linux-kernel - [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able

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Date:   Tue,  3 Oct 2017 12:49:19 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     bigeasy@...utronix.de
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Dean Luick <dean.luick@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        linux-rdma@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>,
        Sebastian Sanchez <sebastian.sanchez@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able_nort()

From: Arnaldo Carvalho de Melo <acme@...hat.com>

sc_buffer_alloc() disables preemption that will be reenabled by either
pio_copy() or seg_pio_copy_end(). But before disabling preemption it
grabs a spin lock that will be dropped after it disables preemption,
which ends up triggering a warning in migrate_disable() later on.

    spin_lock_irqsave(&sc->alloc_lock)
      migrate_disable() ++p->migrate_disable -> 2
    preempt_disable()
    spin_unlock_irqrestore(&sc->alloc_lock)
      migrate_enable() in_atomic(), so just returns, migrate_disable stays at 2
    spin_lock_irqsave(some other lock) -> b00m

And the WARN_ON code ends up tripping over this over and over in
log_store().

Sequence captured via ftrace_dump_on_oops + crash utility 'dmesg'
command.

[512258.613862] sm-3297 16 .....11 359465349134644: sc_buffer_alloc <-hfi1_verbs_send_pio
[512258.613876] sm-3297 16 .....11 359465349134719: migrate_disable <-sc_buffer_alloc
[512258.613890] sm-3297 16 .....12 359465349134798: rt_spin_lock <-sc_buffer_alloc
[512258.613903] sm-3297 16 ....112 359465349135481: rt_spin_unlock <-sc_buffer_alloc
[512258.613916] sm-3297 16 ....112 359465349135556: migrate_enable <-sc_buffer_alloc
[512258.613935] sm-3297 16 ....112 359465349135788: seg_pio_copy_start <-hfi1_verbs_send_pio
[512258.613954] sm-3297 16 ....112 359465349136273: update_sge <-hfi1_verbs_send_pio
[512258.613981] sm-3297 16 ....112 359465349136373: seg_pio_copy_mid <-hfi1_verbs_send_pio
[512258.613999] sm-3297 16 ....112 359465349136873: update_sge <-hfi1_verbs_send_pio
[512258.614017] sm-3297 16 ....112 359465349136956: seg_pio_copy_mid <-hfi1_verbs_send_pio
[512258.614035] sm-3297 16 ....112 359465349137221: seg_pio_copy_end <-hfi1_verbs_send_pio
[512258.614048] sm-3297 16 .....12 359465349137360: migrate_disable <-hfi1_verbs_send_pio
[512258.614065] sm-3297 16 .....12 359465349137476: warn_slowpath_null <-migrate_disable
[512258.614081] sm-3297 16 .....12 359465349137564: __warn <-warn_slowpath_null
[512258.614088] sm-3297 16 .....12 359465349137958: printk <-__warn
[512258.614096] sm-3297 16 .....12 359465349138055: vprintk_default <-printk
[512258.614104] sm-3297 16 .....12 359465349138144: vprintk_emit <-vprintk_default
[512258.614111] sm-3297 16 d....12 359465349138312: _raw_spin_lock <-vprintk_emit
[512258.614119] sm-3297 16 d...112 359465349138789: log_store <-vprintk_emit
[512258.614127] sm-3297 16 .....12 359465349139068: migrate_disable <-vprintk_emit

According to a discussion (see Link: below) on the linux-rt-users
mailing list, this locking is done for performance reasons, not for
correctness, so use the _nort() variants to avoid the above problem.

Suggested-by: Julia Cartwright <julia@...com>
Cc: Clark Williams <williams@...hat.com>
Cc: Dean Luick <dean.luick@...el.com>
Cc: Dennis Dalessandro <dennis.dalessandro@...el.com>
Cc: Doug Ledford <dledford@...hat.com>
Cc: Kaike Wan <kaike.wan@...el.com>
Cc: Leon Romanovsky <leonro@...lanox.com>
Cc: linux-rdma@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>
Cc: Sebastian Sanchez <sebastian.sanchez@...el.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20170926210045.GO29872@jcartwri.amer.corp.natinst.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 drivers/infiniband/hw/hfi1/pio.c      | 2 +-
 drivers/infiniband/hw/hfi1/pio_copy.c | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/infiniband/hw/hfi1/pio.c b/drivers/infiniband/hw/hfi1/pio.c
index 615be68e40b3..3a30bde9a07b 100644
--- a/drivers/infiniband/hw/hfi1/pio.c
+++ b/drivers/infiniband/hw/hfi1/pio.c
@@ -1421,7 +1421,7 @@ struct pio_buf *sc_buffer_alloc(struct send_context *sc, u32 dw_len,
 
 	/* there is enough room */
 
-	preempt_disable();
+	preempt_disable_nort();
 	this_cpu_inc(*sc->buffers_allocated);
 
 	/* read this once */
diff --git a/drivers/infiniband/hw/hfi1/pio_copy.c b/drivers/infiniband/hw/hfi1/pio_copy.c
index 03024cec78dd..c3f48f705c97 100644
--- a/drivers/infiniband/hw/hfi1/pio_copy.c
+++ b/drivers/infiniband/hw/hfi1/pio_copy.c
@@ -162,7 +162,7 @@ void pio_copy(struct hfi1_devdata *dd, struct pio_buf *pbuf, u64 pbc,
 
 	/* finished with this buffer */
 	this_cpu_dec(*pbuf->sc->buffers_allocated);
-	preempt_enable();
+	preempt_enable_nort();
 }
 
 /*
@@ -753,5 +753,5 @@ void seg_pio_copy_end(struct pio_buf *pbuf)
 
 	/* finished with this buffer */
 	this_cpu_dec(*pbuf->sc->buffers_allocated);
-	preempt_enable();
+	preempt_enable_nort();
 }
-- 
2.13.6