From 7568615c1054907ea8c7701ab86dad51aa099888 Mon Sep 17 00:00:00 2001 From: Tony Battersby Date: Fri, 13 Feb 2015 12:09:44 -0500 Subject: [PATCH] sg: fix unkillable I/O wait deadlock with scsi-mq When using the write()/read() interface for submitting commands, the SCSI generic driver does not call blk_put_request() on a completed SCSI command until userspace calls read() to get the command completion. Since scsi-mq uses a fixed number of preallocated requests, this makes it possible for userspace to exhaust the entire preallocated supply of requests. For places in the kernel that call blk_get_request() with GFP_KERNEL, this can cause the calling process to deadlock in a permanent unkillable I/O wait in blk_get_request() -> ... -> bt_get(). For places in the kernel that call blk_get_request() with GFP_ATOMIC, this can cause blk_get_request() always to return -EWOULDBLOCK. Note that these problems happen only if scsi-mq is enabled. Prevent the problems by calling blk_put_request() as soon as the SCSI command completes instead of waiting for userspace to call read(). Cc: # 3.17+ Signed-off-by: Tony Battersby Acked-by: Douglas Gilbert Tested-by: Douglas Gilbert Signed-off-by: James Bottomley --- drivers/scsi/sg.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c index 6ad1480e87b7..208bf3c8a16c 100644 --- a/drivers/scsi/sg.c +++ b/drivers/scsi/sg.c @@ -1335,6 +1335,17 @@ sg_rq_end_io(struct request *rq, int uptodate) } /* Rely on write phase to clean out srp status values, so no "else" */ + /* + * Free the request as soon as it is complete so that its resources + * can be reused without waiting for userspace to read() the + * result. But keep the associated bio (if any) around until + * blk_rq_unmap_user() can be called from user context. + */ + srp->rq = NULL; + if (rq->cmd != rq->__cmd) + kfree(rq->cmd); + __blk_put_request(rq->q, rq); + write_lock_irqsave(&sfp->rq_list_lock, iflags); if (unlikely(srp->orphan)) { if (sfp->keep_orphan) @@ -1762,10 +1773,10 @@ sg_finish_rem_req(Sg_request *srp) SCSI_LOG_TIMEOUT(4, sg_printk(KERN_INFO, sfp->parentdp, "sg_finish_rem_req: res_used=%d\n", (int) srp->res_used)); - if (srp->rq) { - if (srp->bio) - ret = blk_rq_unmap_user(srp->bio); + if (srp->bio) + ret = blk_rq_unmap_user(srp->bio); + if (srp->rq) { if (srp->rq->cmd != srp->rq->__cmd) kfree(srp->rq->cmd); blk_put_request(srp->rq); -- 2.39.5