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block/qapi: take stats->lock when reading BlockAcctStats for query-blockstats

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bdrv_query_blk_stats() reads BlockAcctStats's counters, latency
histogram, and per-interval TimedAverage stats without stats->lock,
while block_account_one_io() updates the same fields under that lock
from an iothread. timed_average_min()/max()/avg() make this worse
than a stale read: they call check_expirations(), which can reset a
window's sum/count/min/max -- a write, not just a read -- so this is
a genuine race with a concurrent writer, not merely a slower reader
like the scalar counters.

Take stats->lock for the whole call, both to close the race and to
make the returned snapshot internally consistent (previously each
field could reflect a different instant relative to concurrent
updates).

block_acct_queue_depth() used to take the lock itself on every call;
since bdrv_query_blk_stats() is its only caller and now already holds
the lock, that would self-deadlock. Make it require the caller to
hold stats->lock instead (documented and asserted).

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Message-ID: <20260724111311.4086859-3-den@openvz.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Denis V. Lunev 2026-07-24 13:13:10 +02:00 • committed by Kevin Wolf
parent 5b0ba385a0
commit 16f94ef4c6
3 changed files with 5 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

3
block/accounting.c
View file

@ -318,10 +318,9 @@ double block_acct_queue_depth(BlockAcctTimedStats *stats,
uint64_t sum, elapsed;
assert(type < BLOCK_MAX_IOTYPE);
assert(qemu_mutex_trylock(&stats->stats->lock) == -EBUSY);
qemu_mutex_lock(&stats->stats->lock);
sum = timed_average_sum(&stats->latency[type], &elapsed);
qemu_mutex_unlock(&stats->stats->lock);
return (double) sum / elapsed;
}

3
block/qapi.c
View file

@ -535,6 +535,8 @@ static void bdrv_query_blk_stats(BlockDeviceStats *ds, BlockBackend *blk)
BlockAcctTimedStats *ts = NULL;
BlockLatencyHistogram *hgram;
qemu_mutex_lock(&stats->lock);
ds->rd_bytes = stats->nr_bytes[BLOCK_ACCT_READ];
ds->wr_bytes = stats->nr_bytes[BLOCK_ACCT_WRITE];
ds->zone_append_bytes = stats->nr_bytes[BLOCK_ACCT_ZONE_APPEND];
@ -624,6 +626,7 @@ static void bdrv_query_blk_stats(BlockDeviceStats *ds, BlockBackend *blk)
= bdrv_latency_histogram_stats(&hgram[BLOCK_ACCT_ZONE_APPEND]);
ds->flush_latency_histogram
= bdrv_latency_histogram_stats(&hgram[BLOCK_ACCT_FLUSH]);
qemu_mutex_unlock(&stats->lock);
}
static BlockStats * GRAPH_RDLOCK

1
include/block/accounting.h
View file

@ -116,6 +116,7 @@ void block_acct_invalid(BlockAcctStats *stats, enum BlockAcctType type);
void block_acct_merge_done(BlockAcctStats *stats, enum BlockAcctType type,
int num_requests);
int64_t block_acct_idle_time_ns(BlockAcctStats *stats);
/* Caller must hold stats->stats->lock. */
double block_acct_queue_depth(BlockAcctTimedStats *stats,
enum BlockAcctType type);
int block_latency_histogram_set(BlockAcctStats *stats, enum BlockAcctType type,