Commit 6e8210ad authored by Gabriel Krisman Bertazi's avatar Gabriel Krisman Bertazi Committed by Greg Kroah-Hartman

blk-mq: Always schedule hctx->next_cpu

commit c02ebfdd upstream.

Commit 0e87e58b ("blk-mq: improve warning for running a queue on the
wrong CPU") attempts to avoid triggering the WARN_ON in
__blk_mq_run_hw_queue when the expected CPU is dead.  Problem is, in the
last batch execution before round robin, blk_mq_hctx_next_cpu can
schedule a dead CPU and also update next_cpu to the next alive CPU in
the mask, which will trigger the WARN_ON despite the previous

The following patch fixes this scenario by always scheduling the value
in hctx->next_cpu.  This changes the moment when we round-robin the CPU
running the hctx, but it really doesn't matter, since it still executes
BLK_MQ_CPU_WORK_BATCH times in a row before switching to another CPU.

Fixes: 0e87e58b ("blk-mq: improve warning for running a queue on the wrong CPU")
Signed-off-by: default avatarGabriel Krisman Bertazi <>
Signed-off-by: default avatarJens Axboe <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent ddf0c377
......@@ -842,7 +842,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
if (--hctx->next_cpu_batch <= 0) {
int cpu = hctx->next_cpu, next_cpu;
int next_cpu;
next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
if (next_cpu >= nr_cpu_ids)
......@@ -850,8 +850,6 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
hctx->next_cpu = next_cpu;
hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
return cpu;
return hctx->next_cpu;
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