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![shaohua.li@intel.com](/assets/img/avatar_default.png)
In some drives, flush requests are non-queueable. When flush request is
running, normal read/write requests can't run. If block layer dispatches
such request, driver can't handle it and requeue it. Tejun suggested we
can hold the queue when flush is running. This can avoid unnecessary
requeue. Also this can improve performance. For example, we have
request flush1, write1, flush 2. flush1 is dispatched, then queue is
hold, write1 isn't inserted to queue. After flush1 is finished, flush2
will be dispatched. Since disk cache is already clean, flush2 will be
finished very soon, so looks like flush2 is folded to flush1.
In my test, the queue holding completely solves a regression introduced by
commit 53d63e6b0d
:
block: make the flush insertion use the tail of the dispatch list
It's not a preempt type request, in fact we have to insert it
behind requests that do specify INSERT_FRONT.
which causes about 20% regression running a sysbench fileio
workload.
Stable: 2.6.39 only
Cc: stable@kernel.org
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
443 lines
13 KiB
C
443 lines
13 KiB
C
/*
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* Functions to sequence FLUSH and FUA writes.
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*
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* Copyright (C) 2011 Max Planck Institute for Gravitational Physics
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* Copyright (C) 2011 Tejun Heo <tj@kernel.org>
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*
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* This file is released under the GPLv2.
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*
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* REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
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* optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
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* properties and hardware capability.
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*
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* If a request doesn't have data, only REQ_FLUSH makes sense, which
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* indicates a simple flush request. If there is data, REQ_FLUSH indicates
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* that the device cache should be flushed before the data is executed, and
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* REQ_FUA means that the data must be on non-volatile media on request
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* completion.
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*
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* If the device doesn't have writeback cache, FLUSH and FUA don't make any
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* difference. The requests are either completed immediately if there's no
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* data or executed as normal requests otherwise.
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*
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* If the device has writeback cache and supports FUA, REQ_FLUSH is
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* translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
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*
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* If the device has writeback cache and doesn't support FUA, REQ_FLUSH is
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* translated to PREFLUSH and REQ_FUA to POSTFLUSH.
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*
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* The actual execution of flush is double buffered. Whenever a request
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* needs to execute PRE or POSTFLUSH, it queues at
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* q->flush_queue[q->flush_pending_idx]. Once certain criteria are met, a
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* flush is issued and the pending_idx is toggled. When the flush
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* completes, all the requests which were pending are proceeded to the next
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* step. This allows arbitrary merging of different types of FLUSH/FUA
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* requests.
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*
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* Currently, the following conditions are used to determine when to issue
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* flush.
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*
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* C1. At any given time, only one flush shall be in progress. This makes
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* double buffering sufficient.
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*
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* C2. Flush is deferred if any request is executing DATA of its sequence.
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* This avoids issuing separate POSTFLUSHes for requests which shared
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* PREFLUSH.
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*
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* C3. The second condition is ignored if there is a request which has
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* waited longer than FLUSH_PENDING_TIMEOUT. This is to avoid
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* starvation in the unlikely case where there are continuous stream of
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* FUA (without FLUSH) requests.
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*
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* For devices which support FUA, it isn't clear whether C2 (and thus C3)
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* is beneficial.
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*
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* Note that a sequenced FLUSH/FUA request with DATA is completed twice.
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* Once while executing DATA and again after the whole sequence is
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* complete. The first completion updates the contained bio but doesn't
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* finish it so that the bio submitter is notified only after the whole
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* sequence is complete. This is implemented by testing REQ_FLUSH_SEQ in
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* req_bio_endio().
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*
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* The above peculiarity requires that each FLUSH/FUA request has only one
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* bio attached to it, which is guaranteed as they aren't allowed to be
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* merged in the usual way.
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*/
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/bio.h>
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#include <linux/blkdev.h>
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#include <linux/gfp.h>
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#include "blk.h"
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/* FLUSH/FUA sequences */
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enum {
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REQ_FSEQ_PREFLUSH = (1 << 0), /* pre-flushing in progress */
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REQ_FSEQ_DATA = (1 << 1), /* data write in progress */
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REQ_FSEQ_POSTFLUSH = (1 << 2), /* post-flushing in progress */
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REQ_FSEQ_DONE = (1 << 3),
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REQ_FSEQ_ACTIONS = REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
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REQ_FSEQ_POSTFLUSH,
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/*
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* If flush has been pending longer than the following timeout,
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* it's issued even if flush_data requests are still in flight.
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*/
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FLUSH_PENDING_TIMEOUT = 5 * HZ,
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};
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static bool blk_kick_flush(struct request_queue *q);
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static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
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{
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unsigned int policy = 0;
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if (fflags & REQ_FLUSH) {
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if (rq->cmd_flags & REQ_FLUSH)
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policy |= REQ_FSEQ_PREFLUSH;
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if (blk_rq_sectors(rq))
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policy |= REQ_FSEQ_DATA;
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if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
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policy |= REQ_FSEQ_POSTFLUSH;
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}
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return policy;
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}
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static unsigned int blk_flush_cur_seq(struct request *rq)
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{
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return 1 << ffz(rq->flush.seq);
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}
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static void blk_flush_restore_request(struct request *rq)
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{
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/*
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* After flush data completion, @rq->bio is %NULL but we need to
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* complete the bio again. @rq->biotail is guaranteed to equal the
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* original @rq->bio. Restore it.
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*/
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rq->bio = rq->biotail;
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/* make @rq a normal request */
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rq->cmd_flags &= ~REQ_FLUSH_SEQ;
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rq->end_io = NULL;
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}
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/**
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* blk_flush_complete_seq - complete flush sequence
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* @rq: FLUSH/FUA request being sequenced
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* @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
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* @error: whether an error occurred
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*
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* @rq just completed @seq part of its flush sequence, record the
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* completion and trigger the next step.
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*
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* CONTEXT:
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* spin_lock_irq(q->queue_lock)
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*
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* RETURNS:
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* %true if requests were added to the dispatch queue, %false otherwise.
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*/
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static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
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int error)
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{
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struct request_queue *q = rq->q;
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struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
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bool queued = false;
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BUG_ON(rq->flush.seq & seq);
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rq->flush.seq |= seq;
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if (likely(!error))
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seq = blk_flush_cur_seq(rq);
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else
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seq = REQ_FSEQ_DONE;
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switch (seq) {
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case REQ_FSEQ_PREFLUSH:
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case REQ_FSEQ_POSTFLUSH:
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/* queue for flush */
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if (list_empty(pending))
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q->flush_pending_since = jiffies;
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list_move_tail(&rq->flush.list, pending);
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break;
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case REQ_FSEQ_DATA:
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list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
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list_add(&rq->queuelist, &q->queue_head);
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queued = true;
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break;
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case REQ_FSEQ_DONE:
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/*
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* @rq was previously adjusted by blk_flush_issue() for
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* flush sequencing and may already have gone through the
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* flush data request completion path. Restore @rq for
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* normal completion and end it.
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*/
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BUG_ON(!list_empty(&rq->queuelist));
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list_del_init(&rq->flush.list);
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blk_flush_restore_request(rq);
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__blk_end_request_all(rq, error);
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break;
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default:
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BUG();
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}
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return blk_kick_flush(q) | queued;
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}
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static void flush_end_io(struct request *flush_rq, int error)
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{
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struct request_queue *q = flush_rq->q;
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struct list_head *running = &q->flush_queue[q->flush_running_idx];
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bool queued = false;
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struct request *rq, *n;
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BUG_ON(q->flush_pending_idx == q->flush_running_idx);
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/* account completion of the flush request */
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q->flush_running_idx ^= 1;
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elv_completed_request(q, flush_rq);
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/* and push the waiting requests to the next stage */
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list_for_each_entry_safe(rq, n, running, flush.list) {
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unsigned int seq = blk_flush_cur_seq(rq);
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BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
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queued |= blk_flush_complete_seq(rq, seq, error);
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}
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/*
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* Kick the queue to avoid stall for two cases:
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* 1. Moving a request silently to empty queue_head may stall the
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* queue.
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* 2. When flush request is running in non-queueable queue, the
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* queue is hold. Restart the queue after flush request is finished
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* to avoid stall.
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* This function is called from request completion path and calling
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* directly into request_fn may confuse the driver. Always use
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* kblockd.
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*/
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if (queued || q->flush_queue_delayed)
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blk_run_queue_async(q);
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q->flush_queue_delayed = 0;
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}
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/**
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* blk_kick_flush - consider issuing flush request
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* @q: request_queue being kicked
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*
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* Flush related states of @q have changed, consider issuing flush request.
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* Please read the comment at the top of this file for more info.
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*
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* CONTEXT:
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* spin_lock_irq(q->queue_lock)
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*
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* RETURNS:
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* %true if flush was issued, %false otherwise.
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*/
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static bool blk_kick_flush(struct request_queue *q)
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{
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struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
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struct request *first_rq =
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list_first_entry(pending, struct request, flush.list);
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/* C1 described at the top of this file */
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if (q->flush_pending_idx != q->flush_running_idx || list_empty(pending))
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return false;
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/* C2 and C3 */
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if (!list_empty(&q->flush_data_in_flight) &&
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time_before(jiffies,
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q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
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return false;
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/*
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* Issue flush and toggle pending_idx. This makes pending_idx
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* different from running_idx, which means flush is in flight.
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*/
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blk_rq_init(q, &q->flush_rq);
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q->flush_rq.cmd_type = REQ_TYPE_FS;
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q->flush_rq.cmd_flags = WRITE_FLUSH | REQ_FLUSH_SEQ;
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q->flush_rq.rq_disk = first_rq->rq_disk;
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q->flush_rq.end_io = flush_end_io;
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q->flush_pending_idx ^= 1;
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list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
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return true;
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}
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static void flush_data_end_io(struct request *rq, int error)
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{
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struct request_queue *q = rq->q;
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/*
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* After populating an empty queue, kick it to avoid stall. Read
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* the comment in flush_end_io().
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*/
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if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))
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blk_run_queue_async(q);
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}
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/**
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* blk_insert_flush - insert a new FLUSH/FUA request
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* @rq: request to insert
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*
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* To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
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* @rq is being submitted. Analyze what needs to be done and put it on the
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* right queue.
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*
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* CONTEXT:
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* spin_lock_irq(q->queue_lock)
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*/
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void blk_insert_flush(struct request *rq)
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{
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struct request_queue *q = rq->q;
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unsigned int fflags = q->flush_flags; /* may change, cache */
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unsigned int policy = blk_flush_policy(fflags, rq);
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BUG_ON(rq->end_io);
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BUG_ON(!rq->bio || rq->bio != rq->biotail);
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/*
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* @policy now records what operations need to be done. Adjust
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* REQ_FLUSH and FUA for the driver.
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*/
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rq->cmd_flags &= ~REQ_FLUSH;
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if (!(fflags & REQ_FUA))
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rq->cmd_flags &= ~REQ_FUA;
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/*
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* If there's data but flush is not necessary, the request can be
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* processed directly without going through flush machinery. Queue
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* for normal execution.
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*/
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if ((policy & REQ_FSEQ_DATA) &&
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!(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
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list_add_tail(&rq->queuelist, &q->queue_head);
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return;
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}
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/*
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* @rq should go through flush machinery. Mark it part of flush
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* sequence and submit for further processing.
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*/
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memset(&rq->flush, 0, sizeof(rq->flush));
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INIT_LIST_HEAD(&rq->flush.list);
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rq->cmd_flags |= REQ_FLUSH_SEQ;
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rq->end_io = flush_data_end_io;
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blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
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}
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/**
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* blk_abort_flushes - @q is being aborted, abort flush requests
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* @q: request_queue being aborted
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*
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* To be called from elv_abort_queue(). @q is being aborted. Prepare all
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* FLUSH/FUA requests for abortion.
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*
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* CONTEXT:
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* spin_lock_irq(q->queue_lock)
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*/
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void blk_abort_flushes(struct request_queue *q)
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{
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struct request *rq, *n;
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int i;
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/*
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* Requests in flight for data are already owned by the dispatch
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* queue or the device driver. Just restore for normal completion.
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*/
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list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
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list_del_init(&rq->flush.list);
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blk_flush_restore_request(rq);
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}
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/*
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* We need to give away requests on flush queues. Restore for
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* normal completion and put them on the dispatch queue.
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*/
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for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
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list_for_each_entry_safe(rq, n, &q->flush_queue[i],
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flush.list) {
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list_del_init(&rq->flush.list);
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blk_flush_restore_request(rq);
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list_add_tail(&rq->queuelist, &q->queue_head);
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}
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}
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}
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static void bio_end_flush(struct bio *bio, int err)
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{
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if (err)
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clear_bit(BIO_UPTODATE, &bio->bi_flags);
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if (bio->bi_private)
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complete(bio->bi_private);
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bio_put(bio);
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}
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/**
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* blkdev_issue_flush - queue a flush
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* @bdev: blockdev to issue flush for
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* @gfp_mask: memory allocation flags (for bio_alloc)
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* @error_sector: error sector
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*
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* Description:
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* Issue a flush for the block device in question. Caller can supply
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* room for storing the error offset in case of a flush error, if they
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* wish to. If WAIT flag is not passed then caller may check only what
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* request was pushed in some internal queue for later handling.
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*/
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int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
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sector_t *error_sector)
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{
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DECLARE_COMPLETION_ONSTACK(wait);
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struct request_queue *q;
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struct bio *bio;
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int ret = 0;
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if (bdev->bd_disk == NULL)
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return -ENXIO;
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q = bdev_get_queue(bdev);
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if (!q)
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return -ENXIO;
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/*
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* some block devices may not have their queue correctly set up here
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* (e.g. loop device without a backing file) and so issuing a flush
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* here will panic. Ensure there is a request function before issuing
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* the flush.
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*/
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if (!q->make_request_fn)
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return -ENXIO;
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bio = bio_alloc(gfp_mask, 0);
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bio->bi_end_io = bio_end_flush;
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bio->bi_bdev = bdev;
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bio->bi_private = &wait;
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bio_get(bio);
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submit_bio(WRITE_FLUSH, bio);
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wait_for_completion(&wait);
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/*
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* The driver must store the error location in ->bi_sector, if
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* it supports it. For non-stacked drivers, this should be
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* copied from blk_rq_pos(rq).
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*/
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if (error_sector)
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*error_sector = bio->bi_sector;
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if (!bio_flagged(bio, BIO_UPTODATE))
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ret = -EIO;
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bio_put(bio);
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return ret;
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}
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EXPORT_SYMBOL(blkdev_issue_flush);
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