On Thu, 2018-09-20 at 11:48 +-0800, Ming Lei wrote: +AD4 On Wed, Sep 19, 2018 at 03:45:29PM -0700, Bart Van Assche wrote: +AD4 +AD4 +- ret +AD0 -EBUSY+ADs +AD4 +AD4 +- if (blk+AF8-requests+AF8-in+AF8-flight(q) +AD0APQ 0) +AHs +AD4 +AD4 +- blk+AF8-freeze+AF8-queue+AF8-start(q)+ADs +AD4 +AD4 +- /+ACo +AD4 +AD4 +- +ACo Freezing a queue starts a transition of the +AD4 +AD4 queue +AD4 +AD4 +- +ACo usage counter to atomic mode. Wait until atomic +AD4 +AD4 +- +ACo mode has been reached. This involves calling +AD4 +AD4 +- +ACo call+AF8-rcu(). That call guarantees that later +AD4 +AD4 +- +ACo blk+AF8-queue+AF8-enter() calls see the pm-only state. +AD4 +AD4 See +AD4 +AD4 +- +ACo also http://lwn.net/Articles/573497/. +AD4 +AD4 +- +ACo-/ +AD4 +AD4 +- percpu+AF8-ref+AF8-switch+AF8-to+AF8-atomic+AF8-sync(+ACY-q- +AD4 +AD4 +AD4-q+AF8-usage+AF8-counter)+ADs +AD4 +AD4 +- if (percpu+AF8-ref+AF8-is+AF8-zero(+ACY-q-+AD4-q+AF8-usage+AF8-counter)) +AD4 +AD4 +- ret +AD0 0+ADs +AD4 +AD4 +- blk+AF8-mq+AF8-unfreeze+AF8-queue(q)+ADs +AD4 +AD4 Tejun doesn't agree on this kind of usage yet, so the ref has to be +AD4 dropped before calling blk+AF8-mq+AF8-unfreeze+AF8-queue(). I read all Tejuns' recent e-mails but I have not found any e-mail from Tejun in which he wrote that he disagrees with the above pattern. +AD4 Also, this way still can't address the race in the following link: +AD4 +AD4 https://marc.info/?l+AD0-linux-block+ACY-m+AD0-153732992701093+ACY-w+AD0-2 I think that the following patch is sufficient to fix that race: diff --git a/block/blk-core.c b/block/blk-core.c index ae092ca121d5..16dd3a989753 100644 --- a/block/blk-core.c +-+-+- b/block/blk-core.c +AEAAQA -942,8 +-942,6 +AEAAQA int blk+AF8-queue+AF8-enter(struct request+AF8-queue +ACo-q, blk+AF8-mq+AF8-req+AF8-flags+AF8-t flags) if (success) return 0+ADs - blk+AF8-pm+AF8-request+AF8-resume(q)+ADs - if (flags +ACY BLK+AF8-MQ+AF8-REQ+AF8-NOWAIT) return -EBUSY+ADs +AEAAQA -958,7 +-956,8 +AEAAQA int blk+AF8-queue+AF8-enter(struct request+AF8-queue +ACo-q, blk+AF8-mq+AF8-req+AF8-flags+AF8-t flags) wait+AF8-event(q-+AD4-mq+AF8-freeze+AF8-wq, (atomic+AF8-read(+ACY-q-+AD4-mq+AF8-freeze+AF8-depth) +AD0APQ 0 +ACYAJg - (pm +AHwAfA +ACE-blk+AF8-queue+AF8-pm+AF8-only(q))) +AHwAfA +- (pm +AHwAfA (blk+AF8-pm+AF8-request+AF8-resume(q), +- +ACE-blk+AF8-queue+AF8-pm+AF8-only(q)))) +AHwAfA blk+AF8-queue+AF8-dying(q))+ADs if (blk+AF8-queue+AF8-dying(q)) return -ENODEV+ADs Bart.
