-fc13457f74dcf054b0d17efb7b94b46fdf17f412.zip | |
blk-mq: document the need to have STARTED and COMPLETED share a byte
For memory ordering guarantees on stores, we need to ensure that
these two bits share the same byte of storage in the unsigned
long. Add a comment as to why, and a BUILD_BUG_ON() to ensure that
we don't violate this requirement.
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
| -rw-r--r-- | block/blk-mq.c | 6 | ||||
| -rw-r--r-- | block/blk.h | 7 |
2 files changed, 13 insertions, 0 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c index a7b46b754a7c..076cbab9c3e0 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2923,6 +2923,12 @@ EXPORT_SYMBOL_GPL(blk_mq_poll); static int __init blk_mq_init(void) { + /* + * See comment in block/blk.h rq_atomic_flags enum + */ + BUILD_BUG_ON((REQ_ATOM_STARTED / BITS_PER_BYTE) != + (REQ_ATOM_COMPLETE / BITS_PER_BYTE)); + cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL, blk_mq_hctx_notify_dead); return 0; diff --git a/block/blk.h b/block/blk.h index fda5a4632aba..6ac43dfd68a7 100644 --- a/block/blk.h +++ b/block/blk.h @@ -122,8 +122,15 @@ void blk_account_io_done(struct request *req); * Internal atomic flags for request handling */ enum rq_atomic_flags { + /* + * Keep these two bits first - not because we depend on the + * value of them, but we do depend on them being in the same + * byte of storage to ensure ordering on writes. Keeping them + * first will achieve that nicely. + */ REQ_ATOM_COMPLETE = 0, REQ_ATOM_STARTED, + REQ_ATOM_POLL_SLEPT, }; |